fatbillybob

Anyone want to chip in on the ramifications of T1 suspension math please feel free. Another thread got me thinking about this stuff. I'm pretty ameteur so you big brain guys don't save it all to yourselves. I'm trying to learn to drive my T1 car like the rest of the T1 guys. One weird quirk is that everyone uses low tire pressure reducing the spring rate of the tires to a suboptimal place according to hoosier engineers. Why do our cars work well with low tire spring rates?

I feel that the rear of my car is too stiff. I don't really oversteer but I do get sudden breakaway. Some say that this can be controled with double adjustable shocks. I got stock Sachs T1's.


Suspension leverage or motion ratio:

Rear lower knuckle to chassis pickup point measured 18"
rear spring contact pad to chassis pickup point measured 11"
front lower knuckle to chassis pickup point measured 16"
front spring contact pad to chassis pickup point measured 10"

Rear motion ratio= 18/11 =1.64

front motion ratio=16/10= 1.60

I saw specs for T1 front spring =650lbs rear spring=800lbs We need to confirm this.

front wheel rate=650/(1.60 squared)=253.9 lbs/in
rear wheel rate=800/(1.64 squared)= 297.4 lbs/in

my corner weights:
LF 882LBS RF 843
LR 757 RR 713

Wheel frequency= 187.8 * square root of (wheelrate/sprung weight)

Wheel frequency=
LF= 187.8*Sq.root of (253.9/882)=187.8*0.5367=100.8CPM=1.68HZ
RF=103.1CPM=1.72HZ
LR=117.6CPM=1.96HZ
RR=121.1CPM=2.0HZ

target wheel freq for non-aero racecar 100-125 or 1.7-2 hz with the front 10% less than the rear to avoid pitch oscillation. I am about 15%

I think this implys that I could soften the rear or up the front wheel rate discounting the effect of the T1 roll bars? That could explain my feeling that the rear is too stiff and maybe why people say the rear of the car needs more rebound in the shocks?

So how does the wheel frequency change with the wheel rate added by the T1 sway bars? Is the T1 bar really hollow? If so anyone know the inner diameter? We can then calculate what the T1 sways add to the equation. Did the T1 sways huge front bar increase the wheel rate back up to get that 10% front to rear ratio to control the pitch?

Well maybe this will be food for thought for somebody. That is about the end of my math ability.

mAydAy121

Awesome job and a great start!

The front-rear frequency split at 15% is fine. Those frequencies are well within the acceptable range, if anything possibly on the low side according to some. Higher ride frequencies will result in a gain of control in transience (more responsive...think Buick vs Corvette) but a loss of overall mechanical grip. You're better off staying softer and adding sway bar for peak steady state grip.

Do not confuse pitch sensitivity and roll stiffness! Swaybars do not play into the natural frequency of the chassis because they only have an effect in roll, where ride frequency is entirely in regards to pitch.

Your roll rate or roll stiffness is what you have to look at to analyze a swaybar's contribution to the handling of the car.

A great place for some more reading on this topic would be the Tech Tips section of OptimumG's website. http://optimumg.com/OptimumGWebSite/.../TechTips.html Claude Rouelle, the president, is one of the world's best vehicle dynamicists. They've got some great write ups regarding springs and shocks.

If you're really interested in this stuff I could recommend some great books too (although most of them are very, very intense).


I would analyze your roll stiffnesses, and if you really, really want to get into some intense stuff, look into some yaw moment analysis. The problem is, the deeper you dig, the greater the necessity of tire data becomes (which often isn't available in lower divisions!), especially running different front and rear widths.

mgarfias

The other thread got me thinking too. It occurred to me that you guys are running light pressures to drop the wheel rates because the C5 has gotten lighter since the T1 package was put together. What was the target weight 10 years ago, and what is it now?

Are the heavier C6 guys dropping the air pressures too, or running higher?

SIK02SS

the first two posts has me like:

but this post I get!

To my knowledge, the front running C6 guys have low pressures too, same with the Vipers

mgarfias

Interesting. Maybe you could try swapping in your FE4 springs and doing some back to back testing?

John Shiels

I drive and SOTP it and time it.

gkmccready

You've got the motion ratio equations backwards, and the lengths seem off a bit.

For the shock (as in with coilovers), the front is approximately 0.69 and the rear is around 0.71 at droop and 0.53 at bump -- call it 0.60 average. If you take the spring off and measure shock travel versus wheel travel you'll be more accurate for this case.

When trackboss and I gathered the leaf data it appeared closer to 0.55 front, and 0.4 rear... front BJ length is 400mm, spring pad is 295mm (0.55 motion ratio), shock mount is 330mm (0.69 motion ratio). Rear BJ length is 440mm, spring pad is 280mm (0.40 motion ratio), shock mount is 345mm (0.55 motion ratio).

For coilovers you also need to look at the angle of the spring -- in the front that's 61deg, and in the rear it varies a few degrees through travel, but call it 53deg. I make the assumption that the leaf always works at an effective 90deg angle.

So... double check your motion ratios since they're key to putting all this together...

gkmccready

Swaybars shouldn't control pitch, they should rotate freely in that axis. Swaybars control roll. If you hit a dip straight on with both wheels at the same time the swaybar should add no rate...

John Shiels

that is near a miracle.

mAydAy121

What tires are you running in T1? It is possible that the tires may just run better at lower pressures. If tire data is not available, its hard to say. A tire company recommending a pressure doesn't mean much if they haven't done some quality tire testing such as at Calspan to analyze the behavior of the tire....

Checking the math is a good move, but yours appears to be right. The frequencies seem about in line so I didn't give much thought to it. How you define motion ratio is a matter of convention, some define a motion ratio as spring travel divided by wheel travel, some define it as wheel travel divided by spring travel. You just have to make sure you choose your convention and carry through with it.

If you define your motion ratio as wheel travel divided by spring travel you have to divide by the motion ratio squared as you did here. If you define it as spring travel over wheel travel you have to divide by the square of the inverse of the motion ratio.

davidfarmer

I think everyone has found that Hoosiers STICK better with dangerously low tires pressures. They may destroy themselves, but they are fastest when they are nearly rolling themselves to a pulp!

gkmccready

I have 582# front, 793# rear in my notes for C5 T1 springs. And C6 T1 I have 582# and 850#. It would be good to confirm them again, though. Also, with my motion ratios the wheel rates work out to 197# and 134# for 650#/800#. Unless I've been quoting entirely screwed up wheel rates for the last two years...

Solofast

Why are you squaring the motion ratio to arrive at the wheel rate? I would have thought that should be a simple ratio and not a squared function.

davidfarmer

motion ratios and static ratios are not the same. Motion ratios are based on the frequency formula...... frequency=square root of (stiffness/mass).

So if you change to coil overs from leafs for instance, the static loading and dynamic loading will be completely difference since the pickup points are different on the control arm.

gkmccready

Because all the books say:

WheelRate = SpringRate * MotionRatio^2 * sin(SpringAngle)

or cos(SpringAngle) if you measure from below like Eibach:

http://performance-suspension.eibach...sion_worksheet

jtmck

[QUOTE=gkmccready;1576864519]Because all the books say:

WheelRate = SpringRate * MotionRatio^2 * sin(SpringAngle)

or cos(SpringAngle) if you measure from below like Eibach:

If you mathamatically measure the MR by, length of arm and length to coil over mount, the angle of the coil, I think the above is correct.

But if you measure the real upward movement of the coil related to the movement of the tire as a MR, you would only square the MR times the spring and discount the spring angle.

I have 700# front coil overs, with .70 MR using the movement of the spring measurment.
700 X (.70)squared equals 343 for a wheel rate.
Am I doing it wrong??

Timz06

Here is some simple T1 (ST2) math-

Aero+ New A6 + Good Driver= Fast Time

gkmccready

I think you still need to account for the spring angle. In theory measuring the MR based on the ball joint and shock mount should be the same as measuring the wheel movement versus shock movement... when you measure the wheel-v-shock you will find that through the range of motion you're measurements will vary and you should take the average of all those measurements. The same is true of the shock angle, ghoffman measured a significant change in shock angle in the rear through the full travel.

Again, I'm just some guy that reads a lot and tries to measure and understand. Find a real suspension guy or a good book to double check. :-)

FWIW, I measured a 68deg shock angle at droop in the front, but at full bump the angle is 61deg. In the rear it's 62deg at droop, and 49deg at bump. By my math, with a 0.69 MR, your 700# front spring is operating with a wheel rate of 309# at droop, and 291# at bump.

Just to bring this back to the OPs question about T1 suspension which uses leaf springs... add this variable... the spring pad slides on the lower control arm... as the arm arcs upward the spring pad effectively slides further outboard... that effectively gives a leaf spring more leverage as you move in to bump -- the exact opposite of what a coil spring sees on these cars as the spring angle changes.

fatbillybob

I just looked the formula up in a book. I'm not smart enough to dream this stuff up myself. I considered the transverseleaf to be 90 degrees to the travel so in my pee brain that makes the spring force on the Y axis. I would also think that this would be a calculus problem with a coil spring at angle as the MR would change as the spring angle changes i.e. more variables to take into account. I do think that eventhough the spring pads changes length the error or +/- relative to my numbers are probably reasonably close for the discussion. A lower 582lbs/in front T1 spring rate which I have also seen reported does us no good and just makes the front to rear wheel frequency difference greater than the 15% which is already there. This lower front spring rate just makes what I "feel" (the rear too stiff) worse. Obviously the whole westcoast T1 crew is perfectly happy and fast on the T1 suspension. I however, lack the skill to drive it or lack other parts of the set-up equation. For example I lack the double adjustable shocks. How many seconds is that worth on the typical 2.5 mile track?

gkmccready

Did you account for sprung vs unsprung weight? The springs only control the sprung mass at each corner, so you need to remove the unsprung weight when calculating your spring frequencies, right? And the rear wheel+tire package is much larger/heavier than the front (typically/stock).