as i understand it, increasing the track width at the front only will increase grip at the front, leading to a more neutral state. is this true??

nate

 

What's your setup?

Have you exhausted all attempts at playing with tire pressures in the rear?

 

Yes spacers will help the car rotate a little. If you use them make sure that you have enough both enough tire clearance and enough stud for the lugs to grab. I have about 7 or 8 turns on my lug nuts with a 1/4" spacer installed.

You can also try:
Higher rear tire pressures.
Toeing the rear out a little.
More front chamber, if it is adjustable.
Larger front tires over rear tires.

The list gets much bigger if you are not running stock class.

 

Still stock legal:

-trail brake
-lift
-left foot brake
-smaller front sway bar

As for spacers, I think they help more with allowing a little larger tire up front. That's what I used them for the the ITR. Alone, I don't know how much they would help unless (like was said before) you had exhausted every other option.

 

There are easier ways to get the car to rotate without spacers.

Pressure bias
less gas in the rear (or more)
make rear stiffer
make front softer

 

maybe i should clarify. i want to get the car to rotate better without sacrificing rear grip. ie, i want to increase front grip. current setup is this 600fr/500rr, 24mm front bar, 22mm rear bar. 1/16 to 1/8 toe in rear, the same but out at the front. i tried softening the front, but i would have to increase front ride height to get sufficient travel for the softer springs which would decrease front camber and decrease front grip. a smaller front bar sacrifices too much roll, and again the camber change at the front reduces grip. no general rules here please, the car is close now, i just need to tweak it a little more. thanks for the help so far, keep the ideas coming.

nate

 

Try swapping the springs front and rear. Also set rear toe to zero.

Aside from that, playing with tire pressures should be all you need (and it doesn't affect the front stiffness as you requested).

 

tried the 500's up front. at my ride height the car was riding the bumpstops. so the 600's must stay up front. front tire pressures are optimized, the rears have been raised above optimum for testing purposes, which makes me beleive i need more rear spring. though i have yet to be convinced that increasing rear spring rate increases front grip. seems to me like it just reduces rear grip. i could be wrong, or maybe the loss in total grip is more then made up for in being able to get on the gas earlier. (a dubious theory at best) suspension travel is an issue, but since most autocrosses are on rather smooth pavement that hasn't been much of a problem. the 600's don't allow much suspension movement, so only having 1/2 inch or so of front wheel bump only hurts me on bumpy surfaces.

nate

 

How low do you have your car? I'd think that if you need 600lb springs in the front to stay off the bump stops on a smooth auto-x course, you either need shorter shocks or to raise the car. you may be sacrificing optimal suspension setup for a slightly lower CG.

 

Because I corded two of my V700s during a track event at VIR, I ran the remainder of the event with (2) V700s up front and near full tread BFG Comp TA ZR street tires on the rear. Rotation? Ohhhhh yeah! My lap times were probably a little slower, but it was fun as heck! Drove it like a rally car.

 

I think the toe in the rear is a big part of your problem. Set it to zero and work from there. I'll second Jerry on the front shocks. If they are too low, you'll bottom out and have incredible plow. Sounds like your spring rates are high enough to avoid that, though. I'd really consider doing the higher rear rate bias "thing" as it really will wake the balance of the car up.

I'd really honestly just start with the alignment and give that a try. Toe in on the rears for an autocross car is the devil.

 

shortened shocks are in the future, though somebody just told me that i might ave problems with the c/a hitting the strut tower. any truth to that?

i tried playing with rear toe earlier in the season and it seemed to affect corner entry a lot more then corner exit. it may have been the setup on the car then though, so i'll have to try that again. i'm rather timid to try toe and higher rear spring rates because i think that will reduce rear grip, instead of increase rear grip. can anybody explain how increasing rear spring rates will increase front grip??

nate

 

>>maybe i should clarify. i want to get the car to rotate better without sacrificing rear grip. ie, i want to increase front grip.<<

Given the issues you're having with your setup, it probably won't be possible to "dial in" front grip to the point where the car produces a reasonable amount of steady-state oversteer. IMO, decreasing rear grip is about the only option that's going to give you more rotation than you currently have. Tire pressure, toe, sway bars, shocks, etc. are all different ways to get less grip out back. But it's gonna be difficult because your Front/Rear spring rate ratios are substantially off-target for producing steady state oversteer on a FWD. With 600 lb fronts (that's a really HIGH front rate for autox IMO), you need to be looking at 800+ for the rears. It's the ratio between front and rear rates that matters, and 2f/3r is usually the target.

 

You may want to think of adjusting the front sway bar to a softer setting (if it is adjustable) If not, I would look at a smaller front bar. This will increase front grip.

an answer to your question for higher spring rates in the rear, with higher spring rates, you are transfering less weight to the rear of the car when it is accelerating, however, with less weight transfer, the rear will grip less as well. so it is a trade off for the balance you want.

 

the bar change isn't something i want to do either. by reducing the front bar size i'll loose transitional response and the car will roll more. contrary to what other's believe, i don't think a rear sway controls body roll much at all once you are running the level of spring rates that i am. i see it this way. if you take the cg of the car and look at it's direction of travel it wants to push to the outside front in a turn. newton's law, an object in motion will remain in motion unless an outside force acts upon it. as the car turns the cg wants to continue in a straight line. as the car yaws the straight line moves to the outside of the car, or right about were the outside front tire is. as the car continues to yaw, the path the cg wants to take moves closer and closer to the perpendicular of the cars direction of travel (steady state, middle of turn) and at that point the rear sway bar starts to take more effect and controlling body roll. so the game plan is as follows. i'm going to toss some 700lb rear springs into the car and get some 5mm spacers for the front. if that doesn't work i'll have to look into some more drastic measures. i'll let you all know how it works out. thanks for the help.

nate

 

Well, increasing rear spring rates does not really increase front grip, nor does it really decrease rear grip either, but it can result in a higher overall cornering speed and change the car's tendency to oversteer/understeer. In a (somewhat large) nutshell...

Lets take an imaginary car weighing 2,000lbs with a 60/40 front/rear weight distribution. Static weights for each wheel would be as follows, and for simplicty sake's we'll assume for the purpose of this example that the left-right weight distribution is 50/50...

LF600--600RF
LR400--400RR
----------------
1000--1000 (total=2000)

Now, in an imaginary steady state right hand --> corner taken at 50mph max before the front would start to slide, let's say given a certain front and rear roll stiffness the total side-to-side weight transfer would be such that the left side of the car would now see 1600lbs and the right side would see 400lbs:

1000--200
600---200

Note that the total front weight (1200) and rear weight(800) has not changed, nor has the weight distribution (60/40). Also note that the total weight transfered from left to right is 600lbs (left is now 1600lbs)Now, let's say it just so happens that the maximum load the right front tire can handle before washing out is 1000lbs and say 750lbs for the rear, hence the front is overworked vs. the rear.

Ok so how can we change this behaviour using springs/antiroll bars? By using a stiffer rear spring we can cause a bigger weight transfer to occur at the rear so that in the same corner at 50 mph we would have:

900--300
700--100

Again, note that the total front weight (1200) and rear weight(800) has not changed, nor has the weight distribution (60/40), Also note that the total weight transfered from left to right is STILL 600lbs (left is STILL 1600lbs) BUT, now the LF tire is seeing 900lbs and LR is seeing 700lbs both of these figures below their maximum capacity. Which means that we can now safely increase our cornering speed...

So now with our new found confidence in our new setup we increase our cornering speed to 55mph, and end up with something like this:

970--230
750--50

Since the front can handle 1000lbs, we are ok there. But the rear tire has reached it's limit at 750 lbs and... well...what happens? we get a quick panoramic view of the parking lot as the car snaps into oversteer... So, we haven't increased the front grip nor the rear grip, nor have we decreased either one. All we did is changed how weight got transfered front vs. rear...

Note that there are several ways to achieve the same result, ie reduced weight transfer in the front. You can widen your track with the use of spacers, lower the center of gravity (CG), move the roll center closer to CG, change the F/R weight distribution etc... but all these with the exception of using spacers to widen track are rather complicated to do. Changine springs/swaybars is cheap and easy and that's the reason it's popular...

Again, all of the above assumes steady state cornering and the numbers used are just that, numbers to explain the basics. This is a very complex topic, and gets even more complex when you try to figure out what will happen under trail-braking, exiting the corner under full power, lift-throttle under maximum cornering load etc...

Hope this made sense



[Modified by jsi, 4:15 PM 9/27/2002]

 

In a perfect world, you don't want to reduce grip at one end of the car to try to improve the other end...But in a perfect world no one would race FWD cars! So, in order to make the front tires work less and control understeer, a good plan is to get the rear of the car to rotate a little when the driver induces it. The front tire angle of attack will be slightly less if the rear slip angles increase, = less front steering input = lower front tire temps. So we can use any combination of stiffer rear springs shock valving, stiffer rear bar, higher rear tire pressures and alignment tricks...or all of the above if you really like the *** out there. I had some experience in World Challenge in 1995 with a Prelude. Up front we ran 600 springs, soft shocks and no front sway bar. Out back we ran stiff shocks, 1,250 springs and 32mm sway bar. It worked, but if you had to lift mid corner you'd better be ready to plant the throttle and counter steer! Such a set up would probably be undrivable on an autocross course!

 

OMG!!!!!!!!! that is the best job i've ever seen at explaining how this works! it makes so much sense now. somebody make this post a sticky. again, thank you. i'll be trying the spacers and the higher spring rates for the next event. it seems to me that changing the springs is a better approach then reducing grip by changing the alignment, so i'll try it without any other changes to the car.

nate

 

ur welcome

I have to admit, my understanding of this is basic at best and there are people on here a lot more knowledgeable on the subject than I am...

As I said, this is just a very simple explanation of the basics behind front vs. rear roll stiffness. There is a lot more to it though as this does not explain how stiffness changes will make the car react under corner entry/exit, wheel rates, roll rates etc since under real race conditions steady state cornering almost never exists... The best thing to do is pick up some good books on racecar suspensions/tuning over the winter an read up!

js

 

JSI wrote:

>>Now, let's say it just so happens that the maximum load the right front tire can handle before washing out is 1000lbs and say 750lbs for the rear, hence the front is overworked vs. the rear.<<

Uhhh. Where does this concept come from? Other than the load bearing ability of the tire as listed on the sidewall, tires do not have a weight limit. As a matter of fact the more weight you apply to a tire the more grip you get. Elementary physics - the amount of lateral force a tire can generate is proportional to the coefficient of friction and the amount of force applied to it downwards.

If your logic were true then Formula One cars would avoid downforce like the plague for fear of "overloading" their tires. We all know that an F1 car produces tons of downforce at speed. 'splain that lucy.

 

do a bunch of hack autocrossers like us have access to the kind of tires that F1 racers run on? can we afford to spend that much $$ on tires that tire engineers pour their heart and soul into? you're comparing apples to oranges with your above comment

 

To simplify the explanation, I was referring to weight and the resultant HORIZONTAL force as one, 1000lbs being a horizontal force. Adding weight does increase grip but only to a certain point. But the extra MASS also increases the horizontal force. Eventually, sooner rather than later, adding more mass will only result in a greater horizontal force that far outweighs the added grip.

Aerodynamic downforce adds grip through weight but since it has no mass it will not create any horizontal force that will cause a loss of grip, so the more, the better.

 

yeah! what he said!

 

From what I learned from the past discussion, roll stiffness does not affect the amount of weight transfer so increasing springs and bars in the back should not lead to this result. The geometry of the car controls in the weight transfer department: track width and ride height. Stiffer end (either front or rear), however, would cause the inside tire of that end to lift (unload) first with the same amount of weight transfer.

From what I learned from the past discussion, it can be done with front and rear ride height and track width differential. The wider and lower the car it is, the less weight transfer there is. Less weight transfer means more overall grip because tires have the most grip when weight is split L50/R50 left to right. The larger the L/R weight differential, the less overall grip the tires would provide.

From what I learned from the past discussion, roll center and CG can't really be brought closer together unless we change the suspension geometry of a Honda. The roll center and CG would move together in almost the same ratio as ride height changes.

Has anyone who has done corner weighting before can tell me how much ride height differential would affect static weight distribution? My intuitive concept tells me that higher rear height would induce more weight in front. If my concept is correct, how much affect does it have? I'm looking for "% increase in front weight per 1/4" increase in rear height" kind of answer.

Not saying that you are wrong jsi. But what you said seems to be inconsistent from what I learned before. Just want to clarify and have a better understanding. Thanks!


[Modified by Wai, 3:14 PM 9/30/2002]

 

Changing roll stiffness does not change how much TOTAL weight will get transfered from side to side as a whole. However, changing roll stiffness DOES change how much will get transfered side to side at each axle, front or rear. So if you increase weight transfer at one end of the car, you reduce it at the other but the total remains the same.

I basically mentioned this in my original post:

Note that there are several ways to achieve the same result, ie reduced weight transfer in the front. You can widen your track with the use of spacers, lower the center of gravity (CG), move the roll center closer to CG, change the F/R weight distribution etc... but all these with the exception of using spacers to widen track are rather complicated to do. Changine springs/swaybars is cheap and easy and that's the reason it's popular...