Author Topic: Spinning Street Roadster  (Read 26291 times)

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Offline jl222

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Re: Spinning Street Roadster
« Reply #15 on: July 26, 2008, 02:48:44 PM »
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


Offline Sumner

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Re: Spinning Street Roadster
« Reply #16 on: July 26, 2008, 03:16:33 PM »
............... 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

Offline jl222

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Re: Spinning Street Roadster
« Reply #17 on: July 26, 2008, 04:27:32 PM »
............... 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

Offline DallasV

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Re: Spinning Street Roadster
« Reply #18 on: July 26, 2008, 05:51:21 PM »
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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Offline maguromic

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Re: Spinning Street Roadster
« Reply #19 on: July 26, 2008, 09:20:24 PM »
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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Offline Sumner

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Re: Spinning Street Roadster
« Reply #20 on: July 26, 2008, 10:46:31 PM »
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

Offline Dynoroom

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Re: Spinning Street Roadster
« Reply #21 on: July 27, 2008, 02:29:52 AM »
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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Offline 1 fast evo 2

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Re: Spinning Street Roadster
« Reply #22 on: July 27, 2008, 01:33:21 PM »
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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Offline Sumner

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Re: Spinning Street Roadster
« Reply #23 on: July 27, 2008, 05:42:26 PM »
...............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.

............................
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

Offline jimmy six

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Re: Spinning Street Roadster
« Reply #24 on: July 27, 2008, 06:32:04 PM »
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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Offline Glen

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Re: Spinning Street Roadster
« Reply #25 on: July 27, 2008, 06:49:48 PM »
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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Offline 836dstr

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Re: Spinning Street Roadster
« Reply #26 on: July 29, 2008, 09:30:08 PM »
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!

Offline Glen

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Re: Spinning Street Roadster
« Reply #27 on: July 29, 2008, 10:02:49 PM »
See Penny Cook in the sales trailer she has the log and pins
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Offline Glen

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Re: Spinning Street Roadster
« Reply #28 on: July 29, 2008, 10:03:10 PM »
See Penny Cook in the sales trailer she has the log and pins
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Offline 836dstr

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Re: Spinning Street Roadster
« Reply #29 on: July 29, 2008, 11:53:37 PM »
Thanks Glen,

Hopefully there will only be one pin needed.

Tom