Landracing Forum
Tech Information => Technical Discussion => Topic started by: Cajun Kid on June 30, 2008, 10:43:22 AM
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How much caster should I have ? Also what is the best front to back weight balance? Is it 50/50 ?
I assume I should check the weight and balance with me in the car ( I weigh 365 so I know that makes a big differance) I am however setting kinda far back and towards the center of the car.
I have never driven a straight axel car, so at 120mph plus it felt like it was just wondering a bit, hard to keep it right on the line ?
I think there was a 10 to 15 mph cross wind, With my area brick and side area profile, maybe the cross wind had more to do with it than the caster.
Charles
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search the forum under "Caster" there's lots on this one.....
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Cajun Kid, do you have a panhard bar on the front axle? If you don't, it will wander a little, at least my Willy's coupe did when I first built it. I run about 10 degrees caster on my Studebaker and about 51 % of the weight is on the front end.
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I have a panhard bar on front. My caster with a dial type degree checker thingee is about 4 degrees...
I will weigh the car later this week on all fours... My guess is it is well over 50% up front and with me in it more on the left side too.
I have no idea what the front to back weigh balance should be ? I assume the left to right I should get exact with me in it. ?? Am I correct ?
Thanks
Charles
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I weigh my car with me and all my safety gear in the drivers seat, the correct amount of gas in the tank and the engine full of water and the intercooler full also. I adjust ballast so left and right weights are near the same.
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What is the right front to back weight distribution? is 50/50 it?
Does my ride height or body style have a bearing on front to back weight distribution ?
I also assume if I need to add weight to the right rear I need to keep it as low and far back as possile?
Would rear weight be better behind the axle? maybe as far back as the rear spreader bar or push bar area ?
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What is the right front to back weight distribution? is 50/50 it?
Does my ride height or body style have a bearing on front to back weight distribution ?
I also assume if I need to add weight to the right rear I need to keep it as low and far back as possible?
Would rear weight be better behind the axle? maybe as far back as the rear spreader bar or push bar area ?
You don't want the weight way behind the rear axle if you are going to run on the salt. Back there it will act like a pendulum if the car starts to go around. Read this about CP and CG.....
http://purplesagetradingpost.com/sumner/bvillecar/bville%20-%20LSR%20Thoughts-4.html
......... or someone else's thoughts on it.
Also to figure the weight you need on the rear axle you can use the " Drag Force-HP-Thrust-Weight " spreadsheet here...........
http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html
........ hope that helps,
Sum
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Cajun Kid,
With a straight axle you should probably be running something in the 10-15 degrees of caster. The closer you get to 20 degrees the more you need steering stops.
Tom
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IMHOP 836 is in the caster ballpark- but check your TOE-IN also!
Toe out or zero toe CAN make a vehicle seek or "wander" as you described.
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Cajun
On the 222 camaro we run 64% on rear tires 36% on fronts,car is glued in at 288mph +. What do you think lakesters,streamliners,and rear engine modified roadsters are running? Formula 1 cars run around 65-35 %.
JL222
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222 - gonna throw some S##t in the game before I go nite-nite. I wanna emphasize - no criticism intended, cause there be a whole lot of DIFFERENT solutions out there!
From what weight balance you've described I feel you must run a lot of body rake and run LOW to the ground with a good air dam.
I woulda thought that thing would weathervane MAJOR. I.E. swap ends.
(The old CP/ CG thing.)
REX, BLUE, DAHLGREN? Opinions? - Even Willie!
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I have set up my roadster weights 65 - 35 and with 8 degrees of caster. I don't want to start a debate but I think anything more is a crutch for the set up. I know lots of guys run 12-14 degrees, but what happens when you have to correct the wheel. One of the rear corners picks up and when you go to correct that. The car comes around. Just build a simple model out of wood and try the different caster angles and see what happens to the chassis. But then again I dance to a different drummer.
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ok you asked for it --i think in order to figure out a problem you have to understand how everything involved works --im not sure im the one to try to explain it all but here goes --many things effect the steering on the front of a car some are easy to fix some arnt the easy ones are --tire pressure and tire wear easy to check- next is camber and caster these can be checked with a degree wheel--then toe in and toe out check with a tape measure--then scrub radis probibly the most important next to caster have someone turn the steering wheel lock to lock and visually check if you can see that its off its too for off--and last the ackerman principle unless you have the spindles on wrong you will be ok in lsr ---all of that should be easy to understand --lets start with tire pressure you all know what happens when you drive on a low or flat tire--tire wear if your tires are worn on the outside or the inside it will cause the car to wonder if they wore that way on your car you have a alignment problem---camber on a street car the camber is usually set different on the left as on the right because of the crown in the road on flat ground the camber should be the same --caster and rake are the same thing it is the angle of the king pins the angle will be slanted back at the top in degrees if you have a arms and ball joints you wont have a kingpin but the angle is still there its a line through the top and bottom ball joint --there is another line that goes through the kingpin or ball joints this one angles out and when it reaches the ground should be in the center of the tire if it is you are in good shape that is the scrub radis you all know what happens to your car when you put a set of offset wheels on the front handling goes out the window--toe in and toe out if the front wheels are closer in the front than they are in the back you have toe in ---some other notes --when cars are built new they are made for comfort not speed --i am going to make some reccomendations just what has worked for me through the years ---caster 10 to 16 degrees if you are going to error do it on the high side--camber 0 degrees if not make sure both sides are the same---toe in none if you dont have new parts 1/8 toe in --scrub radis as close to 0 as possible (center of tire) no thats not the end the rearend in the car can effect steering it can be in out of square with the front suspension --the rear end housing can be bent and cause toe in toe out most that ive worked on i had to straighten first especially if they have been welded on --rear tire dia. if they are different especially with a spool rearend can make your car handle bad --there are things i forgot im sure but remimber im a motorcycle guy and i run 55 degrees of caster (rake) on my bike so what the hell do i know just some thouhgts willie buchta
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Willie, thanks good and "understandable" info...
I got all of that except I am a bit confussed on Scrub Radius.. I have never heard that term. (my past experiance was drag cars and bikes and I don't recall that term or measurement, but is has been 20 years since I raced or worked on a car....)
My caster is at 4 degrees, toe in approx 1/16.
My tire preasure seemed to be low for some reason, cold preasure dropped to 30 psi.
I also think my front tires are to wide ? 195/60/15
Thanks
Charles
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Sum,
I am having trouble with the Car 3 speed RPM gear chart excel format.. It will not let me change the gear ration in rear end. Something about sheet protection,
I know the Works version allows me to do that as I did it at home and MAN it is dead on.. I am my office and do not have MS Works..
Is there a way you can email me the excel file for 3 speed car tranny's
Charles
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Sum,
I am having trouble with the Car 3 speed RPM gear chart excel format.. It will not let me change the gear ration in rear end. Something about sheet protection,
I know the Works version allows me to do that as I did it at home and MAN it is dead on.. I am my office and do not have MS Works..
Is there a way you can email me the excel file for 3 speed car tranny's
Charles
Hi Charles, sorry for the problems. I don't have excel, but make the spread sheets in works and then save them in excel format. I just load that one into works using the excel format and it seemed to work.
If it is blocking the cell with "protection" you should be able to go to the cell and turn that off using the "format" and then "protection" tools at the top of the spread sheet.
Maybe someone else wouldn't mind loading it and letting me know if there is a problem.
Thanks,
Sum
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Sum,, I will try that.. Thanks
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charles dont think that because you find a low tire that you are done ---if you were standing in front of your car look at the kingpins you will notice they arnt straight up and down but are angled in at the top and out at the bottom --if you emagine a line through the center of the kingpin to the ground --at ground level the line should be in the center of the tire --if it isnt your scrub radis is off and will cause your car to wonder --in lsr its very important --if you cant figure it out have a front end shop do it someone that is familiar with race cars --when i finish a race car i always get it close then it goes to the alignment shop ---i know there are pluses and minuses in front end alignment and personal preferences but we all still need to stay in the safe zone willie buchta
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Willie, my Caster is about 4 degrees from my angle dial thing. The toe is 1/16th in.
The scrub , I have no idea.. When I get the valves adjusted I will go to front end guy..
Does this scrub radius have something to do with back spacing on the front rims?
If the rims have more back space (narrows the wheelbase) is that a factor to reduce the scrub radius ?
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Willie, my Caster is about 4 degrees from my angle dial thing. The toe is 1/16th in.
The scrub , I have no idea.. When I get the valves adjusted I will go to front end guy..
Does this scrub radius have something to do with back spacing on the front rims?
If the rims have more back space (narrows the wheelbase) is that a factor to reduce the scrub radius ?
Yes the back spacing, tire height, etc. will effect it.
another page to read..........
http://purplesagetradingpost.com/sumner/bvillecar/construction%20page-21.html
Sum
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Charles,
Quick ans, general rules of thumb:
When our driver increased his caster to about 12 degrees on his street roadster it handled better at Maxton in the 135-150 range.
Yes, backspacing is a component in figuring scrub.
Yes, usually increased backspace is needed to narrow the TREAD to get closer to 0 scrub radius.
Nice to meet you at Maxton, good looking car, keep up the good work.
Take care,
Ed
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Charles, another thing with a solid axle you will lose some degree of caster when the front end lifts. The 12 to 15 degree sounds good. Steering stops are not a bad idea but you can probably get by without them. Tire pressure is very important usually best checked early when they are cold. You can do a search on almost anything on the web site. The history over the years is the best, so many answers can be found or someone that can help you. :-D
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I have set up my roadster weights 65 - 35 and with 8 degrees of caster. I don't want to start a debate but I think anything more is a crutch for the set up. I know lots of guys run 12-14 degrees, but what happens when you have to correct the wheel. One of the rear corners picks up and when you go to correct that. The car comes around. Just build a simple model out of wood and try the different caster angles and see what happens to the chassis. But then again I dance to a different drummer.
In cars with a lot of weight on the front end the wheels have more inherent resistance to turning so less caster is needed.
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It's good to check tire pressure cold --- but -- if you've been in line for a long time (say, Bonneville) and one side of the car has been in the sun for hours, the other shaded, it's good to even them off when you're 3 or 4 back.
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222 - gonna throw some S##t in the game before I go nite-nite. I wanna emphasize - no criticism intended, cause there be a whole lot of DIFFERENT solutions out there!
From what weight balance you've described I feel you must run a lot of body rake and run LOW to the ground with a good air dam.
I woulda thought that thing would weathervane MAJOR. I.E. swap ends.
(The old CP/ CG thing.)
REX, BLUE, DAHLGREN? Opinions? - Even Willie!
Thank God we didn't know anything about center of pressure-center of gravity theories when we built the 222 car, or we would have a lot of wheelspin [resulting in oversteer and possible spinout] like a lot of other cars i know of.
Yes the car is low and has 1'' of rake under the rocker pannels.
JL222
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Willie, I just did the imaginary line thru the king pins to check scrub radius. The line hits the ground inboard of the center of the tread/tire center line. I would say the line intersects the ground 1/2 way between the inner edge of the tire and the center. So it looks like I have the wheels out to far (not enough back space ???
The rims are 15x7 with 4.5 back space.. I want to go to 15x 5 rims
What can I do to get the center of the tires in? Or am I close enough now ?
Charles
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charles i would try to get the scrub down it sounds like you have about 1 1/2 inch i would try to get it down to about 1/2 inch (at least below 1 inch) you can do that by changeing rims -tires-machine work --first though take some accurate mearurements you may be closer or farther than you think ---what happens when you have to much scrub on uneven ground the wheels will try to steer the car its worse if you hit a pot hole or loose salt --i think that more caster will help too --on our dra cars we would run 10 to 12 degrees--------------------- just some thoughts willie buchta
sumner had the same problem you might want to check his website
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Thank God we didn't know anything about center of pressure-center of gravity theories when we built the 222 car, or we would have a lot of wheelspin [resulting in oversteer and possible spinout] like a lot of other cars i know of.
Yes the car is low and has 1'' of rake under the rocker pannels.
JL222
Like someone has mentioned earlier there is more to it than just CP/CG. Obviously your combination has worked for you, but I still wouldn't say that it is correct for everyone. You do have a lot of mass up front in your car and in the middle which I'm sure helps your situation. If you took a lighter car and just hung weight at the rear bumper I don't think that would be a good deal. Now a lot of weight in the back doesn't hurt a thing if the car is never upset and doesn't have wheel spin. Your car must be in that operating mode now. The CP is only going to come into effect if the car does start spinning the wheels and is upset to the point that it wants to turn. A lot of cars that don't get into this situation don't need to worry about it probably.
The point you made about the F1 cars I feel actually probably shows what does happen with the weight in the rear. These cars along with a lot of other circle track cars use a lot of aero work for downforce and to help them handle. Their main purpose is to corner and not go straight so they work toward a neutral balance where they aren't pushing or loose. We would like our cars to push if anything. Once the F1, NASCAR cars do start to spin you can see them spinning like a top in most cases exactly what we don't want to happen.
John when some of us bring up CP and CG we aren't criticizing your setup as it has worked for you and you are happy with it, we just want some of the people that are considering this for the first time to take a look for what has worked for a lot of other people, in fact airplanes use the concept every day :-). They should also take a look at your car and make a decision what is going to work with their car, their HP, their body style and the speeds they expect to run.
c ya,
Sum
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Willie, when you say get the scrub down, I assume you mean bring the wheels in more towards the center of the car ? Narrow the overall width (track)
I can do that by increasing the backspacing or keeping the same back spacing and narrow the rim ? Correct?
The reason I ask is I have 15x7 rims now. I want to go to 15x5 but that narrow I can not increase the back space, so by reducing the rim width (from space, does that help too ?
Sorry to be so bothersome,, but scrub radius is a new term/concept to me,,,
Charles
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sorry charles with a scrub radius of 0 it would be in the center of the tire any thing outside of that would be positive anything inside the center of the tire would be negetive willie buchta
yes more backspace narrower will also work if you have a 6 in wide tire and rim now and you changed the tire and rim to a 4 inch with the same backspace you would move the scrub closer to 0 --for every inch that you narrow the tire and rim but keep the backspace the same you move the scrub radius in by 1/2 in --i know it sounds goofey but say you have a 6 inch wide tire and rim with a 3 1/2 in backspace and 1 1/2 inches of positive scrub if you narrow the rim and tire to 4 inches and keep the 3 1/3 in backspace you will now have 1/2 inch of positive scrub much much better
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.............The reason I ask is I have 15x7 rims now. I want to go to 15x5 but that narrow I can not increase the back space, so by reducing the rim width (from space, does that help too ?........
You are just going to have to see where the center of the tire is with the new wheels. If it is more inboard that is good. The other thing that would help is if the tire is larger in diameter. That would move the projected point as projected through the king pin out and that would be good.
I feel that increasing the castor will also move the point out as it rotates the point upwards and thus it hits the ground further out. I can't get anyone else to comment on this though, so it is just my view. Anyone??
c ya,
Sum
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Sum, Wiilie, Anyone, I have drawn the same conclusion. Caster should help a small fraction to move the line out, however, I can not visualize how the tire height will help scrub radius? The center of the tire contact patch will not move in or out as the tire height grows or shrinks ???
Front Tire height... I am at 24.2 now,,, I am thinking of going to 26 tall, but If i do that It will raise the car 1" higher off the ground. not sure If I want to do that. More room for more air under car. But Alt class allows for a Step Pan (not sure if that is good idea on my car.. What do you think ?
Hmm, but taller front tires will also "slow the steering" correct ? Would that help to going taller and narrower ?
I feel the short fat tires seem to follow the irregularities in the pavement/concrete more than skinny tires may ? am I right ?
See, now that I have rum my car once, I am now searching for ways to make it handle better and safer as my initial runs confirm that the motor can do lots more, the question is can the car do it ??
I am hooked,,, this LSR stuff is a blast,, and you guys and gals on this site and at the track make the experiance even more enjoyable,,, sooy to ask so many questions, but I need to learn.
Charles
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......... I can not visualize how the tire height will help scrub radius? The center of the tire contact patch will not move in or out as the tire height grows or shrinks ???...........
The center of the tire patch will be the same place, but as you raise the axle the projected point 0f the king pin inclination will move out.
|\
| \
| \ |\
| \ | \
Notice that the left inclination has moved further from the vertical line than the right one. The left has the axle higher due to a larger diameter tire than the right example.
c ya,
Sum
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charles jack the front of the car up about 3 inches then follow the line to the floor and you can see how much farther the lines have gotten apart --thats what will happen with a taller tire --if you put on a taller tire it will move the scrub --if you change the backspace on the rim ---if you run a narrower rim with the same backspace those are things you can do without machine work --if you run a taller tire to fix your problem and the car is to high LOWER IT that wont change the scrub with a straight axel and very little with a arms as long as you dont lower it by putting the small tires back on LOL when you get this fixed dont stop there check everything else that effect handleing --if you need a list i think all of us on this site could think of about everything ===take all of your measurement accurately otherwise you are just guessing willie buchta
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Thank God we didn't know anything about center of pressure-center of gravity theories when we built the 222 car, or we would have a lot of wheelspin [resulting in oversteer and possible spinout] like a lot of other cars i know of.
Yes the car is low and has 1'' of rake under the rocker pannels.
JL222
Like someone has mentioned earlier there is more to it than just CP/CG. Obviously your combination has worked for you, but I still wouldn't say that it is correct for everyone. You do have a lot of mass up front in your car and in the middle which I'm sure helps your situation. If you took a lighter car and just hung weight at the rear bumper I don't think that would be a good deal. Now a lot of weight in the back doesn't hurt a thing if the car is never upset and doesn't have wheel spin. Your car must be in that operating mode now. The CP is only going to come into effect if the car does start spinning the wheels and is upset to the point that it wants to turn. A lot of cars that don't get into this situation don't need to worry about it probably.
The point you made about the F1 cars I feel actually probably shows what does happen with the weight in the rear. These cars along with a lot of other circle track cars use a lot of aero work for downforce and to help them handle. Their main purpose is to corner and not go straight so they work toward a neutral balance where they aren't pushing or loose. We would like our cars to push if anything. Once the F1, NASCAR cars do start to spin you can see them spinning like a top in most cases exactly what we don't want to happen.
John when some of us bring up CP and CG we aren't criticizing your setup as it has worked for you and you are happy with it, we just want some of the people that are considering this for the first time to take a look for what has worked for a lot of other people, in fact airplanes use the concept every day :-). They should also take a look at your car and make a decision what is going to work with their car, their HP, their body style and the speeds they expect to run.
c ya,
Sum
[/quote
The formula 1 cars i know about were the turbo charged 90 cu in. type that produced 1200-1350 hp [qualifying motors] in the 1980s.
Not only did they go around corners they had to exit the corner,which was a big problem because some of them tripled
in power from 8000 -13000 rpm. 65%-35% weight distribution [static weight] + rear wing. wheelspin was a problem to harness as the drivers said pressing the throttle was like firing a shotgun. Wheelspin resulted in slower lap times and worse
starting position.
Cajuns question was what some people were running, not what we thought he should run.If you notice i don't tell people what to do, just what we do.
Hope to see you at bville JL222
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Got my car weighed today.
Without Driver
Total Weight Ready to Race is 2521
Front Weighs 1256 (49.8%) and Rear is 1265 (50.2%)
Left Weighs 1258 (49.9%) and Right Weighs 1263 (50.1%)
LF is 595 RF is 661
LR is 663 RR is 602
With Driver the distribution changes to 52% Left and 52% Rear
Do I need more % of weight on the front ?