Tuning Your Suspension for Oversteer/Understeer

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andysapp
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Tuning Your Suspension for Oversteer/Understeer

Post by andysapp »

I found some interesting info regarding suspension tuning:

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Granted... most of you on this forum won't really be looking to tune for understeer...
but good knowledge to have nonetheless.
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Post by OutDriveU »

great! this is a basic but much overlooked tuning guide. especially the tyre pressures. i always run with +10 psi in the rear compared to front . makes allot of diff
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Post by xonn »

Most of the chart looks pretty good but I disagree with the recommendations as far as tire pressure goes. Assuming that you have a neutral handling car and that all tires are currently at the same pressure, increasing tire pressure in the rear will not increase understeer. It'll promote oversteer because the rear tires will be faster to loose grip than the front tires.

-----|--------|-------------------------------|---------|
----/_______\-----------------------------|______|

-Normal pressure----------------------Higher pressure

dashes are for spacing

Now granted, my diagram is less than perfect, but I think illustrates my point well enough. When you significantly increase the a tire's pressure, say to 50 psi or so, you're effectively decreasing the contact patch of the tire and making it more prone to loose grip when pushed harder.

A common setup with FF cars to help them rotate a bit better is to run higher pressure in the rear than the front so that at the extremes, the rear of the car looses grip before the front thus inducing oversteer. The same is true for FR and AWD cars. If you want to make a car more prone to rotate (ie oversteer) run more pressure in the rear tires than in the front. and vice versa for understeer. Of course there are a myriad of other factors that should/can be considered such as the stiffness of the struts in rear vs the front and the type of sway bars that you're using. None of those individually influence a cars handling more than the tires though. Sorry for the long post...
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Post by andysapp »

I agree...

I played around with PSI setting a lot this weekend in Miami.
The drifts were MUCH easier to control using a higher PSI in the rear tyres.
With a lower PSI, the slides felt too mushy to control correctly. Once I upped the pressure it was all good. The stiff swayBar in the rear was also a great factor for control.

I also had a LOT of rubber to try and break away from (when we were "competing" - lol). I was using the Azenis 235.40.zr18's... I'd really like to try stretching a smaller tyre on there. I think I heard someone this weekend say they were running 205's strrreeeetched over 18's.
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Post by xonn »

Running a smaller tire in the rear would definately make it easier to get the car sideways. I'm not sure how much control you'd have once you got it there though. I know that one of the better known japanese tuners (Signal I think), ran 205 tire in the rear and 225 in the front for a time attack rsx that they were building. It was a FF car but the same concept applies. Less grip in the rear == easier to induce oversteer. How as the Miami drift event btw? :)
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Post by OutDriveU »

Shit thats true I didnt look hard enough but WTF. I run higher in the rear like I said as this increases the spring rate of the tyre. That chart is surley bassackwards
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Post by andysapp »

I don't remember where exactly I found this... but I think it might be an Alex Pfeiffer quote off Drifting.com... Enjoy!
--

The limit of the car's handling is based solely on the tires. Period. You exceed the limit of the tires, you either a) skid or b) spin.

The limit of an individual tire's grip varies with brand, design, tread pattern, compound, wear, etc. In a static environment, it is very easy to find the tire's limit. But when they are actually on your car, it is a different story.

Assuming constant values for the coefficient of friction of the road, and coefficient of stiction of the tire, you can affect how early you reach the tire's limit with two things: weight distribution, and slip angle. Slip angle refers to the difference between where the front wheels are pointed, and where you actually want the car to go. There is always some slip angle; it isn't a bad thing until you are asking the tires for a greater angle then they can handle.

Both weight distribution and slip angle are strongly dependent on two things: your driving ability, and the car's chassis setup. I'm sure you already know that the best money you can spend to make the car faster is to start autocrossing and going to driver's schools - impriove the nut behind the wheel! Assuming that you are already doing that, any suspension upgrades will change (hopefully for the better) how the slip angles and weight distribution are acting dynamically.

To specifically discuss swaybars, consider the two primary changes you will notice after installing them: decreased body roll, and changed handling balance.

Decreased body roll: Tires need to be in contact with the road to grip! And they need something (weight!) keeping them on the road to grip under load. Ever notice that FWD cars can chirp their tires easily under acceleration? It isn't the raw power of four-cylinder fury, but rather the lack of weight over the front wheels when the weight shifts back under acceleration allows the tires to overcome the minimal grip they have on the road.

When you are cornering, and the weight shifts to the outside, your inside tire is doing less work. In extreme cases (not directly applicable to your car), where that inside tire is lifting off the road, you are completely losing 50% of your front end grip! Yikes! by keeping the weight closer to the center of the car and more equally loading the tires, you will be able to maintain adhesion for a longer period of time, under higher cornering forces. It doesn't improve the tire's ability to grip, just gives it the chance to do it's job. No point in paying for performance tires if they aren't working at 100% of their ability!

Handling balance: How the car enters a corner directly affects the slip angles applied to the front tire. Oversteer, understeer, these terms get tossed around a lot. Too much in my opinion, since most drivers only know what they mean from Gran Turismo, and are clueless as to how to actually attune their driving to these behaviors. In general, understeer means the front has less grip than the rear, and oversteer means the rear has less grip than the front. In a perfectly neutral-handling car, high slip angles will induce understeer, while poor braking/turn-in technique will induce oversteer. The level of neutality, or "balance," is determined by static weight distribution, and static front-rear rake.

But is the car static when it enters a corner? Heck no, it's a rapidly changing dynamic system. If you overcook a corner and get on the brakes as you turn in, the weight will shift forward, and the rear wheels will get "light", and they will lose grip more easily. On the other hand, if you turn in too early (or early apex, if you are at the track), the car will reach its limit of front end grip before you reach the end of the corner, and start to "plow". The front tires have lost grip, due to excessively high slip angles, and they are giving up, and your car is heading toward the outside of the corner, and maybe toward an accident!

Swaybars are just one way that you can tune handling balance to help avoid the effects of oversteer and understeer. Shocks, springs, tire weidth/pressure are others, and perhaps the most important is driving ability. Regardless, in addition to the decreased body roll characteristics of larger diameter sway bars, the different adjustment holes allow you to create your own balance of front and rear body roll. These adjustments will directly affect how the car responds when you turn the wheel. In time, you can learn to use them to minimize slip angles for your driving style, while keeping the car's weight stable in lateral transitions.

To sum it up, the suspension is a system. If you change a single component, the effects can be complicated and far-reaching throughout the system. Swaybars can absolutely improve the car's handling, and allow it to reach the limit of adhesion with higher cornering speeds, as long as you recognize that nothing can change the laws of physics.
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Post by Siegel Racing »

Pretty much everything about a suspension has a sweet spot. The grip vs whatever graph is a bell-curve. There is an "ideal" (max-grip) pressure for each tire. That pressure is where the most rubber is on the road. Below that, the tire rolls over and ends up more on the sidewall. As you add pressure, there will be a point that the most rubber is on the ground. As you keep adding (over the ideal pressure) you start to stand the tire up; only the the center of the tire is on the ground.

GENERALLY you will want your front tires at this ideal pressure. If you really using the tire, remember, it gets hot. When gas (air) gets hot, it expands. You can easily gain 10+ psi in a tire between cold and hot pressures. You want your hot pressure to be ideal (totally different on different tyres, even different depending on suspension.) You want the rear SOMEWHERE ELSE. Higher or lower that ideal pressure will result in less grip. Under, the tire rolls over and grip goes as you get onto the corner of the tire. Higher and you only have the center of the tire on the ground, and you have increased the effective spring rate. Generally I like lower in the rear on the street (high pressures on the street wear out the center of the tire QUICK), but higher pressures in the rear on the track is generally a little more predictable, progressive slide.

For instance, what I found worked well on the STi's was 44 front 36 rear on the street. 55 rear on the track. The "Max inflation pressure" refers to the pressure the tire carries the most load at, not that it instantly explodes if you go over it. Some tires require 120+ psi to seat...

Anyway, good luck, have fun, sticky side down,
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Post by marshun »

also for a quicker turn in you'd want a little negative toe in your front suspension. and caster forward some. i dont remember if thats negative or positive. im thinkin positive.

keep in mind how tall your tires are (sidewall), how wide, if you've stretched smaller tires or not. the wider pants also helps the car stay more stable during the drifts.

thats all i can think of off the top of my head.

anyone know koguchi's settings for his suspension? i ask because he has an s13 and so do i. :)
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Post by mattology »

Siegel Racing wrote:Pretty much everything about a suspension has a sweet spot. The grip vs whatever graph is a bell-curve. There is an "ideal" (max-grip) pressure for each tire. That pressure is where the most rubber is on the road. Below that, the tire rolls over and ends up more on the sidewall. As you add pressure, there will be a point that the most rubber is on the ground. As you keep adding (over the ideal pressure) you start to stand the tire up; only the the center of the tire is on the ground.

GENERALLY you will want your front tires at this ideal pressure. If you really using the tire, remember, it gets hot. When gas (air) gets hot, it expands. You can easily gain 10+ psi in a tire between cold and hot pressures. You want your hot pressure to be ideal (totally different on different tyres, even different depending on suspension.) You want the rear SOMEWHERE ELSE. Higher or lower that ideal pressure will result in less grip. Under, the tire rolls over and grip goes as you get onto the corner of the tire. Higher and you only have the center of the tire on the ground, and you have increased the effective spring rate. Generally I like lower in the rear on the street (high pressures on the street wear out the center of the tire QUICK), but higher pressures in the rear on the track is generally a little more predictable, progressive slide.

For instance, what I found worked well on the STi's was 44 front 36 rear on the street. 55 rear on the track. The "Max inflation pressure" refers to the pressure the tire carries the most load at, not that it instantly explodes if you go over it. Some tires require 120+ psi to seat...

Anyway, good luck, have fun, sticky side down,

I run nitrogen in my tires that i use for drifting so the pressure stays the same no matter what
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Post by andysapp »

WHOA!!!
Really? That's crazy!

There's no change of a Hindenberg incident is there?
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(I think that was nitro-filled... or was it CO2?)
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Post by shuya »

andysapp wrote: (I think that was nitro-filled... or was it CO2?)
hydrogen
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Post by andysapp »

Haha... LOL, you can tell I'm up on my science!
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Post by andysapp »

This thread is popular today for some reason.
:?:
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Post by wicked clown »

andy basicly summed it all up in his long post, FOERST and foremost you have to know how to react to conditions. i learned this FOERST hand in my racecar. we would make a change and it wouldnt do anything. it is because i wasnt reacting to the changes. now, i can feel out a 1/4 lb. tire pressure diff. on the track. a 1/4 turn on the springs will throw my car to a whole new level of either good handling or turn it into a major pain in the ass to drive. i run nitrogen in my tires now so that i dont have to adapt as the race goes in and pressures build, and it has helped me a ton. i went on a 50 lap race and the tire pressures never changed, only the tire wore down, and it wore evenly and i ended up winning that race. i see ppl wrenching on their cars all the time and they never see a diffrence in the handling. just get a lot of seat time, learn your car and what all the changes will do to it. you will get there. sometimes it just takes patience.
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