Landracing Forum
Tech Information => Steering - Suspension - Rear End => Topic started by: CobraBall on July 25, 2008, 05:22:24 PM
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I am familiar with a Bonneville STR class '27 Ford/ 32 Chassis w/ a small block V8. The roadster had a nasty habit of spinning above 170mph :-o . Here are some of my observations, thoughts & ideas regarding this racecar.
The trunk was completely filled with a water tank, fuel tank, 3 batteries and lead ballast. The ballast & batteries were over or aft the rear axle.
The front suspension was an off-the-shelf aftermarket street rod traverse leaf sold axle. Most street rod steering arms are designed to provide front tire clearance resulting in a negative Ackerman. About 20 degrees caster. Scrub radius unknown.
The required rear fenders had been trimmed/shortened on the front edge.
Besides using a body that was designed for 50 mph tops, IMHO several other problems contributed to the tendency to spin.
• The CP was forward of the CG.
• Too much added weight aft of the rear axle.
• Negative Ackerman caused outside front tire to skid.
• Open rear end
Possible fixes:
- Mount a Moon style water tank in front of the grille.
- One battery
- No added weight behind the rear axle
- Locked rear end.
- Go to a no Ackerman setup.
- Extend the front of the rear fender down to the original stock shape/position
These are my observations and ideas. Please criticize, suggest, correct, ignore, etc.... :-D
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Whats the vehicle number so I can look up it's history. We record every spin plus all unusual runs at the lakes and on the salt. We have a history of repeat vehicles
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Geez Louise Glen, you mean to tell me there's a spin police? Man I hope I don't get on that list any more. :-o
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Some vehicles have bad habits as do some drivers. :-o
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Geez Louise Glen, you mean to tell me there's a spin police? Man I hope I don't get on that list any more. :-o
You didn't spin, you just got really sideways. :-D
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In 1997, when I asked former Breedlove aerodynamicist Don Baumea to explain why some cars spun out on the salt, here's the essence of how he explained it:
It's not a matter of horsepower, torque, or pounds of ballast. It's pounds of thrust versus pounds of drag. A vehicle has a certain amount of pounds of aerodynamic drag holding it back, and it is propelled forward by a given amount of pounds of thrust that is applied at the contact patch of the tires on the driven wheels.
As the vehicle's speed increases, and its aerodynamic drag increases exponentially, it is at the point where the amount of drag exceeds the amount of thrust that you have instability. You reach a point where the air is simply preventing the vehicle from accelerating.
In itself, adding more power will not help unless the power reaches the ground as thrust at the contact patch of the tires on the driven wheels, and that calls for a corresponding increase in ballast to keep the tires planted and hooked up.
There's more to it of course, and my understanding of all this is quite limited, but I wanted to share what I could.
Dave
http://www.roadsters.com/
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are you saying----you get up against it and spin the tires willie
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In 1997, when I asked former Breedlove aerodynamicist Don Baumea to explain why some cars spun out on the salt, here's the essence of how he explained it:
It's not a matter of horsepower, torque, or pounds of ballast. It's pounds of thrust versus pounds of drag. A vehicle has a certain amount of pounds of aerodynamic drag holding it back, and it is propelled forward by a given amount of pounds of thrust that is applied at the contact patch of the tires on the driven wheels.
As the vehicle's speed increases, and its aerodynamic drag increases exponentially, it is at the point where the amount of drag exceeds the amount of thrust that you have instability. You reach a point where the air is simply preventing the vehicle from accelerating.
In itself, adding more power will not help unless the power reaches the ground as thrust at the contact patch of the tires on the driven wheels, and that calls for a corresponding increase in ballast to keep the tires planted and hooked up.
There's more to it of course, and my understanding of all this is quite limited, but I wanted to share what I could.
Dave
http://www.roadsters.com/
You pretty much nailed it and once the tires spin and you have that weight in the back like was mentioned it can want to act like a pendulum and swing the car around and of course once the weight gets in the front it now has a spinning momentum and will go past the front back to the back again, and again.
Under ideal conditions what was described will result in the spinning tires that starts the whole thing off, but if anything upsets the car and unloads a tire it can happen sooner than when it should.
I have a spread sheet on my site that will give you a good idea of how much weight you need on the tires.....
http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html
..... the "Drag Force-HP-Thrust-Weight" one. Also using that spread sheet if you know the frontal area of your car/bike and the HP it took to run a speed from previous runs you can narrow in on the Cd of your vehicle by figuring backwards through the spread sheet.
Likewise if you use the know the HP it took you to run a certain speed you can use the "Horsepower Needed" to figure the HP you will need to run a new speed. Then you can go back to the "Drag Force-HP-Thrust-Weight" one and using the new HP figure figure how much weight you will now need on the rear tires.
If you don't have figures from running already don't let that stop you. You can go to Dr. Mayfield's site....
http://www.mayfco.com/tbls.htm
........................ and see if he has the Cd of your vehicle. Then go back to the spreadsheet and plug in the Cd, frontal area and HP you think you can make at the rear wheels and see what speed you should be able to run. Plug in the Cd, frontal area and then different mph until the HP figure comes close to what you think you have. Then go down the spread sheet again and look for the weight you will need.
Clear as mud :?,
Sum
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are you saying----you get up against it and spin the tires willie
That is what he is saying, just like putting your front tire against a building and some water under the back tire,
Sum
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are you saying----you get up against it and spin the tires willie
That is what he is saying, just like putting your front tire against a building and some water under the back tire,
Sum
That is when you find out who the really good drivers are. LOL
Tom G.
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I know I am capable of flunking---but to day on the dyno----my eng jumps over 100'# just above where it falls back to after the shift.. explains why I am subject to getting really busy shortly after it settles down after the shif...
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I am familiar with a Bonneville STR class '27 Ford/ 32 Chassis w/ a small block V8. The roadster had a nasty habit of spinning above 170mph :-o . Here are some of my observations, thoughts & ideas regarding this racecar.
The trunk was completely filled with a water tank, fuel tank, 3 batteries and lead ballast. The ballast & batteries were over or aft the rear axle.
The front suspension was an off-the-shelf aftermarket street rod traverse leaf sold axle. Most street rod steering arms are designed to provide front tire clearance resulting in a negative Ackerman. About 20 degrees caster. Scrub radius unknown.
The required rear fenders had been trimmed/shortened on the front edge.
Besides using a body that was designed for 50 mph tops, IMHO several other problems contributed to the tendency to spin.
• The CP was forward of the CG.
• Too much added weight aft of the rear axle.
• Negative Ackerman caused outside front tire to skid.
• Open rear end
Possible fixes:
- Mount a Moon style water tank in front of the grille.
- One battery
- No added weight behind the rear axle
- Locked rear end.
- Go to a no Ackerman setup.
- Extend the front of the rear fender down to the original stock shape/position
These are my observations and ideas. Please criticize, suggest, correct, ignore, etc.... :-D
CobraBall
Don't forget about aerodynamic lift as one of your problems. Years ago Jim Hall (Chaparrals) and chev, figured out the high speed instability they were having (and all others) was aero lift causing the suppension to go into droop and unloading the tires
and voila! the spoiler was born, then the adjustable spoiler then the wing and adjustable wing. Of couse you can't have a spoiler in roadster class. Make sure that your rear end is square with the chassis and is not toed in our out,same with front but
make sure you have a little toe in.We run 1/8''. Talk to the record holders in roadster class,i know a couple that have gone 255 and 260 with out a bunch of weight and they run suspension. A few blown roadsters have gone 285 to almost 300, but i'm not sure on the weight but suspect a bunch.
Back to aero lift, even though this was figured out in the mid 1960s a lot of coupes and sedans don't do anything to combat it even though they could. Some just do not know.
What kind of suspension do you have.The wrong type of setting can cause problems.
I have a book from Alston Chassis-( Door Slammers: The Chassis Book by Dave Morgan) and Tune To Win by Carroll Smith.
Smiths book has all the different handling problems and how to deal with them. These books will give you a lot of knowledge
about setting up a race car.
Also what kind of tires are you running ,not to wide i hope.
Good luck JL222
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Geez Louise Glen, you mean to tell me there's a spin police? Man I hope I don't get on that list any more. :-o
But you could belong to the Spin Out Club and get a spin out hat pin. Is anybody still giving those out?
JL222
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El Mirage Ladies Aux, EMLA, the pin is yours for a small donation. Ask at the sales trailer at any meet.
DW
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There is also a cute little sticker (or at least there used to be) that you can put on your car.
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In 1997, when I asked former Breedlove aerodynamicist Don Baumea to explain why some cars spun out on the salt, here's the essence of how he explained it:
It's not a matter of horsepower, torque, or pounds of ballast. It's pounds of thrust versus pounds of drag. A vehicle has a certain amount of pounds of aerodynamic drag holding it back, and it is propelled forward by a given amount of pounds of thrust that is applied at the contact patch of the tires on the driven wheels.
As the vehicle's speed increases, and its aerodynamic drag increases exponentially, it is at the point where the amount of drag exceeds the amount of thrust that you have instability. You reach a point where the air is simply preventing the vehicle from accelerating.
In itself, adding more power will not help unless the power reaches the ground as thrust at the contact patch of the tires on the driven wheels, and that calls for a corresponding increase in ballast to keep the tires planted and hooked up.
There's more to it of course, and my understanding of all this is quite limited, but I wanted to share what I could.
Dave
http://www.roadsters.com/
I don't buy that unless your driving a brick. Why not? In Cobraballs situation there are plenty of roadsters going a hell of a lot faster than 170 mph with out spining and without carrying a lot of weight i know one roadster who holds several records weighs less than the 4000 lbs of ballast another roadster carries.
If hitting the aero wall causes the tires to spin we would have a lot more spins. 1 would think 95% cars hit the aero wall
otherwise why have the short course?
What do you think is limiting speed if its not aero drag?
JL222
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............... If hitting the aero wall causes the tires to spin we would have a lot more spins. 1 would think 95% cars hit the aero wall otherwise why have the short course?
What do you think is limiting speed if its not aero drag?
JL222
John I think we are saying the same thing. Hitting the aero wall does not mean the tires will necessarily spin and even if the tire does spin that doesn't mean the car will necessarily spin.
If there is not enough weight on the drive tires for traction to get the hp required to the ground at a given speed the tires can spin. Can we agree on that? Now if you have plenty of traction then you will hit the aero wall like you mentioned and the tires will never spin. Or before you hit the aero wall you might spin the tires because there is not enough weight on them to go faster even though you have the HP. A good driver might be able to go a little further than a not so good driver, but sooner or later the physics are going to win. The third scenario is that you run out of traction and HP at exactly the aero wall where the car can not go faster due to aero drag.
I still think Dave's explanation is right on. I should of worded my response to Willie better as I can see where it could be read to mean anytime you get to the aero wall the tires will spin. Sorry about that.
Now go out there and run over 300, I think you have the weight needed and the HP needed, but please be safe doing it,
Sum
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............... If hitting the aero wall causes the tires to spin we would have a lot more spins. 1 would think 95% cars hit the aero wall otherwise why have the short course?
What do you think is limiting speed if its not aero drag?
JL222
John I think we are saying the same thing. Hitting the aero wall does not mean the tires will necessarily spin and even if the tire does spin that doesn't mean the car will necessarily spin.
If there is not enough weight on the drive tires for traction to get the hp required to the ground at a given speed the tires can spin. Can we agree on that? Now if you have plenty of traction then you will hit the aero wall like you mentioned and the tires will never spin. Or before you hit the aero wall you might spin the tires because there is not enough weight on them to go faster even though you have the HP. A good driver might be able to go a little further than a not so good driver, but sooner or later the physics are going to win. The third scenario is that you run out of traction and HP at exactly the aero wall where the car can not go faster due to aero drag.
I still think Dave's explanation is right on. I should of worded my response to Willie better as I can see where it could be read to mean anytime you get to the aero wall the tires will spin. Sorry about that.
Now go out there and run over 300, I think you have the weight needed and the HP needed, but please be safe doing it,
Sum
Sum
No- were not saying the same thing your solution is add more weight. I'm saying look at other things like alignment - suspension settings-to high spring rate and aero lift. talk to the guys that are going 75 mph faster than Cobra balls at the same weight level and see what there doing, buy some books so you can understand what there talking about.
As i've said before if you like spinning the tires put in an unsprung rearend or too high a springrate with the instant center set wrong and spin the tires to your hearts desire.
I believe cars spin out from aero lift than from hitting the aero wall or we would have more spins. Watch those videos on Utube the cars are going along just fine when sudenly there flying with no increase in rpm from wheelspin.
we've gone to great lenghts in our build dairy to show why our car hooks up( but nothing sinks in but the weight).If we didn't have soft suspension and a low instant center we could spin our tires too, like other cars several mph slower.
See you at bville JL222
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Roadsters just like to spin, They have better aero backwards than forwards so they are always trying to swap ends. Our roadster is a solid rear end so I can't speak alot about suspension but the more you drive the car the more you learn how to keep it straight. One slide in one direction is OK, front end darting is ok, but when you get into that rythmical left and right hula dance....very bad. when horsepower is hooked up to the salt it just plows in a straingt line, when you run out of HP it's like a forklift on ice. Just make small incrimental changes to weight and suspension and figure out what works best to keep it from wanting to swap ends the least. But remeber roadsters allways want to swap ends.
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but when you get into that rhythmical left and right hula dance....very bad.
Most of the time it spins in this scenario is because the car has too much castor and one of the back tires lifts and it comes around. Too much castor is a crutch for a badly set up car. I agree with jl222 that just adding weight is not the answer. There are other things that are going on with the car like aero lift and can be fixed to get it going straight without adding weight.
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but when you get into that rhythmical left and right hula dance....very bad.
Most of the time it spins in this scenario is because the car has too much castor and one of the back tires lifts and it comes around. Too much castor is a crutch for a badly set up car. I agree with jl222 that just adding weight is not the answer. There are other things that are going on with the car like aero lift and can be fixed to get it going straight without adding weight.
I don't think we are saying you fix everything with weight and that you can ignore the rest of the way the car is setup. At least I didn't mean to imply that. We are talking about the fact that it takes a certain amount of weight for the car to have traction to apply the HP needed at a given speed.
My lakester is setup to have a number of spring rate settings and shock settings both independent of each other and I agree with the sprung soft theory. I've also paid attention to the scrub radius the best I can. I am running about 18 degrees of caster with my frontend and feel comfortable with that after studying a number of other similar cars. I wouldn't by any means recommend 18 degrees across the board for every car.
c ya,
Sum
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Every car is different, but they all have lift. It depends on the class you run and the options available for your model if running in the Production, coupe, or altered classes. The #265 gen3 Firebird weighed over 5,000 lbs & the #135 gen4 Firebird weighed over 6,000 lbs. due to totally different aero package. Both of these cars have run over 300 mph. But the question is if you're running a roadster the rules affect what you can do as the street roadsters can't vent the trunk deck like the fuel/gas roadsters can for example. When you can't do something with aero you most likely need to do something with weight. Corner weight & balance are a few of the things you will want to keep track of as you develop what works for you. Body rake is another thing to look at in a roadster as this can effect the down force to the tires. If your car has suspension don't forget to put the car in the air and run everything through it's travel to make sure it's not binding or changing settings to drastically. Just some thoughts.
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In 1997, when I asked former Breedlove aerodynamicist Don Baumea to explain why some cars spun out on the salt, here's the essence of how he explained it:
It's not a matter of horsepower, torque, or pounds of ballast. It's pounds of thrust versus pounds of drag. A vehicle has a certain amount of pounds of aerodynamic drag holding it back, and it is propelled forward by a given amount of pounds of thrust that is applied at the contact patch of the tires on the driven wheels.
As the vehicle's speed increases, and its aerodynamic drag increases exponentially, it is at the point where the amount of drag exceeds the amount of thrust that you have instability. You reach a point where the air is simply preventing the vehicle from accelerating.
In itself, adding more power will not help unless the power reaches the ground as thrust at the contact patch of the tires on the driven wheels, and that calls for a corresponding increase in ballast to keep the tires planted and hooked up.
There's more to it of course, and my understanding of all this is quite limited, but I wanted to share what I could.
Dave
http://www.roadsters.com/
You pretty much nailed it and once the tires spin and you have that weight in the back like was mentioned it can want to act like a pendulum and swing the car around and of course once the weight gets in the front it now has a spinning momentum and will go past the front back to the back again, and again.
Under ideal conditions what was described will result in the spinning tires that starts the whole thing off, but if anything upsets the car and unloads a tire it can happen sooner than when it should.
I have a spread sheet on my site that will give you a good idea of how much weight you need on the tires.....
http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html
..... the "Drag Force-HP-Thrust-Weight" one. Also using that spread sheet if you know the frontal area of your car/bike and the HP it took to run a speed from previous runs you can narrow in on the Cd of your vehicle by figuring backwards through the spread sheet.
Likewise if you use the know the HP it took you to run a certain speed you can use the "Horsepower Needed" to figure the HP you will need to run a new speed. Then you can go back to the "Drag Force-HP-Thrust-Weight" one and using the new HP figure figure how much weight you will now need on the rear tires.
If you don't have figures from running already don't let that stop you. You can go to Dr. Mayfield's site....
http://www.mayfco.com/tbls.htm
........................ and see if he has the Cd of your vehicle. Then go back to the spreadsheet and plug in the Cd, frontal area and HP you think you can make at the rear wheels and see what speed you should be able to run. Plug in the Cd, frontal area and then different mph until the HP figure comes close to what you think you have. Then go down the spread sheet again and look for the weight you will need.
Clear as mud :?,
Sum
Sum - I have a quick question for you. In the thrust weight required program the number you come up with when finished- is that total car weight or as I suspect weight over the driven wheels.
Also what if the car is awd ?
Or say I convert to fwd where should the weight be added in fwd- ahead of the axle or forward in the cabin?
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...............Sum - I have a quick question for you. In the thrust weight required program the number you come up with when finished- is that total car weight or as I suspect weight over the driven wheels.......
Over the drive wheels.
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Or say I convert to fwd where should the weight be added in fwd- ahead of the axle or forward in the cabin?
If you have to give up all wheel drive and convert I would do it to the fwd as that is a win win situation as far as I'm concerned. The weight in the nose would be fine in my estimation as that is moving the CG further forward. In this situation even if you were to spin the tires it is just going to go straight ahead. Maybe Romero would chip in here with his thoughts and his experience since he has real world experience. George Johnson has already taken advantage of front wheel drive with his record setting lakester and John Burk on here is building a fwd streamliner.
Anyone else with fwd experience want to offer an opinion?
c ya,
Sum
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Dyno is right on. Each one is different and each one acts it. I knew our roadster would never spin and it didn't until we put in 700hp where 300 hp use to be and it spun at 175. We were at Bonneville with not a lot of ways to fix it. I asked my son what happened first and he said it went slightly right, then left, then a big right. I added 1/8" to the right side length and he went back out had set the record at 186; went straight as an arrow according to him. Next year 196, next year 202, next 216. When the car ran out of HP at 215 it was a hell of a ride and I just hung on. we did add 400 lbs after the spin.
Running out of HP causes crazy things to a flat face car; they don't like it and they tell you. If you try to drive one at part throttle to get your licenses you may quit. Driving a twin turbo smallblock roadster at 150 and 175 can be an eye opening experience. The key is to accelerate slow and just get to the license speed not hold it for a mile.
Others have said if you have a locked rear and spin, take it out. If you spin without one, put one in...Good Luck
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Sure wish I had a $10. bill for every roadster I have seen spin from the tower, in 25 years it's been a bunch plus a lot of wide tire short wheel base sports cars. :roll:
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Getting hit by a wind gust of 25 MPH from the South between to 2 and 3 mile at about 170 MPH can also do it when you twitch the steering to try to correct a little. This happened in 2006 on Wednesday and the Course was shut dowm the rest of the day.
I forgot to get a spin pin!
I slowed down steering a bunch after that scenic tour!
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See Penny Cook in the sales trailer she has the log and pins
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See Penny Cook in the sales trailer she has the log and pins
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Thanks Glen,
Hopefully there will only be one pin needed.
Tom
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As a newbie V4F/STR racer with a sticker, I have to ask a newbie-with-a-sticker question. At what speed do aerodynamics (CP/CG, lift, etc.) become a legitimate factor in a roadster? I'm trying to discern whether my issues are aero, chassis/suspension, me... or all of the above. There are no stupid questions, right?
Be kind, Pedro
Godspeed Cliff Gullett
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We were going about 185 on our first run with a posi in our C/STR when we got legitimate factors and spun for the first time in our 7 years. What legitimate factors -- don't know. Took JD's advise --"if you spin with a posi, take it out. If you spin with an open, put in a posi" or words to that effect.
Took it out -- next two runs were record runs at 210.
(By the way, you'd truckstop if you saw the diversity of wheel weight readings on this car.)
Stan Back
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As a newbie V4F/STR racer with a sticker, I have to ask a newbie-with-a-sticker question. At what speed do aerodynamics (CP/CG, lift, etc.) become a legitimate factor in a roadster? I'm trying to discern whether my issues are aero, chassis/suspension, me... or all of the above. There are no stupid questions, right?
Be kind, Pedro
Godspeed Cliff Gullett
How about more specifics about the car and the condition? Total weight, front/rear distribution, HP, spun at what speed, spun under acceleration or braking, etc..
Sum
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OK, here goes-
1928 Dodge Roadster V4F/STR #60. New car. Total of 6 trips down the salt the year.
Weight 2393 # with driver. 53.1 % rear weight distribution. No ballast.
Car was traveling 117 mph (according to the GPS) just before the 2 mile and it just, literally, turned around. I could definitely feel the wind causing the car to wander, but it literally, just turned around. It was not an overcorrection... as I didn't even have time to correct. Again, it just turned around. Conditions at time of spin. Big wind. About 29 mph. I was the last run before they shut it down for the day because of the wind.
But, that being said, the car was a handful every run before. The rear end seemed to want to hop from side to side. After discussing this with Ron Joliffe in impound he suggested more rear tire (Goodyear Front Runner 27 X 4.5) pressure (was 45 psi). I did increase it to 60 psi for my return run, and it did seem to help the hula dance a bit, but it did not eliminate it. FYI- have a background in sports car racing, autocross and hill climbing so I have a pretty good understanding of vehicle dynamics, but I have never experienced such instantaneous, abrupt oversteer as I did on the last run of the week (the one where I got the sticker from Lee Kennedy!). Kind of an embarrassing way to start your landspeed career.
Since returning, I have done the following-
Checked the rear suspension. Ford 9" (Currie) Rear axle is square in the chassis. No bind. I have about 1 1/2 in travel in compression but very little in rebound (had to jack the coilovers up at the last second to add the fenders for STR. Wondering if this zero rebound situation "lifted' the rear end. I have ordered new coilovers to allow for more rebound.
Front end. Solid front axle. 6" drop. All new components. Transverse spring 1/16 inch toe in. 9 degrees caster. Chassis mounted rack, but negligible bump steer. No front panhard. 1 1/2 compression, plenty of rebound. No bind. 15 X 4" wheels 1 3/4 back space. 4" scrub radius.
I've read every posting I could find on suspension since (hindsight...) and based on what I read, this week I changed steering arms to greatly reduce Ackerman, added a dead perch to eliminate any shackle deflection, and added another 1/16 inch toe in (total 1/8).
Any suggestions are greatly appreciated!
Thanks, Pedro
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As described by Roadster above, spins appear to become a big factor when the rear tires break loose. Our roadster did that before I added about 600#. No more breaking loose.
The next fun time is when you run out of HP or when the torque falls off and you have just past the 4 and have a mile to go. When hooked up and pulling things are great and you are a very happy driver. When the HP runs out and you are no longer accerating hard those flatnose roadsters tell their driver "let's see if you are a real driver and take you from the race track to the dance floor".
My first words in my head just after passing the 4 mile was "holy s--t". and don't spin it out. Ran out of HP. Gained 1.5 mph for the last mile and I can tell you that's not enough for a good ride anly a wide awake one.
The answer for us.....more gear.............Good Luck
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............... 4" scrub radius.............
That is the first thing I would get right and as JD has said a bad handling car can appear to not be so bad handling while accelerating, but become a lot more noticeable at a more constant speed. It looks like from your speeds (on SCTA site) you were at pretty much the same speed for a lot of your run.
I don't know what your HP is, but I'll bet you have enough weight for the HP you are making. If you can get more weight up front and it doesn't hurt your mph it sure won't hurt anything.
You didn't really mention that you were spinning the tires, so I don't think that is what upset the car.
I glad you posted since we can all learn from what is happening to others.
c ya,
Sum
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I am intrigued by the "running out of HP" comments in this post and others. The car was definitely "out of gear". The cars was maxed out by the 2 mile every run, and even more so on the "spin run" due to the wind. And now that you mention it, it also seemed to "hula dance" the most near max rpm. We never tried more gear because, heck, we're new at this (we never planned to qualify for a record on our break-in run), and didn't even consider that the V4F could pull more than we had. We'll have more gear in October.
I am now very aware that no one recommends R&P steering on the salt. Too late to modify before October though. Good winter project.
As for Caster. I have 9 degrees. Is more a "crutch"? How much more? Consensus (if possible)?
I still must ask. Is CP/CG an issue at 120 mph?
Keep the great ideas coming!
Thanks, Pedro
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...............I still must ask. Is CP/CG an issue at 120 mph?..........
It could be an issue at any speed, but my personal feeling is that this mainly comes into play to help the car go straight once it is upset (rear wheels/tires get loose for whatever reason and the back wants to come around) and is why I asked if you thought you were spinning the tires. Don't use CP/CG as a crutch if at all possible, but then again a short wheelbase roadster needs all the help it can get as it is about as close to a "top" on wheels as you can get and that is mostly because it is hard to get the CG very far ahead of the CP if in fact you can do that.
You say you don't have any ballast, so I guess that means there are no heavy items behind the rear axle such as a battery?? If so try moving them ahead of the axle.
Get the scrub radius better and I'll bet you do better. There again it will get worst when you are in a low accelleration mode.
c ya,
Sum
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Pedro --
Sent you a PM.
Stan
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Pedro
We have run 288mph with a 279mph exit speed and run 65% weight on rear tires, batteries and weight boxes behind rear tires. car engineered by pros [Alston]
Car is coupe with spoiler to combat lift -3000lbs on rear tires.
All cars with out spoilers our wings have lift and i believe roadster are among the worst, i wish more of the really fast roadster guys would post what they were running for weight and wheel %.
JL222
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In a quick look I saw no mention of what type of rear end (Ford) you are running - is it open, spool or "locker" - As J.D. pointed out this is a factor - poorly functioning "lockers" can give you fits darting left and right. Real chassis guys will give you reasons why open or spool is better. Pat
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OK, more answers, more questions...
Stan I did get your PM. Thank you. I'll call you today or tomorrow for the wheel info.
Sum, I do not know if the rear tires were spinning or not. Being a newbie with a loose car, I was a bit more focused on the car going straight than the tach. I will have tach recall next time out.
Pat, The rear is open. We never thought spinning the tires might even be an issue in a V4F when we built the car. Heck, we hadn't even fired the engine 72 hours before we loaded it on the trailer.
There is no added weight behind the rear axle. Battery and H20 tank are in the cockpit.
The overwhelming consensus at this point seems to be more gear and less scrub.
Still whats the magic caster #?
Are Moon tanks of any aero benefit? Surprised I don't see more of them on STRs.
Thanks guys!
Pedro
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Pedro: Welcome to the V4F world.
The #1931 coupe had 10% slippage (RPM vs speed) on its best run this year.
Tony Thacker wind tunnel tested the Thacker & Shine STR in Hot Rod here: http://www.hotrod.com/roadtests/hrdp_0604_thacker_shine_a_v8_roadster_testing/index.html (http://www.hotrod.com/roadtests/hrdp_0604_thacker_shine_a_v8_roadster_testing/index.html)
The conclusion on their car was that the Moon tank provided less aero. YMMV
Mike
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Pedro
You don't think lift is a factor?
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Pedro: Welcome to the V4F world.
The #1931 coupe had 10% slippage (RPM vs speed) on its best run this year.
Tony Thacker wind tunnel tested the Thacker & Shine STR in Hot Rod here: http://www.hotrod.com/roadtests/hrdp_0604_thacker_shine_a_v8_roadster_testing/index.html (http://www.hotrod.com/roadtests/hrdp_0604_thacker_shine_a_v8_roadster_testing/index.html)
The conclusion on their car was that the Moon tank provided less aero. YMMV
Mike
Hi Mike
Thanks for the link,thats the most info i have ever read about roadster aero , very enlightning - roll cage levering weight on rear end - cd-.521-lift on front -on and on-great! Now going to reread.
JL222
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Great info, I guess. I think that when the tunnel isn't treated to the unobstructed spinning front wheels, your results may vary.
Doesn't that car still take a spin on the salt once in a while?
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Hi Mike
Thanks for the link,thats the most info i have ever read about roadster aero , very enlightning - roll cage levering weight on rear end - cd-.521-lift on front -on and on-great! Now going to reread.
JL222
I went looking for the article on the net based on a comment by "interested bystander" in this thread: http://www.landracing.com/forum/index.php/topic,2943.0.html (http://www.landracing.com/forum/index.php/topic,2943.0.html)
The article is discussed (and questioned some) here: http://www.landracing.com/forum/index.php/topic,1014.0.html (http://www.landracing.com/forum/index.php/topic,1014.0.html)
I'm still trying for figure it all out, so any info helps.
Mike
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The loading of the rear axle creating a stable situation. Peter Stevens-Mclaren FI designer.
Pro advice.
JL222
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One other important item I forgot to mention. This pertains to open and locked rear end alike. The rear tire circumferance must be perfect. I mean perfect. Any diviation, even 1/32", will cause a problem at speed. I true my own tires on my car (too long to go on about here) mark them for the same mount spot every time they are off. Same for wheel runout. I use forged wheels when I can because they are perfect. I've beaten every steel wheel straight before I mount tires......Good Luck....JD
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A roadster with driveshaft torque unequaly loading the rear tires seems like a tight rope walker in a variable cross wind . IRS with a torque tube that returned the torque directly to the engine without twisting the chassis would fix that .
John