Spring Rate VS Suspension Travel

[Guest]

My dilemma, I am not quit sure how to properly determine maximum amount of roll to maintain efficient suspension geometry VS over springing a car resulting in lateral skipping across an autocross course surface irregularities and bumps.

What is the most efficient way to measure camber changes during changes in suspension compression and rebound? With changes in wheel direction? Flipping my upper control arms is an option to increase my caster and camber is adjustable at the upper control arm ball joint..

Strictly autocross, I presently run 450lbs springs in front and 500lbs in the rear on a 2400lbs front wheel drive vehicle approaching the dampening limit of stock Koni’s. Wheel rates, 225 in front and 275 in back.

I get occasional bottoming on the front bump stops on R tires, in high load corners with surface irregularities. I devised a method of dropping the lower mounting position of my Koni damper bodies, which provides approximately 2.5 cm of additional compression travel on top.

Should I consider sending the dampers for shortening to reduce bump stop contact? Or re valveing along with increasing my over all spring rates maintaining the roll balance and eliminating bump stop contact? I do not covet an over sprung vehicle with reduced lateral adhesion. What are some of the preeminent indicators a vehicle is over sprung?

A stiffer front anti sway bar will upset the balance, reduce overall lateral grip and induce additional under steer, not a consideration.

[ December 29, 2001: Message edited by: DB2-R81 ]</p>

[Vancouver Special]

That's a lot of questions.

First thing first. What's your corner weight (with driver)?

I am surprised you actually know your wheel rate. The front seems way too soft. How did you obtain your wheel rate (just want to make sure they are correct)?

Joe

[DB2-R81]

Hello Joe,

Corner weights are 877LF, 806RF, 475LR, 436RR I have removed around 50lbs since last weigh in and went up 1 turn on the left rear, 8 turns per inch.

Motion Ratio
Take the spring off the damper and put the damper back on. With the wheel/tire removed, move the wheel hub from full droop to full compression in 1" increments. Measure and record corresponding strut displacements. With the data, correct the wheel hub displacement numbers by the wheel off-set to get the theorectical wheel displacement.

Wheel VS Spring Rate

Integra G2 motion ratios:
Front: 1.5:1 (for one inch of shaft travel you get 1.5 inches of wheel travel.
Rear: 1.35:1

Wheel rate = (spring rate / (motion ratio squared))

Front wheel rate = spring rate / 2.25

So 400 lb front spring gives a wheel rate of 180,
and a 600 lb spring gives a wheel rate of 270.

Rear wheel rate = spring rate / 1.82

[ December 29, 2001: Message edited by: DB2-R81 ]</p>

[Vancouver Special]

Great, you have got the basics figured out.

With your current set up, your wheel rate is only 0.25x corner weight for the front. For the rear it's 0.58x corner weight. No wonder you are bottoming out in the front. Most competition cars' suspension frequencies are around mid to high 2 cycles per second. That equals to wheel rate around 0.6 - 0.8X the corner weight. Currently, there is a lot of discussion among the Formula Ford guys as well as the D/E Mod guys from SCCA about going to 1-1.5X corner weight. I think for autox, 1X is about the upper limit and 1.5X is overkill. For a street driven car, I would say 0.5x the corner weight is about the maximum one would want to go. This would mean 430# front wheel rate. With the motion ratio you provided, spring rate up to 1000# could be used. To prevent understeer, you might need to reduce the front sway bar size somewhat. This would reduce the front roll rate which would balance out some of the increase from the stiffer springs. The net result is that at corner entry, under brake (when sway bar has no effect), the stiffer springs would reduce chassis movement, hence less chance of bottoming, also less suspension geometry changes. You will need better quality 2-way adjustable shocks to go with the set up. They will provide an increase in rebounce damping (needed for the stiffer springs) without increasing the bump rate (which might be the current cause for your car skipping over rough pavement). Don't forget, overly stiff rebounce will also feel like overly stiff spring.

As far as shorter shocks are concerned, I think all cars with reduced ride height should use them. With shorter shocks, proper length bump rubbers could be used and that would provide a gradual increasing in spring rate rather than a sharp increase. You can alway use hard plastic shims to shim the bump rubber down but you can't do it the other way around with a longer shock.

Joe

[DB2-R81]

Hello Joe,
Sounds good, Thanks for your help so for, I am reviewing the discussion on Yahoo SCCA-DMOD-EMOD discussion group.

Just one more question, front to rear wheel rates rates? I was under the assumption with a 65/35 f/r weight-distribution, you want even body-roll which means wheel-rates that are 65/35 as well. Actually even more because the roll-axis inclination points down towards the front so you want even higher wheel-rates in front, perhaps even 70/30 for the same amount of body-roll front & rear.

But these suggestions were passed on to me from an Integra Type R owner with similar chassis characteristics.

On a car very comparable to yours:

400F gives wheel rate of 180 & 1.53 cps
600F gives wheel rate of 270 & 1.88 cps
800F gives wheel rate of 355 & 2.15 cps
1000F gives wheel rate of 444 & 2.41 cps
1200F gives wheel rate of 533 & 2.64 cps
1400F gives wheel rate of 622 & 2.85 cps
1600F gives wheel rate of 711 & 3.04 cps

At a front corner weight of about 750, it should be obvious that a front wheel rate anywhere close to front corner weight is ridiculous. Such ideas make more sense in the world of Formula Fords and Sports Racers. If you run 1000 plus front spring rates, don't worry about reducing your front sway bar size - it won't be doing much work anyway, and you will need at least 1200R and a large rear bar to keep from plowing - and your new problems will be skipping and darting. Don't listen to anything that comes out of the real race car world - it's not relevant to our stupid cars. Unless somebody is talking from experience with cars very similar to ours (and Rabbits and Golfs are not even similar enough), you must treat their information as potentially hazardous.

250R gives wheel rate of 140 & 1.7 cps
600R gives wheel rate of 330 & 2.6 cps
800R gives wheel rate of 440 & 3.0 cps
1200R gives wheel rate of 660 & 3.7 cps

As has been mentioned in the past, 600F / 900R is an excellent setup for mortal human road course work. Combo's as high as 1000F / 1400R (with big rear bar) can be used by talented and gifted drivers.

The short answer to your original question is that you must discover what you need. As to shock body length, you can figure out what you want by starting with desired static ride height, examine a-arm and tire clearance (with an allowance for bushing deflection - which will give you more travel than you'll get out of your shaft alone) and figure out how much bump travel you have to work with, then using reasonable numbers for max longitudinal and lateral weight transfer you can select reasonable starting points for rates (that will give you your static deflection and damper free length.

For road racing use on DOT rubber a G3 Integra must have a front spring rate of 450 or higher. It must be higher if you run on banking, or if you have large surface irregularities in corners. You might find success with most of your existing setup by increasing the rear spring rate and bar, and then play with camber to rebalance grip - that might keep you off the front bump stops - unless they are in the trailing brake / turn-in zone.

I've tried to explain so many times that the heavy rear high rate bias brings rear camber change under better control and results in improved grip that offsets the tendency to excess oversteer that concerns so many people. Such concerns also overlook the import of specific driving techniques - after turn in the throttle only goes down and the steering wheel only unwinds.

[ December 30, 2001: Message edited by: DB2-R81 ]</p>

[Vancouver Special]

There are two issues here - spring rate and roll rate. Wheel rate is affected by both if it is a one wheel bump or the vehicle is in roll. However, under brake as in corner entry, stiff enough springs are required to prevent the chassis's downward movement in using up most of the available suspension travel. At moderate wheel rate (say 0.5X wheel rate as per my original suggestion as an upper limit), there is no way a car like an Integra would have enough roll stiffness from the front springs alone to render the front sway bar useless. There will be enough suspension movement to generate twist in the bar (just think about a total weight transfer under lateral and diagonal weight transfer which equals or exceeds 0.5X corner weight, which would produce over 1" wheel upward movement if there is no sway bar). How high you go in front spring rate is up to you. My original assessment is that somewhere less than 1000# is the "upper" limit. The other person's comment that 600/900 is the correct set up doesn't mean much because sway bar rates would skew that spring combination since total wheel rate (in roll) is a combination of spring and sway bar rates. You could easily be running 800/700 in spring with a much smaller front bar and a larger rear bar to get similar handling. 0.5X corner weight is no where close to "harsh" in terms of suspension stiffness and the car should not "bounce off the road" unless the suspension is running out of travel, which is a different problem all together.

Regarding your question why shouldn't the F/R wheel rates be the same as the F/R weight bias inorder to maintain the same roll rate. The answer is that the roll moment is a function of roll centre height and they are usually different front and rear because of suspenion geometry. Also, car designers also have to consider the car's balance when it's fully loaded with passengers and cargo. I suspect you will find that once your car is fully loaded, it will be far from 65/35 percent. With softer rear springs, the muffler might be dragging on the ground.

Joe

[brianlofthouse]

Phantom, as you mentioned here, since the swaybars are a factor in the total roll rates, does your "guideline" wheel rate of 0.5 x corner weight (for street driven autox cars) include swaybars? I think you've also suggested that swaybars should be ~25% of the spring/swaybar total, is that right?

I'm looking at ordering GC coilovers soon, and want to get the spring rates sorted out first.

[Vancouver Special]

Just to clarify two things:

1) My comment regarding 0.5X corner weight is a "guideline" only, since most people don't know their sprung and unsprung weight at the corner. I also stated that 0.5X in my view is the "upper" limit rather than a "target" wheel rate. To be more exact, suspension frequency should be used. Some cars (as Dennis Grant found out about his Talon), the un-sprung weight could be much higher than originally thought. Most cars have a typical 1:5 ratio between un-sprung/sprung weight. If your car falls within that range, a 0.5X wheel rate would give a low-2 CPS (cycle per second) ride frequency. At this frequency, the car is firm but certainly I wouldn't call it harsh. I know quite a few individuals on this list are running at that frequency on their street cars. The trick is to ensure that there is enough suspension travel rather than riding on the bump stop. To answer your question, this ride frequency (or wheel rate/corner weight ratio) does not take sway bar roll stiffness into consideration.

2) I don't think I "suggested" 25% of the total roll stiffness should come from the sway bar. Instead, I think I said "...there are designers out there who think sway bar stiffness should not exceed 25% of the total roll stiffness....". My personal view is that 25% as an "upper limit" is a good suggestion. However, there are factors that need to be considered. For example, for a car that runs with an open diff, I think it should run with as little sway bar as possible at the drive end. I would try to get as much roll stiffness from the springs (without going too stiff, like beyond mid-2 CPS) before going to add sway bar rate for roll.

One thing about the ground control system is that you are at the mercy of the shock you choose. If the shock body is too long, you will need to up the rate to prevent bottoming. Once it bottoms, even 1.5 cps spring rate would feel harsh.

Joe

[DBM]

this is a good thread. Thank you for all of the info Joe. I too am in the market for new springs/dampers. My MR2 has an open diff, medium sized front factory sway bar and tiny factory rear sway bar (11mm). This is good, i'll stick with the stock bars and save prep points/money.
The problem my car has is also bottoming of the front suspension. This is usually easilly cured by using the rear strut inserts in the front strut casings. The rear inserts are 25mm shorter than the fronts, so the casing can be shortenned for more travel. I have done this on two of my friends cars, but not mine. Go figure.
Anyways, Brian, have you come to any conclusion on what rates you'll be looking into? With our cars having pretty much identical strut suspension at all 4 corners (although rear axle carriers and drive shafts weigh more), and our FR weight distrobution, one would think that the rear spring rates should only be a small margin higher than the fronts, but with the larger front sway bar, we have to run a fair bit stiffer in the rear. The factory rates are almost 2X higher in the rear than front. The leads to bottoming of the front suspension (which all MR2 owners know). I'm thinking about 375# rear, 200# front, with the shortenned struts up front. I'll have to calculate suspension frequency with those rates...they are just a rough guestimate at the moment. My car is quite light now compared to stock (1900ish compared to 2400ish), so maybe lower rates might be OK. i'm kinda worried that the added travel could possibly cause some funky dynamic suspension geometry changes when operating that far out of the factory range. We really gotta get a closer look at the front suspension of our cars eh? One more ice race for me, and then my car can go up on jack stands untill the end of may.
Yeah, I definately gotta look into this some more.

[ March 11, 2003, 07:05 PM: Message edited by: Dennis Beck ]

[brianlofthouse]

Right, increasing my front shock travel will be a priority at the time I install the GC kit.

Also, FYI I drive an A/SP SC MR2 with a quaife diff.

Sorry if I misquoted you Joe! I realized after posting that 0.5x was your suggested "upper limit guideline", and that unsprung weight is a factor for a given car (and I'll try and measure it on my car, but is it tricky with struts?).

Sorry again about the misquote on the 25% swaybar thing too. FWIW if my calculations are right, my current swaybars are ~%65 of my spring/swaybar totals.

Joe (real quote this time):
"I would try to get as much roll stiffness from the springs (without going too stiff, like beyond mid-2 CPS) before going to add sway bar rate for roll."

- actually, I was wondering if it would be realistic to try that and see if body roll is reasonable without any swaybars at all? (I'm not sure if that's what you meant, or if you meant using factory bars)

Since I'll eventually need to convert the wheel rate to spring rate, are wheel/spring-rate ratios of strut suspensions essentially based on the strut angle from vertical, since the LCA pivot is directly under the strut? I do plan to measure strut angles soon, (along with control arm pivots, to calculate roll centers), but is there a "ballpark" motion ratio for struts? (like 0.9 or something) A few lbs error in spring rates shouldn't matter too much if the front/rear balance is maintained, right?

[ March 12, 2003, 03:30 PM: Message edited by: BrianL ]

[Vancouver Special]

Originally posted by BrianL:
...I'll try and measure it on my car, but is it tricky with struts?)....

...see if body roll is reasonable without any swaybars at all? (I'm not sure if that's what you meant, or if you meant using factory bars)....

...is there a "ballpark" motion ratio for struts? (like 0.9 or something)....

To measure un-sprung weight, put car on jack stands. Remove strut, put bathroom scale under wheel hub, then add 1/2 of strut/spring weight and add wheel/tire, lug nuts weight. Subtract this weight from your corner weight and you have your sprung weight for frequency calculations.

Try OEM sways with your new spring set up. Don't be afraid to play with your shock settings before going to stiffer bars. A car with high roll stiffness might "feel" racy but it's more difficult to drive at the limit in quick transient situation.

As to ball park motion ratio, you can probably measure it quicker while the strut is off then trying to estimate strut angle and geometry calculations.

Joe

[DG]

When you measure unsprung weight, don't forget to disconnect the sway bars.

Don't ask how I figured this out.

DG

[brianlofthouse]

thanks again Joe!

DG - thanks for pointing that out, that's one of those DoH! moments I guess... (actually the last time I had my car corner weighted I left the swaybars connected, which isn't quite as bad)

[brianlofthouse]

Since I want to get up to date corner weights to choose spring rates, should I weigh the car with me (or my weight) in the driver seat?

[Vancouver Special]

If you are doing CW, you should be in the car. However, for selecting spring rate, the two sides will not be the same anyway (since you are not in a single seater). You can just use the average of the two corners, it's close enough.
Good luck.

Joe