Go Back  RX7Club.com - Mazda RX7 Forum > Tech and Performance > Race Car Tech
Reload this Page >

Curing understeer - racer feedback please

Race Car Tech Discuss anything related to road racing and auto X.

Curing understeer - racer feedback please

Old 04-27-06, 10:24 AM
  #1  
Full Member
5 Year Member
Thread Starter
 
Join Date: Nov 2005
Location: San Diego
Posts: 138
Likes: 0
Received 0 Likes on 0 Posts
Curing understeer - racer feedback please

Ok, so I just got adjustable shocks.

If I want to decrease understeer, should I make the rear stiffer than the front, or the front stiffer than the rear?
SammyD is offline  
Old 04-27-06, 10:39 AM
  #2  
RX7 lover
5 Year Member
 
Join Date: Jun 2005
Location: UK
Posts: 1,160
Likes: 0
Received 1 Like on 1 Post
There's a grid around somewhere that shows what to do with your geometry/suspension to cure what handling problems. I think it was either petite or pineapple racing. Chcek their websites or try searching the forums for it.
BobfisH is offline  
Old 04-27-06, 10:42 AM
  #3  
Mr. Links
15 Year Member
iTrader: (1)
 
Mahjik's Avatar
 
Join Date: Aug 2001
Location: Kansas City, MO
Posts: 27,592
Likes: 0
Received 4 Likes on 4 Posts
Understeer?

What size tires/wheels are you running, and what tire pressures? It's not very common to fight understeer with these cars.
Mahjik is offline  
Old 04-27-06, 10:50 AM
  #4  
Goodfalla Engine Complete
iTrader: (28)
 
Monkman33's Avatar
 
Join Date: May 2005
Location: Kennewick, Washington
Posts: 3,203
Received 11 Likes on 9 Posts
could be alot of things.
tire pressure too low (so the tire will roll over easily and you're seeing how well your sidewall grips)
front sway bar too stiff compared to the rear
if you are running aftermarket wheels, did you go to a narrower width? or did you go to staggered widths between frnt and rear, if so, how much.
what kind of tires are you running? are they worn out? warmed up properly? (depending on tire type)
Monkman33 is offline  
Old 04-27-06, 11:03 AM
  #5  
Full Member
5 Year Member
Thread Starter
 
Join Date: Nov 2005
Location: San Diego
Posts: 138
Likes: 0
Received 0 Likes on 0 Posts
Guys - please focus on the original question for now.

To reduce understeer, should the front shocks be set stiffer than the rear, or should the rear shocks be stiffer than the front.

Thanks.
SammyD is offline  
Old 04-27-06, 11:15 AM
  #6  
Senior Member
5 Year Member
 
axr6's Avatar
 
Join Date: Dec 2005
Location: Auburn, CA
Posts: 342
Likes: 0
Received 0 Likes on 0 Posts
The FD, like most other factory car, has a built in understeer. This is for the safety of the average non-racing trained driver. What it does, in case you enter into a corner too fast, it wants to go straight (push, or understeer) forcing you to turn the steering wheel more, thereby crossing the tires up more, which in turn, will rub some speed off and allow you to make the turn, within limits of error. It also makes the car's handling uncomfortable and non precise. It is a "safe" handling mode and the manufacturer's build them this way for liability reasons.

I have never had a car that did not initially understeer, much over my liking. The best compensation is not with shock settings but with a proper size, or adjustable, rear sway bar. By installing a larger, or stiffer bar in the rear, you can tune the handling towards neutral, regardless to tire size differences between the fronts and rears.

Be careful in the process; the car's handling will improve but, it will also be less tolerant to mistakes and will spin around easier.

I would not use shock settings to compensate for major understeer as you can get yourself into far more trouble with a non compliant dampening than the minor benefit you may get from it by small improvement in understeer.

If you have not done it yet, be sure to replace front and rear rubber sway bar mounting bushings with stiff poly, getting rid of the factory rubber ones. Those alone will make the car handle with more precision and they are cheap. Trade off is a bit harsher ride as you loose some independent suspension action between the left and ride sides of the car.

Last edited by axr6; 04-27-06 at 11:21 AM.
axr6 is offline  
Old 04-27-06, 11:24 AM
  #7  
Goodfalla Engine Complete
iTrader: (28)
 
Monkman33's Avatar
 
Join Date: May 2005
Location: Kennewick, Washington
Posts: 3,203
Received 11 Likes on 9 Posts
that was a good post. /\
Monkman33 is offline  
Old 04-27-06, 12:07 PM
  #8  
Racecar - Formula 2000
 
DaveW's Avatar
 
Join Date: Sep 2002
Location: Bath, OH
Posts: 3,310
Received 27 Likes on 24 Posts
Originally Posted by SammyD
Ok, so I just got adjustable shocks.

If I want to decrease understeer, should I make the rear stiffer than the front, or the front stiffer than the rear?

Here's a link to a PDF on general suspension adjustment effects, including damper adjustments:

http://www.neohio-scca.org/comp_clin...ut%202005a.pdf

Dave
DaveW is offline  
Old 04-27-06, 12:25 PM
  #9  
Lives on the Forum
 
rynberg's Avatar
 
Join Date: Dec 2001
Location: San Lorenzo, California
Posts: 14,717
Likes: 0
Received 2 Likes on 2 Posts
Yes, softer shock settings up front vs the rear would reduce understeer, but you should really listen to some of the statements made in this thread.

If you are really having a trouble with understeer, either something in your setup is creating it (big stagger difference in tire sizes or huge front anti-roll bar) or it's your driving technique.
rynberg is offline  
Old 04-27-06, 01:05 PM
  #10  
Lives on the Forum
15 Year Member
 
DamonB's Avatar
 
Join Date: Feb 2001
Location: Dallas
Posts: 9,617
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by axr6
I have never had a car that did not initially understeer, much over my liking. The best compensation is not with shock settings but with a proper size, or adjustable, rear sway bar.
Rule #1:

Don't try to fix a problem at one end of the car by screwing up the other end of the car.

If one end of the car isn't working then work on that end. Handicapping the "good" end should only be done as a last resort.
DamonB is offline  
Old 04-27-06, 01:43 PM
  #11  
Full Member
5 Year Member
Thread Starter
 
Join Date: Nov 2005
Location: San Diego
Posts: 138
Likes: 0
Received 0 Likes on 0 Posts
Ok, thanks everyone! I just wanted to get a better understanding of the pysics behind all this.

So a fatter roll bar would essentially stiffen the rear end, resulting in a lesser degree of understeer. I belive it.

I would have thought the opposite, however. You get more bite on the front end by putting more weight on the front tires. If you stiffen the front shocks, and soften the rears, then the front wheels would be fighting more of the weight of the car leaning over, right? So I figured that the front would get more 'bite' on the road. I guess it just doesn't work that way, but I'm not sure why.
SammyD is offline  
Old 04-27-06, 01:44 PM
  #12  
Eats, Sleeps, Dreams Rotary
10 Year Member
 
7racer's Avatar
 
Join Date: Apr 2001
Location: Dallas, Texas
Posts: 3,737
Likes: 0
Received 0 Likes on 0 Posts
Sammy,

you didn't answer the question. What's your current setup that you are getting so much understeer.
7racer is offline  
Old 04-27-06, 01:47 PM
  #13  
Full Member
5 Year Member
Thread Starter
 
Join Date: Nov 2005
Location: San Diego
Posts: 138
Likes: 0
Received 0 Likes on 0 Posts
Wow, some interesting stuff on this link, namely:

-Rubber friction coefficient decreases as contact pressure increases.

http://www.neohio-scca.org/comp_clin...ut%202005a.pdf

I would have never guessed that to be the case!

I've read in driving manuals about weighting the front end of the car under braking to get more bite before turn-in. This appears to be a direct contradiction?
SammyD is offline  
Old 04-27-06, 01:49 PM
  #14  
Full Member
5 Year Member
Thread Starter
 
Join Date: Nov 2005
Location: San Diego
Posts: 138
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by 7racer
Sammy,

you didn't answer the question. What's your current setup that you are getting so much understeer.
I have a bad wheel set up, so I went back to stock. I'm upgrading the tire sizes on the wheels, so no biggie.

This post is really about me trying to understand the physics of car handling more than specifically fixing my car. There's been a lot of great info posted =)
SammyD is offline  
Old 04-27-06, 02:02 PM
  #15  
development
10 Year Member
 
dubulup's Avatar
 
Join Date: Aug 2002
Location: Lafayette, LA
Posts: 5,714
Likes: 0
Received 2 Likes on 2 Posts
Originally Posted by axr6
The best compensation is a proper size, or adjustable, rear sway bar.

If you have not done it yet, be sure to replace front and rear rubber sway bar mounting bushings with stiff poly, getting rid of the factory rubber ones.
Is the R1/2 front sway bar identical to the base/touring?

Where can I find these polyurethane sway bar mounting bushings? I quick internet search didn't yield anything...without the purchase of front and rear bars.

TIA
dubulup is offline  
Old 04-27-06, 02:18 PM
  #16  
Senior Member
5 Year Member
 
axr6's Avatar
 
Join Date: Dec 2005
Location: Auburn, CA
Posts: 342
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by SammyD
Ok, thanks everyone! I just wanted to get a better understanding of the pysics behind all this.

So a fatter roll bar would essentially stiffen the rear end, resulting in a lesser degree of understeer. I belive it.

I would have thought the opposite, however. You get more bite on the front end by putting more weight on the front tires. If you stiffen the front shocks, and soften the rears, then the front wheels would be fighting more of the weight of the car leaning over, right? So I figured that the front would get more 'bite' on the road. I guess it just doesn't work that way, but I'm not sure why.

Like so much in life, car handling is a compromise. It was shown in experiments that in order to get max tire grip, you want to allow the car to roll over onto the outside tires and increase the tire to ground contact force. Great, if you only going to run in circles and go for max G-force numbers. But, if you need to make transitions from left to right turns, as in most cases of race and winding road driving, you need to minimize the body roll and the resulting weight tranfers. It takes time to properly and smoothly transfer chassis weight from one side to the other. During these periods of weight transfer the car is at its most unbalanced states, due to various suspension movements and variations in those settings.

I had found that for the FD the stock front bar, combined with poly bushings were quite sufficient for roll resistance, particularly for any street speeds. At the same time, the rear bar is much too small to provide the needed balance between the front and the rear. With that small rear bar the weight transfers on the rear-end of the car will be greater, resulting in greater grip which, will not allow the rear end to come around in a neutral manner. So, in effect, with the larger rear bar you make the compromise of actually reducing the outside tire grip in order to obtain chassis balance.

Also, must remember that with soft bars and lots of roll, most of the tire-to-pavement contact is with the outside tires. You can, and actually will, lift the inside tires in a turn. With lot of roll stiffness, you reduce the outside tire contact pressures but, increase the inside tire contact pressures in the process, simply because you do not transfer all that much weight onto the outside tires.

With the proper chassis balance and roll resistance the car will transition much smoother and be altogether much faster and feel more precise for road courses. The ultimate amount of roll resistance truly depends on the driver's preference and driving style, as well as on environmental factors, such as track, tire selection and temperatures, etc...

Trying to adjust with you shocks, assuming a stock sway bar setup, you'd end up placing your rear shocks on max stiffness while keeping the front ones soft which could really get tricky and unpredictable in bumpy corners.
axr6 is offline  
Old 04-27-06, 02:19 PM
  #17  
RX7 lover
5 Year Member
 
Join Date: Jun 2005
Location: UK
Posts: 1,160
Likes: 0
Received 1 Like on 1 Post
Originally Posted by SammyD
Wow, some interesting stuff on this link, namely:

I've read in driving manuals about weighting the front end of the car under braking to get more bite before turn-in. This appears to be a direct contradiction?
no, because the reason friction decreases with area of contact patch is because the weight is spread over a larger area hance pressure per square inch decreases creating less friction.

Braking increases the weight on the front of the car, thus increasing pressure per unit of area and thus friction.
BobfisH is offline  
Old 04-27-06, 02:26 PM
  #18  
Senior Member
5 Year Member
 
axr6's Avatar
 
Join Date: Dec 2005
Location: Auburn, CA
Posts: 342
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by dubulup
Is the R1/2 front sway bar identical to the base/touring?

Where can I find these polyurethane sway bar mounting bushings? I quick internet search didn't yield anything...without the purchase of front and rear bars.

TIA

Bushings can be purchased at most local speed shops, or even at places like JC Whitney's online catalog. Try searching places, like Summit racing. Measure your bar diameters and order the correct size. I just got some a few months ago for the front bar, pretty sure it was JCWhitney's. They normally arrive with the proper clamps that usually fit the stock mounting bolt patterns.

Last edited by axr6; 04-27-06 at 02:48 PM.
axr6 is offline  
Old 04-27-06, 02:39 PM
  #19  
Senior Member
5 Year Member
 
axr6's Avatar
 
Join Date: Dec 2005
Location: Auburn, CA
Posts: 342
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by SammyD
Wow, some interesting stuff on this link, namely:

-Rubber friction coefficient decreases as contact pressure increases.

http://www.neohio-scca.org/comp_clin...ut%202005a.pdf

I would have never guessed that to be the case!

I've read in driving manuals about weighting the front end of the car under braking to get more bite before turn-in. This appears to be a direct contradiction?

That is called "trail breaking" It is done to transfer weight onto the front tires to increase their grip while, at the same time, take weight OFF the rear tires to decrease their grip and thus, allow the rear end to rotate around.

What needs to be understood is that in the vast majority of car setup cases you can NOT obtain a perfect chassis balance that will be ideal for slow, as well as, fast corners. What setup work great at autocross speeds could give you the scare of your life in a 140MPH sweeper. Thus, what many racers prefer is to set up the car to be near neutral for the fastest road course corners where most time is gained or lost and just deal with the understeer in slow corners. Trail braking is one of the ways to deal with slow corner entry understeer.

Last edited by axr6; 04-27-06 at 02:42 PM.
axr6 is offline  
Old 04-27-06, 02:44 PM
  #20  
Full Member
5 Year Member
Thread Starter
 
Join Date: Nov 2005
Location: San Diego
Posts: 138
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by BobfisH
no, because the reason friction decreases with area of contact patch is because the weight is spread over a larger area hance pressure per square inch decreases creating less friction.

Braking increases the weight on the front of the car, thus increasing pressure per unit of area and thus friction.
Hey, what you say is true, but the comment I was referring to was not with regards to the contact patch, rather the adhesion properties of rubber in general. The link says rubber friction coefficient decreases with contact PRESSURE. Not area. I find this hard to believe, however.
SammyD is offline  
Old 04-27-06, 02:53 PM
  #21  
Lives on the Forum
15 Year Member
 
DamonB's Avatar
 
Join Date: Feb 2001
Location: Dallas
Posts: 9,617
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by axr6
It was shown in experiments that in order to get max tire grip, you want to allow the car to roll over onto the outside tires and increase the tire to ground contact force.
Nope. Any car will generate the most grip when each tire is loaded equally. That means minimal weight transfer and is why racecars are built with the CG as low as possible along with nearly 50/50 weight distribution. This results in minimal weight transfer and tires loaded as equally as possible at all times.

Originally Posted by axr6
...you need to minimize the body roll and the resulting weight tranfers.
Nope. Weight transfer has zero to do with body roll. You can adjust the amount of roll all you like and the effect on weight transfer will be zilch.

Originally Posted by axr6
With that small rear bar the weight transfers on the rear-end of the car will be greater
Nope. Bigger (stiffer) bars create greater weight transfer. Smaller bars create less weight transfer.

Originally Posted by axr6
So, in effect, with the larger rear bar you make the compromise of actually reducing the outside tire grip in order to obtain chassis balance.
Now you've got it right. Bigger bars saturate the outside tire more quickly and this results in less grip on the outside tire. Bigger bars transfer more weight across the chassis, not less.

Never handicap the end of the car that IS working. Try to fix the end of the car that ISN'T working. Only handicap the good end as a last resort.

Originally Posted by axr6
Also, must remember that with soft bars and lots of roll, most of the tire-to-pavement contact is with the outside tires. You can, and actually will, lift the inside tires in a turn.
Nope. Stiffer bars are more apt to lift the inside tire. Cars with little roll resistance and lots of grip will merely ride the bumpstops on the outside of the turn, they will not lift the inside tire because there's not enough force through the bar to pick the tire up off the ground.

Originally Posted by axr6
With lot of roll stiffness, you reduce the outside tire contact pressures but, increase the inside tire contact pressures in the process, simply because you do not transfer all that much weight onto the outside tires.
Nope again.

Originally Posted by axr6
Trying to adjust with you shocks, assuming a stock sway bar setup, you'd end up placing your rear shocks on max stiffness while keeping the front ones soft which could really get tricky and unpredictable in bumpy corners.
Depends. It's all balance. "Stiffer" and "softer" are relative terms, not absolutes. Without a shock dyno nobody knows what the absolutes are.

Last edited by DamonB; 04-27-06 at 03:02 PM.
DamonB is offline  
Old 04-27-06, 02:59 PM
  #22  
Lives on the Forum
15 Year Member
 
DamonB's Avatar
 
Join Date: Feb 2001
Location: Dallas
Posts: 9,617
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by SammyD
The link says rubber friction coefficient decreases with contact PRESSURE. Not area. I find this hard to believe, however.
It's true. Note that as cF decreases that does not necessarily mean total grip decreases. That does sound contradicting but it's not. Increasing the load on a tire increases the grip even though it decreases cF. The increase in load (and the resulting increase in grip) is enough to overpower the slight decrease in cF so the end result is still more grip. As you continue to increase load on a tire grip of that tire continues to increase, albeit at a lesser and lesser rate.

https://www.rx7club.com/suspension-wheels-tires-brakes-archive-112/17-vs-18-racing-mind-narrow-vs-wide-tires-292194/
DamonB is offline  
Old 04-27-06, 03:06 PM
  #23  
-
 
Join Date: Jul 2003
Location: Wayne, NJ 07470
Posts: 409
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by SammyD
Wow, some interesting stuff on this link, namely:

-Rubber friction coefficient decreases as contact pressure increases.

http://www.neohio-scca.org/comp_clin...ut%202005a.pdf

I would have never guessed that to be the case!

I've read in driving manuals about weighting the front end of the car under braking to get more bite before turn-in. This appears to be a direct contradiction?
No, it's not a contradiction. If you double the downforce on a tire you do get more traction.

But

You do not get double the traction. Insead of 200% traction you get about 180% or 190%.

Oversteer and understeer are terms describing the relation between front and rear traction.

If one end of the car is stiffer, it will get more weight transfer. It will actually remove some of the weight transfer from the softer end of the car. Therefore the softer end of the car will end up with more traction relative to the stiffer end of the car.

When one end of the car is stiffer than the other end of the car, you are actually trading some of the traction from one end to the other. For example the soft end of the car might have 40% of the load but the load actually generates 45% of the traction while the stiff end of the car might get 60% of the load but the load only generates 55% traction . That's oversteer if the stiff end is in back.

In general you get more traction with softer suspension but if the suspension is too soft the tires roll over and you lose the traction you've gained. Or if the soft shocks allow the tires to bounce too much, again you lose the traction that you've gained.

Or if one end of the car is so stiff that it lifts the inside wheel off the ground, it cannot take any more load off the soft end of the car.

A soft car will give you lots of traction but then the car will have slow response. A stiffer car will have very fast reactions but have less traction. (well, it would if it was possible to have a car with perfect geometry)

ed
edmcguirk is offline  
Old 04-27-06, 03:13 PM
  #24  
development
10 Year Member
 
dubulup's Avatar
 
Join Date: Aug 2002
Location: Lafayette, LA
Posts: 5,714
Likes: 0
Received 2 Likes on 2 Posts
Originally Posted by DamonB
Nope. Any car will generate the most grip when each tire is loaded equally. That means minimal weight transfer and is why racecars are built with the CG as low as possible along with nearly 50/50 weight distribution. This results in minimal weight transfer and tires loaded as equally as possible at all times.
I believe he was stating results from tests, not how race cars should be built.

Nope. Weight transfer has zero to do with body roll. You can adjust the amount of roll all you like and the effect on weight transfer will be zilch.


Nope. Bigger (stiffer) bars create greater weight transfer. Smaller bars create less weight transfer.
Don't these comments contradict each other? bars reduce roll which has zilch to do with weight xfer, yet stiffer bars create greater weight xfer

Last edited by dubulup; 04-27-06 at 03:15 PM.
dubulup is offline  
Old 04-27-06, 03:23 PM
  #25  
Lives on the Forum
15 Year Member
 
DamonB's Avatar
 
Join Date: Feb 2001
Location: Dallas
Posts: 9,617
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by edmcguirk
No, it's not a contradiction. If you double the downforce on a tire you do get more traction.

But

You do not get double the traction. Insead of 200% traction you get about 180% or 190%.
Exactly. If grip increased at a 1:1 ratio with load we would absolutely never have to concern ourselves with weight transfer. If it were a 1:1 ratio we wouldn't worry about weight transfering across the chassis in a corner because as the inside tire lost grip the outside tire would gain the amount of grip lost from the inside and the total sum would still be equal. It doesn't work that way. As weight transfers across the chassis the sum of the tires' grip is less than when each tire is equally loaded. This is fundamental. Anything you can do to reduce weight transfer will increase total grip.

As weight transfers across the chassis the outside tire becomes more highly loaded, its cF goes down and that tire saturates and slides more quickly. If we could keep the weight equally balanced at all the tires all the time then no single tire would saturate before the other and the vehicle would experience the greatest amount of grip possible from those tires since each one would be loaded as little as possible. That means the CG as close to the road as possible and the track as wide as possible. Nothing else will effect the amount of weight transfer.
DamonB is offline  

Thread Tools
Search this Thread
Quick Reply: Curing understeer - racer feedback please


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: