The real reason our cars understeer

Racefiend":1snor55j said:

So I continued making measurements of the front suspension, and I must say I'm pretty dissapointed in Toyota. Here's what I found:

The shocks have 5.75" of total stroke.

With the car sitting level, with driver, the front suspension compresses 3-7/16" (which leaves 2-5/16" of compression travel)

The bump stops hit at 3.75" of compression, which leaves a whopping 5/16" of compression before you start smacking the bump stop.

The bump stop itself adds 50 lbs/inch @ .5" of compression, and 100 lbs/inch at .75". At this point they're pretty much fully compressed, so stiffness goes up exponentially from there.

Since I completely disassembled/drained the front shocks, I decided to do the bounce test on the front to see what the front spring rates were. I got 68 cycles/min (cpm). This equates to about 126 lbs/inch. The FAQ states the front should be 174 lbs/inch for all-tracs up to 1991 and 123 lbs/inch for later years. Mine is a 1990, so either the FAQ is wrong, or someone swapped front springs on my car at some point.

From my thread here:

http://alltrac.net/phpBB2/viewtopic.php ... bd149c6e04

we know the front sway bar adds ~50 lbs/inch of wheel rate during cornering.

So we know the front wheel rate is about 175 lbs/inch during cornering. However, this is in effect only for the first 5/16" of body roll. At this point, the bump stops hit. So at 13/16" of body roll, the wheel rate is now 225 lbs/inch. At 1-1/16" of body roll, the front wheel rate jumps to 275 lbs/inch. Beyond that much body roll, the wheel rate goes up tremendously as the bump stop is compressed beyond the point I could even get it squished. Now, the all-trac weighs about 2100 lbs up front, so it is quite easy to transfer enough weight to the outside tire to compress it over 1" during a hard turn (I'll have exact figures later).

So what does this all mean? Well, during hard cornering, the front outside wheel rate jumps dramatically, causing the understeer of the car. Things get worse when you lower the car. This explains why during my measurements the rear wheel rates were proportionately way higher than the front, and technically should have caused serious oversteer. We are left with an interesting paradox. If you stiffen the rear springs/swaybar, you decrease the understeer by compensating for hitting the bump stops up front. If you stiffen the front springs/swaybar instead, you will also reduce understeer by keeping the front suspension from compressing the bump stops as much.

I've taken measurements of the stock strut inserts, and will be calling Bilstein to try and find a shorter stroke replacement for lowering the car. I still need to put my car on scales again to find out the center of gravity so I can figure out the weight transfer numbers. The quest for the proper suspension continues....

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I don't mean to ride your nuts here, but I don't believe what you've posted in this (and your swaybar thread) is correct. I have nothing to back up this claim, since I no longer have the stock suspension on my 165, except that it doesn't make sense,

Are you sure the front suspension only has 5/16" of stroke before the bumpstops? I find this hard to believe and think you must have measured something incorrectly, or your springs are saggier than granny's tits. This suggests that lowering the car more than 5/16" would mean it was running on the bumpstops, which is obviously not the case.

Additionally, I read your other thread on swaybars and I'm not entirely sure I agree with your conclusion there, either...the conclusion that the stock rear sway bar is stiffer than the front doesn't seem right, from casual observation. I'll get back to you on this when I have some time to think about it, later this week, hopefully.


:smokes:

ion_four":1px3ib1i said:

I don't mean to ride your nuts here, but I don't believe what you've posted in this (and your swaybar thread) is correct. I have nothing to back up this claim, since I no longer have the stock suspension on my 165, except that it doesn't make sense,

Are you sure the front suspension only has 5/16" of stroke before the bumpstops? I find this hard to believe and think you must have measured something incorrectly, or your springs are saggier than granny's tits. This suggests that lowering the car more than 5/16" would mean it was running on the bumpstops, which is obviously not the case.

Additionally, I read your other thread on swaybars and I'm not entirely sure I agree with your conclusion there, either...the conclusion that the stock rear sway bar is stiffer than the front doesn't seem right, from casual observation. I'll get back to you on this when I have some time to think about it, later this week, hopefully.


:smokes:

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I don't see any riding nuts at all. Thats why this is a discussion forum

I'll be happy to explain how I took my measurements. I measured the distance from the outer rim of the wheel to the edge of the fender arch with the suspension at full droop. Then I lowered the car and put a driver in it, and measured the same distance again. The difference was 3 7/16, so that is how much the strut compresses at ride height. I pulled the strut and measured the stroke, which gave me 5.75" (I just double checked it since I have it laying around). Now, that leaves us 2 5/16" of compression left for the strut. The bump stop measures 2.25" in length (just double checked that also), so that leaves us 1/16". However, the strut rod does not compress all the way even with the top of gland nut, it's about 1/4" up, so there you have the 5/16" before the bump stop hits the top of the gland nut. When I put it all together, I double and triple checked my ride height compression measurements again, just to make sure, because I also could not beleive that was the case. But the measurements were the same.

Now, my springs are definitely not new. They have 100k miles on them, but I did not notice the car sag any more than another stock st185. Even if they sagged a whopping 1/2", you till get about .75" of travel before the bump stop hits.

As far as the sway bars are concerned, just at first glance, you would think the front was much stiffer because it is so much bigger. However, there are a few factors which reduce the effective wheel rate of the bar. First of all, the arm of the rear bar extends only ~9.5" to the end link. The front bar, on the other hand, extends about 15" to the end link. The longer the arm, the less tension is at the end of that arm. Think of using a breaker bar on a tight bolt verses using a stubby ratchet. It's the same principle. The other difference is that the rear sway bar mounts directly to the strut. Therefore the spring rate of the bar at the end link is also the spring rate it adds at the wheels. The front bar, however, mounts to the lower control arm, at right around mid length. Just did a quick measurement, since the car is in the air and I'm workin in the garage. A rough measurement gives 14" from lca mount to BJ, and about 7.5" from lca mount to sway bar mount. We know from the motion ratio calculation, that the wheel rate equals: (sway bar rate) X (mount distance/BJ distance)squared. In this case, its: (sway bar rate) x (7.5/14)squared. So it is: (sway bar rate) X .25

As you can see, the effective wheel rate of the sway bar is 25% of it's actual spring rate. And the wheel rate is what really matters. You can easily check this by disconnecting the upper strut mount on both front and rear. Next, grab the hub on the rear and try and move it up and down by hand. It's pretty hard. Next, do the same with the front. It is MUCH easier to do.

hope that helps explain what is going on.

(Glad to see you didn't take it personally, as some people are kind of touchy about criticism!)

I see what you mean about the front and rear sway bars, exactly, now. I realized that arms of the front bar could be longer than the rear, but I would have thought the thickness of the front bar would have overshadowed that...However, the different mounting points (inboard on the front LCA vs. right on the rear strut) would definitely make a difference. Somewhat related: this is why my Civic has stiffer springs than my 165, even though it's a lighter car, because the double-wishbone suspension in front (and rear shock mounted inboard on the LCA in the rear) reduces the effective spring rate at the wheel. So, the spring has to compress less with the same amount of force applied to the wheel.

I'm still not convinced about the suspension travel, though. Your measurements make sense, but the result is absurdly small! I read of an easier and probably more accurate way to determine travel, which you might want to try:

When the suspension is at droop, put a zip tie around the strut shaft, tight enough so that it won't slide back down on its own. Then, put the wheel back on the ground, get in the car. Then, jack the car up so the wheel is off the ground and see how much the zip tie has moved and how much room is left on the shaft. The room left between the ziptie and the top of the strut shaft (or bump stop if it's on the top) is the amount of stroke you have before bottoming out or hitting the bump stop.

Additionally, you can go drive over some bumps or take some hard corners and see if it gets pushed to the top, indicating you're riding the bump stops.

I have never actually measured my suspension, but when installing my bastard KYB/J-Speed/OEM coilover suspension, it looked like I had closer to two inches of suspension travel. This is with a stock length strut insert. I'll have to actually measure this before sticking by it, but it's what I recall.

If you do have that little travel, the poor mans solution would be to measure how much your spring can compress before it binds (sum of the gaps in the coils). If that distance, at ride height, is significantly more than the length of the bump stop, shave the bumpstop down! Of course, if the difference is close, you'd have to take into account the compression rate of the bump stop. Though, if we're talking a spare inch of bumpstop, I'd just eyeball it and cut away!

^^Yup, you could also just slide your bump stop down and see if it ends up at the top of the shaft when you're done. Billy mentioned in my other thread that he performed the zip tie trick and came to the same conclusion, so he cut half of his bump stop off to help aleviate the problem.

I can no longer make any more measurements/tests on the stock suspension as I already replaced it. Maybe someone else with a stock suspension will volunteer to validate/disprove the data?

Ion_four, Racefiend is right about the shock travel issue. When I performed the exact test that you described, I was very depressed with the results. My trac is lowered only 1" and with the bump stop cut in half, just setting the car on the ground and bouncing on it to settle the suspension causes the zip tie to touch up againt the bump stop. Im planning on using my car for lapping this year and will have to redo the shock setup. Im thinking revalved bilsteins and eibach springs cuz we have a ton of experience with those shocks from my last lapping car and love them. Im also assuming that its going to have to be a custom job cuz unless the shocks are shorter on the coilover kits your going to buy off the shelf, you will have the same prob.

Billy, good luck finding some Bilsteins that will be shorter than the front Toyota strut insert. I tried and got nowhere. The problem is they don't have any general info on strut sizes to look up. You have to give them a specific part number and they will give you the dimensions of that strut. The only way would be to get an existing strut and have their shop shorten the body and stroke. Big $$$. I also checked with other major strut suppliers that do have general dimension data and the smallest commercial strut replacement was right about the size of the stock Toyota one. The only thing I found was the Koni race insert, and that is what I went with. Had to cut and shorten the strut housing, but it worked out great. I'll be doing a writeup with pics as soon as I get my car finished and tested.

billy,

That is insane! I just find it hard to believe that Toyota would give the car such little suspension travel...I'll have to do some thinking as to whether or not there's an advantage to that...I doubt it!

Maybe it's just added to the long list of compromises of shoe-horning a turbo/AWD powertrain into a compact car?

Ion_four, its true, very very true. Racefiend has the right idea though and its the only way around the problem......shorter shocks. Yes you loose some droop travel, but who cares. We arent jumping these things (on purpose). Racefiend, I have had bilstein shorten the shocks on my civic (cuz they have the same problem, but not untill you slam it 3-3.5") and it was only about $110.00/shock. I dont consider that expensive at all. I had the piston shortend 3/4" and the body 3/4" so I ended up with 1.5" of extra travel. I was having them custom valved for my sping rates anyway so it was a good time to do it. It was about $300/shock by the time i was done but that was including buying the shock itself, and at the end of the day I had custom valved, shortend bilstiens, and the were amazing on the track. Hey is it true the mr2 shocks fit? Is it true they are shorter than ours? I would love to see what you did with yours. When can you show some pics and specs?

i wish i could find good literature that focuses on mc pherson type suspension setups for performance street/circuit cars.
its almost as if u dont have double wishbone u dont have a suspension worth using.
racefiend where did u learn so much about suspension setups?

u gotta remember 4wd/awd car inherently push(understeer), they are ways around this.
simple things like softer front springs and sway bars
harder rear etc
one thing ive heard and seen that works really well is more rear drive %
ie 50% front and rear will push but 60 rear 40 front will make the rear end pivot thus creating more steering as u have less overall rear grip.
i dont know if this is at all practical for the gt4 im rather new to the gt4 im just giving some ideas of what other cars use

ok i'm sure suspension gurus like racefiend will rip into me for posting without having a clue about what i'm saying but from what i gather (and i admit its not much) a harder rear anti-roll bar will not take away the understeer problem in the way we would like. it would simply cancel it with oversteer. so in a sense you get rid of the understeer but is this the ideal solution? this is what i wish i had answers to. concrete answers

softer front springs would also mean that you would need a larger diameter front antiroll bar which seems to contradict trend in higher performance race cars which seem to prefer more spring rate and less anti-roll bar.

as for changing the torque split ratio front to rear im not sure that its possible to change it. i always assumed it was fixed at 50/50 permanently. but if this is not so i would also expect less push under effect from a 60% rear torque split.
but then again i dunno what the hell i'm talking about right? lol

I think the biggest reason why the car understeers is the weight ratio.

Just out of interest, do those of you with an ST165 experience understeer issues?

any posts on these forums outlining the front/rear weight distribution of the alltracs? 165 185 and 205?

The most typical weight distribution is 60/40. Some have managed to adjust that some. I think mine had a little less weight bias to the front when I corner weighted it.

CMS-GT4":1updapws said:

The most typical weight distribution is 60/40. Some have managed to adjust that some. I think mine had a little less weight bias to the front when I corner weighted it.

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Good thread life breather.

hm, wheer did racefied gone to, did he ever post his custom setup....