The real reason our cars understeer

[Racefiend]

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....

[lumbercis]

Buy some coilovers?

Seriously, the alltrac as it was sold commercially was a GT car at best, not a sports car. If you are "hard cornering" in it you are doing things toyota didn't design the suspension for. It isn't the engineering that is the problem, it is your expectations of what a stock suspension should be capable of.

J.

[Balagast]

+1 the difference when adding my Teins compared to a stock suspension (freshly replaced) was absolutely night and day.

[Racefiend]

Buy some coilovers?

Seriously, the alltrac as it was sold commercially was a GT car at best, not a sports car. If you are "hard cornering" in it you are doing things toyota didn't design the suspension for. It isn't the engineering that is the problem, it is your expectations of what a stock suspension should be capable of.

J.

My expectation is that a manufacturer would design a proper suspension for a car, not band aid a "find whatever parts from the parts bin" suspension by using bump stops to tune it for the car, especially for what is supposed to be the top model of the Celica line. 5/16" of free movement on the shock is ridiculous, especially if it's a GT car, or an econobox, that have softer suspensions than sports cars. You will induce that much roll just driving to Safeway to buy milk, not to mention hitting large bumps or during braking.

Buying coil overs is an easy fix, however most setups use generic shocks with some adjustability for you to tune it to your car, and you're stuck with the spring rates they chose. They are usually overly stiff for street use also. My goal is to design a suspension tuned specifically for the car, for cheaper than most coil-over solutions. Then throw up all the data so anyone can change the numbers to suite their preference for their car.

[lumbercis]

Discussion about Bilstein setup
http://alltrac.net/phpBB2/viewtopic.php ... highlight=

The easiest way to get what you want would be to get koni struts with ground control coilovers. Go to the GC website and you can pretty much order any combination you want.

J.

[brophats]

another big reason i've notice for the cars understeering issue is that the wheel stance is wider in the front than the rear, its 100% noticeable if you put spacers on the rear. i had to use 15mm spacers in the front to clear my wilwood calipers so at the same time i put 25mm in the rear just for looks and it gave the car a completely new dynamic.

demetrius

[smog7]

wider rear track increases understeer. wider front track reduces understeer
see here:
http://www.alltrac.net/phpBB2/viewtopic ... 93&start=0

[Mafix]

might i add the obvious.

your car is 18 years old.

[Racefiend]

Discussion about Bilstein setup
http://alltrac.net/phpBB2/viewtopic.php ... highlight=

The easiest way to get what you want would be to get koni struts with ground control coilovers. Go to the GC website and you can pretty much order any combination you want.

J.

Yes, that's an easy solution, however it does not alleviate all of the problems.

1. You still have a stock sized strut, with much less compression travel than rebound travel. The problem gets worse as you lower the car.

2. You have a strut that is not valved for the specific wheel rates you choose. Koni's are only high speed rebound adjustable.

My solution will be to find a Bilstein monotube that fits the bill for a reduced stroke strut insert, and reduce the OAL of the stock strut tube to suite. I don't really want to reduce the stroke, but to increase the available compression travel there is not much choice. Then send the inserts to Bilstein to have them valved. Adjustability is not a requirement as most people do not need to adjust a properly valved shock.

[klue]

seems you've got it figured out, so get to it!

[lumbercis]

Check out the link to the supraforums thread that is in the Bilstein thread above. He did exactly what you are trying to do.

If you want to talk deep suspension, talk to gtfour77. He seems to have done the most suspension development with his autocross car.

Good luck,
j.

[brophats]

well my face is red, all i know is since i added the spacers the car oversteers substantialy more. but i did widen the front at the same time.

[CodingParadox]

Yeah, I really need to get some more advanced suspension for my alltrac. The SS really doesn't cut it for serious motorsport.

[TomsGT4]

I'd like to see how my set up would fair in your tests. Koni adjustables with stock stock springs up front and a 20mm bar in back. Made a world of difference going into the corners.

Also, would a good quality upper spherical mount effect reduction in understeer?
Considering it should allow you to power-out more as you come through the corners, giving more stability.

[CMS-GT4]

I think going with a custom setup would be the best approach if you want the best possible handling. My HAs do well for what they are, but I am sure better could be done.

Since you want a larger diameter strut, you might concider finding another toyota strut that uses the same mounting points that is larger. I am not sure if there is one, but this might be your best approach. Then you could choose a custom strut insert and get ground control perch and springs.

I have wondered if tein would do custom work to the dampners on their suspension. I know they offer some custom settings and rebuild them here in the states. It would not hurt to ask.

If I had to choose an off the shelf option again, I might concider the DMS setup. I still like the HAs. The price was fair and the car is very solid with almost no roll at all, however I am sure a better dampner could be had.

Also, MR2 inserts will fit and have a shorter rod. This might lead you to something helpful as well.

BTW: I think there are many factors to why these cars understeer. You have just began on this jouney.
The heavy nose is one big factor. Likely (in the USA anyway) not having a rear lsd. Not enough tire for the weight of the car. Not enough front caster. Not enough camber on stock suspension.
I thik if you did all the weight reduction you had planned on doing then got some custom struts with some nce camber/caster plates along with some other suspension changes, you might be happy with the car.

I am not sure ifit is 100% possible to remove understeer from these cars but you can at least raise the limits a bit and get enough nuetral mid corner and corner exit handling to be happy.