[MicaCeli]

This thread to to chronicle my sts2 setup for the Mk1. Since I can't find a ton of info for street tire setup for the Mk1 Mr2 (Ton's on info on CRX's and Miata's) I'm asking help from the more experienced persons on this forum.

Starting point:

Car: 88 Toyota Mr2 NA Hardtop with all power options

Suspension:
Koni Yellow's
Ground Control coilovers with 250Lb's Front 400lbs rear Springs
SusTech Sway bars front(second hole) and rear(full soft).

Tires:
195/50/15 Bridgestone Re01r's on 15x7 +35 offset wheels

Alignment:

Front:
Caster 5
Camber -2.7
Toe right between 1/8 and 1/16 out

Rear:
camber -2.5
Toe 1/16 in


Information up to date:

We?ll take this a step at a time:

Ok, so its a pure competition car. No/little compromise. So the starting point is a target "total roll". As a general rule a strut car doesn't react well to body roll due to camber issues and lateral roll center movement. A target of 1.5~1.75 degrees total roll is typical. Total roll is a function of grip, wheel rate, moment arm length and roll stiffness. Keeping the car flat also minimized the severe bump steer issue the Mk1 chassis has.

We'll start with grip. Current generation STS tires are going to generate something in the 1.1g of grip, vs 1.3+ of say a Hoosier A6. So the STS car will require slightly less roll stiffness than a CSP car to achieve the same total roll. We need some roll to give the driver feedback and prevent the outside tires from being overloaded on turn-in and transition. Shocks will play a key roll here.

Next is ride height because that determines the length of the moment arm (the distance between the roll axis and CG). The CG acts through the moment arm (like a lever) to roll the car about the roll axis. The roll axis is the line drawn through the front and rear roll centers. The CG height is basically fixed in relation to the body but the roll axis is a function of the control arm and strut angle and therefore ride height. The roll axis height drops at a significantly higher rate than ride height. So we have to be careful about how low the car goes.

Before we can choose spring/bar rates we need to set the ride height to determine roll axis height and therefore moment arm length. Ideally, we want the roll axis to be 1 to 2 inches above ground with the rear at about 1" and the front at about 2". Since we won't be modeling the car in software, our target is to simply have the roll centers above ground level and that the front be above the rear so the roll axis is reclined toward the rear of the car.

So our starting point is to deal with the things that we can?t really change easily. Spring/bar rates are free so we can choose rates that work with the rest of the setup. Ride height is basically free so we can raise/lower the car to our advantage. What we can?t change as per the STS rules is the control arm geometry. So we need to set the control arm angle to optimize roll axis location and camber curve. In reality, its not control arm angle but instead the angle of the line between the inner control arm pivot bolt and the center of the outer ball joint. You can clearly see the front ball joint pivot is well above the centerline of the control arm. Same for the rear but somewhat less offset.

To achieve our previously stated geometry, we want the front virtual control arm (the line between the inner pivot and the ball joint center) to be roughly parallel to the ground. In the rear, the ball joint should be slightly above (maybe ??) the inner pivot. Unfortunately, this is likely to set the nose of the car noticeably higher than the rear. This may be adjusted later when we know the severity of the body?s rake. Actual ride height and roll center location will be effected by tire diameter. This virtual control arm angle is about the best compromise location to optimize the camber curve assuming less than 2 degrees of body roll. Its doesn?t address the bump steer problem but that will be addressed later.

-Steve

Very good info..... Did you want to start a new thread on how to set up a MR2 to handle? I know you have the knowledge and I would love to trial and report back on car setup (like the ES MK2 guys)

So what you are saying is that the front arm should be parallel to the ground and the inner pivot on the rear should be LOWER then the outer ball joint by about 1/4"? (Did I read this right, because most of the info on this site states that the arms should never be pointed 'UP' towards the wheel. Just the info I keep getting here.)

This could be done I think and I have enough adjustment to do it.

The normal assumption is; you don't want the virtual control arm past horizontal. However, (and you have me getting ahead of myself a little here) what most people are worried about when they say this is camber gain. They assume, when the control arms are past horizontal, the camber gain starts going negative. This isn't exactly accurate.

The camber curve starts going negative when the angle between the virtual control arm and the strut becomes greater than 90 degrees. Because the strut is not vertical (it leans toward the centerline of the car) when the virtual control arm is parallel to the ground, the angle between the virtual control arm and the strut is less than 90 degrees. So you have a small amount of travel to use before the camber curve goes positive. This is also another reason we want total roll less than 2 degrees. Less deflection minimizes the negative geometry change.

Later, we will discuss the angle of the strut and at that time, will likely raise the rear end to bring the virtual control arm back to level. The rear roll center location may be adjusted my increasing the strut angle from the vertical (this also lowers the roll center).

-Steve

 

[MicaCeli]

So Steve we are going to do this step by step? With you outlining and me doing?

Are we testing after each step?

-Slava

 

[XHead]

So Steve we are going to do this step by step? With you outlining and me doing?

Are we testing after each step?

No.

I am taking this step by step because of available time.

As of now, you don't have enough of the elements to do any usefull testing.

I will post additional steps as time permits. They may come daily, or weekly, depends on my schedule. Right now, my car needs attention after testing sunday. However, I may have another installment today.

By the way, questions are fine. I don't know your level of knowledge and I also know I tend to buck conventional wisdom. But then I have been doing that my entire career.

-Steve

 

[MicaCeli]

Think of me as a newbie with suspension tuning, I think it will also help others tune their Mk1's.

Oh and buck away Conventional wisdom says get a miata or CRX and run it in sts2.....I'm bucking

 

[XHead]

Installment #2:

Now that we have control arm angle (roll axis) set, we will address wheel rates. Realize that spring rates and wheel rates are related but not equal. The wheel rate is the result of the wheel?s mechanical advantage over the spring. This is expressed as a linkage ratio. As an example, the linkage ratio of a typical strut suspension is about 1.1:1. That is, the wheel moves 1.1? for every 1? of strut movement. Now this changes as the suspension moves through its range of travel and typically the mechanical advantage is at its maximum at full droop and goes down as the suspension compresses. While this is a generalization, it is basically the nature of the Mk1?s suspension. Its also important to remember that front and rear suspensions seldom have the same linkage ratio.

Its also important that we understand that a wheel rate includes the action of a swaybar if fitted. Swaybar rates are figured much the same. The bar?s wheel rate is the bar?s spring rate multiplied by the linkage ratio. Most Mk1 swaybars connect to the strut, so the linkage ratio is the same as that of the spring. And finally, the wheel rate is the wheel rate of the spring plus the wheel rate of the bar.

Now we return to the first installment and consider total roll rate. We targeted ~1.5 degrees of total roll. Now we need to figure a total roll rate that yields that amount of roll with the given roll axis height. Total roll rate is the total of the front and rear wheel rates. Front wheel rate + rear wheel rate = total roll rate. So how do we determine a total roll rate? I have developed my own basic starting point that is simple and effective. I have proven this method to work well for me with many different cars in both autocross and road racing. As noted previously, a favorite assumption of many people is to choose spring rates that equal corner weights. The problem with this method is it ignores actual vehicle dynamics. Most such setups would include a front (and sometimes a rear) swaybar. The front swaybar, if sized accordingly could provide enough additional front roll stiffness to give adequate handling balance. For our purposes here, we don?t care if its swaybar, spring rate or a combination of both, we are only concerned with the resulting wheel rate. Later we will decide on the split between spring and bar rates.

Back to determining total roll rates. I noted I had a simple formula for a starting point. That formula is: ? the total vehicle weight divided by the inverse of the weight distribution. An example for a 2200 lbs car with 44/56 front to rear weight distribution (a rough estimate of your car's weight and distribution):

2200 / 2 = 1100 lbs

1100 * .56 = 616 lbs/in front wheel rate

1100 * .44 = 484 lbs/in rear wheel rate

The reason for inversing the weight ratio is to offset the car?s rear weight bias. If each end of the vehicle had the exact same level of grip, the rear would break away first because the higher weight would overcome the available traction earlier. Therefore a rear weight bias car will naturally oversteer therefore more front roll stiffness is necessary to counteract that natural tendency. How much front roll bias varies but all things being equal (and they never are) basing the offset in roll ratio on the inverse of the weight bias gets you pretty darned close.

Now you are probably stunned at the high wheel rates, but we aren?t through yet. First, remember the second paragraph regarding grip? STS tires don?t generate the same level of grip that A6 Hoosier do. As a result, lower spring rates are necessary to produce the desired amount of body roll. The difference in G loading is about 20~25%, so lets reduce our wheel rates by that amount

616 front * .8 = 492.8 lbs/in

484 rear * .8 = 387.2 lbs/in

To round off the rates: 500 lbs/in front and 400 lbs/in rear.

Now lets apply these to the MR2 chassis and its dynamics. Experience and testing are valuable here and I can apply my experience and make a couple of assumptions. First, that the low roll centers resulting from the current ride height and the high CG result in a long moment arm and a lot of body roll for a given wheel rate. Second, that the front has a longer moment arm than the rear. So we need a little more front roll stiffness and a little more total. Therefore we will bump the rates up slightly, especially in the front. I would think that using the wheel rates as spring rates would be enough of a bump and then add say 50 lbs/in to the front. The resulting spring rates:

550 lbs/in front

400 lbs/in rear.

Next we will address swaybars.

-Steve

 

[MicaCeli]

You know, although this will turn my suspension tuning knowledge on it's head, it makes a hole heck of allot of sense with the MR2 being what it is. I've been applying what I have learned from FF and FR cars to the MR2 and lost something in the translation.

This is fantastic, thank you

 

[XHead]

Think about it this way. A mid/rear engined car is a fwd car turned around backwards. On a fwd car, they run softer front springs and bars and REALLY stiff rear springs and bars to get something like a normal handling balance. AND, to reduce inside front wheelspin on corner exit.

So if a mid/rear engined car is a fwd car backwards, why not just turn the fwd setup round backwards and put it on the mid/rear engined car?

Or think about it this way. Control body roll with the end opposite the weight/drivewheels. This creates mechanical grip on the drive axle. Now it doesn't matter which end has the drivewheels/weight.

Funny how sometimes the most obvious things aren't that obvious because its so far outside our existing pool of knowledge.

What I have posted above is basically the process I sat down and put together back in 1994 when I realized I wasn't making any progress using the "conventional" tuning methods (softer springs, big swaybars and stiff shocks). I derived and refinded this method over the following couple of seasons. I even put together a spreadsheet that would let me look at the entire setup on one page. Then I could change a spring rate or swaybar rate and see how that change rippled through the entire setup.

It was an eye opening experience for me. I could try lots of different combinations on paper, pick one to test and note the change in feel and performance. Then compare that to the spread sheet. It didn't take long to correlate results with changes and then make changes and predict results. Accurately. A very powerful too.

I now have a much more sophisticated method in use on the DP car. But then Prepared rules give you so much freedom you need a more sophisticated process. But my simple Lotus 123 spreadsheet was a revelation at the time.

The car was transformed in the space of a single season. After floundering around in the middle of the trophies for my first half dozen trips to Nationals, this process put me on the path that lead directly to my first Championship in '98. Now I don't credit this for my first Championship but it certainly put me in a position to win. Car setup isn't a substitute for learning to win. But you can't win if the car isn't there.

-Steve

 

[MicaCeli]

Seriously it's so hard to find this kind of info anywhere. There are tid-bits here and there and you can pick up so much info that it can be overwhelming. Also it doesn't help that the people that have done well and have 'figured out the formula' will not share it. I understand that they have put in allot of work into figuring this out, like you, and want to keep this information to themselves and let others figure it out on their own.

I believe that knowledge should be shared (with references of course )

Again thank you very much for sharing this info with us.

 

[Warp3]

Subscribed...my car is being prepped for CSP instead, but a lot of this still applies.

 

[MicaCeli]

Although there is good info out there about car setup this is very good for MR2's.

That reminds me I should go out and take measure my sway bars. ST's (Unless someone can measure theirs for me )

Found some info on the net and calculators for it.

http://www.buildafastercar.com/tech/Sway-Bar-Rate-Calculator

Sticky?

 

[mtbmr2]

Although there is good info out there about car setup this is very good for MR2's.

That reminds me I should go out and take measure my sway bars. ST's (Unless someone can measure theirs for me )

Found some info on the net and calculators for it.

http://www.buildafastercar.com/tech/Sway-Bar-Rate-Calculator

Sticky?

FYI: http://www.mr2oc.com/showthread.php?t=174154
(MKI Sway bar specs, in the MKI Suspension forum)

 

[thirdvector]

Steve, we really can't thank you enough. I've read Puhn's and others books (probably need to read them again, lol) and it has never been as clear to me as it is now. Mostly I didn't understand how all that theory could bridge itself to the real world and autocross, where changing the geometry is prohibitive. Thank you for the epiphany. :toast

Now I wanna get out there and measure my stock front anti-roll to see how much spring rate it produces....

 

[MicaCeli]

FYI: http://www.mr2oc.com/showthread.php?t=174154
(MKI Sway bar specs, in the MKI Suspension forum)

Yeah I looked at that Doesn't give me anything but diameter though.

 

[MicaCeli]

I have two auto-x's this weekend and will measure my sways tonight and see if I can close to the calculated wheel rates. I know mine are very off as it is right now.

I will probably do a full redesign when XHead finishes this thread.

 

[ButtDyno]

Although there is good info out there about car setup this is very good for MR2's.

That reminds me I should go out and take measure my sway bars. ST's (Unless someone can measure theirs for me )

Found some info on the net and calculators for it.

http://www.buildafastercar.com/tech/Sway-Bar-Rate-Calculator

Sticky?

FYI buildafastercar is from the same guy who did some of those Excel spreadsheets on IWSTI.com.

 

[XHead]

Yes, buildafastercar.com has a good tool for computing swaybar rates. I have a spread sheet that does that too.

Mica, I didn't get a chance to see your post before now. If you used the wheel rates I gave you and did it with a front swaybar, the car will understeer.

I hope to get the next installment up in a day or two.

-Steve

 

[MicaCeli]

Yes, buildafastercar.com has a good tool for computing swaybar rates. I have a spread sheet that does that too.

Mica, I didn't get a chance to see your post before now. If you used the wheel rates I gave you and did it with a front swaybar, the car will understeer.

I hope to get the next installment up in a day or two.

-Steve

Damn youre good. I didnt even post up what I did yet.

It had a little bit of understeer but I though I would try to start and take some oversteer out of the car. It felt good but deff a little too much understeer for my liking. This was just some prelim stuff that I wanted to do for a little while.

I await the next post.

 

[XHead]

Installment #3:

Now that we have a starting point for individual wheel rates we can now decided how we want to divide the wheel rate between spring rate and swaybar rate. To make an informed decision we must understand the function of the swaybar and how it interacts with the springs. A swaybar is nothing more than a torsion bar (spring) that has either end attached to each wheel of a single axle. It?s the twisting action of the bar that is the torsion spring. Because it only applies when a one wheel of the axle moves independently of the other, it has only moderate effect on ride quality.

To understand the swaybar?s effect on the springs and chassis we must understand how the springs interact with the chassis as well. We already understand how they affect body roll but they also effect ride quality and that is what we will now address. There are three basic principles that apply. First is ride frequency. Ride frequency is the rate at which the chassis reacts to input, a bump in the road. Ride frequency is expressed as Hz. A soft ride frequency would be about 1 Hz or one cycle per second. A cycle being the car passing over a bump, the chassis reacting and then returning to its original state. A stiff ride would be a frequency of about 2 Hz and a very stiff ride would be 2.5 to 3 Hz. We could easily calculate ride frequency based on the sprung weight of the car (total weight carried by each axle ? unsprung weight of each axle = sprung weight) and the wheel rate of the springs on each axle. However, for our exercise here its only necessary that we understand the concept. The final part of the concept is that each axle has its own ride frequency based on the sprung weight and wheel rate of each axle.

Ride frequency is used to determine the second principle favored speed. Favored speed is the road speed at which the two ends of the car return to their original state after the car passes over a bump. To achieve a positive favored speed (a speed greater than zero, and yes you can have negative favored speeds) the front ride frequency must be lower than the rear so that the front returns at the same time the rear does for a given speed. Achieving this effect will produce the best ride quality for that speed and desired stiffness. The favored speed can be set at any target speed by tuning the front and rear spring rates to achieve the ride frequencies that produce the effect at the target speed. This effect works for both stiff and soft rides at most any speed and is why your stiffly sprung car feels smoother as speed increases. Most manufacturers set the favored speed using spring rate to achieve a smooth ride at their desired target speed (usually between 50 and 70 mph), then use swaybars to tune the ultimate handling balance and body roll.

The final principle we need to understand is center of suspension. Center of Suspension (CoS) is the point on the chassis at which, if weight was applied, both ends of the car would compress exactly the same amount. As an example, if the front and rear springs had exactly the same wheel rate then you could push down exactly half way between the two axles and the front and rear would compress the exact same amount. If the front springs were softer than the rear, the center of suspension point would be moved rearward to accommodate the softer front spring rate. The weight of the car then acts through the center of suspension via the Center of Gravity (CG). The difference in the car?s CG and Center of Suspension is a moment arm (which is basically a lever) just as the difference in CG and roll center. So the CG acts through the moment arm to compress the suspension in reaction to a bump. If the CG is behind the CoS, then the rear suspension is compressed more for a given load. The inverse is also true, if the CG is ahead of the CoS, the front suspension is compressed more. And like body roll, the longer the moment arm (the greater the distance between the CoS and the CG), the more the suspension on that end is compressed for a given load.

To minimize excessive body movement in response to bumps, we want the center of suspension as close to the CG as possible. This minimizes the length of the moment arm and therefore the CG's leverage over the CoS. For the Mk1, the CG is near the rear axle so we would have to have front springs that were much softer than the rear to achieve a center of suspension anywhere near the CG. Then to achieve good ride quality we set a desired favored speed and then tune the spring rates to suite. For the Mk1, setting a favored speed of say 50 mph, the front springs would have to be slightly softer, or the rear springs slightly stiffer, that that which would locate the center of suspension exactly at the CG.

This is the method the manufacturers use to tune the ride quality and handling balance of their cars. When we consider what we learned about roll ratios in the earlier installments its easy to see why the manufacturers use a hefty swaybar and soft springs on the front of the Mk1 (or most cars for that matter). Now that we understand how and why Toyota setup the stock Mk1 we can determine where we need to go to improve the car. Clearly we are not as concerned with ride quality because we are setting up a competition car. However, it is necessary that the car be able to track smoothly over bumps so as to not upset the car enough to loose traction.

-Steve

 

[Wangan_X]

i'm confused in the first two installments you said stiffen the front, in the last you say soften it...

 

[XHead]

Hmmm, maybe I didn't make it clear but in #3 I was outlining the process by which a manufacturer chooses spring and swaybar rates. Re-read the last paragraph of #3. In #3 I stated that a softer front spring was necessary to give a smooth ride but the manufacturer would then choose a very stiff front swaybar to provide enough front roll stiffness to tune the handling balance back to neutral/understeer. I haven't re-read the three installments to verify continuety but that was the goal.

The next installment will outline the process for choosing tuning for a pure competition setup and will review the process by which we choose actual spring and bar rates for the racecar.

I am sorry if I confused you. Because I am posting the installments as I complete them, its not possible to go back and edit earlier installments for content before posting. I am really mostly doing this off the top of my head as I don't have the time right now to sit down and draft the entire project with a more top level view.

I am writing in Word and pasting to the forum. So once I get it all done, I will be able to go back and re-organize it to make for an easier read.

-Steve