rear springs and steering feel

[zooracer]

Ok, my rear springs on my ITB swift are very short and stiff when compared to stock. Essentially they just sit there on top of the lower suspension arm, with the top pressed up onto the body (stock placement obviously...)
When I jack the car up, even a little, the rear springs pop off the body and lean over to one side. If I try and lower the car, the springs hang up and do not seat back into original place, so I have to reach in and place them by hand before lowering.
On track the inside rear does not lift like on some FWD's, so I guess this hasnt caused a problem yet.
Should I worry?
Also, I had a fellow swift racer (BMW master mechanic) fabricate the front camber plates/strut mounts. He used a large bearing in the mount for the strut's pivoting movement. I now have a car with such effortless steering that there is no feel. Almost like a cadillac with over-boosted power steering.
Any suggestions?
matt

[joeg]

Matt--For the rear suspension, install a roll bar and/or a cable to limit the droop so the spring stays in position when that end of the car is jacked up. The ITCS allows "Tender springs" for the same problem. Some may argue a cable stay is not a "tender spring", but it can be part of an anti-roll bar, I suppose...

I would check your toe settings on the front before concluding the steering feel has disappeared on account of the sherical bearings in your camber plate.

Cheers.

[Matt Rowe]

Steering feel is also heavily affected by caster settings. I would check and adjust caster before considering any changes to the bearing in the plates.



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~Matt Rowe
ITA Shelby Charger
MARRS #96

[racer_tim]

Matt, I have drilled small holes in the upper spring perch, and safety wired the top spring loop with safety wire to the perch. I assume that the lower perch is quite "large" and if you were to keep the top of the spring from "falling over" that it would "seat" properlly on the bottom?



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Tim Linerud
San Francisco Region SCCA
#95 GP Wabbit
http://linerud.myvnc.com/racing/index.html

[Tom Donnelly]

Tie-wraps work too.

[zooracer]

Thanks for the quick responses. I checked the alignment settings just to make sure, and the front toe was set at 0 degrees. The caster was not adjusted or measured. I will take it to a race shop and have them set it.
Any suggestions for caster? I like lots of front end feel (who doesnt), so a heavy wheel is not a problem.
I will do the drill and safety wire thing.
Thanks for the ideas
matt

[lateapex911]

Matt...

On the rear, I would suggest that you raise the car until the springs are just unseated. Then fab up a cable on your shocks that will limit droop to that point.

As shocks are free in the rules, this is the most legal solution, and it is also easier when it comes time to change springs.

Also, from a dynamic suspension point it it the prefered method as well.

Experiment at the front with your caster. Keep going backwards at the top (increasing negative caster, say from -1 degree to - 2 degrees then to -3 degrees) until it gives you the feel you are looking for. The FWD guys may have some specific figure that will work in some FWD applications, and I would think that might be a good place to start. In general, the more neg caster (-3 degrees, for example), the more self centering the steering becomes, and the better the steering feel.

To me, FWD cars are a bit of an odd duck, and wheel offsets have a lot to do with it as well, not to mention scrub radius, etc.

An experienced suspension engineer will be able to really get you on the money here!

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Jake Gulick
CarriageHouse Motorsports
ITA 57 RX-7
New England Region
[email protected]

[Greg Amy]

No, no, no! Don't screw with the suspension and limit travel. Bad, bad kitty!

I don't know your car, so I'm talking generally, but limiting your suspension geometry with straps and such is Not Good for handling performance. The last cars to try that were the old swing-axles Beetles and Corvairs, and that was just to keep the suspension from jacking the car over.

Optimally, instead of limiting travel, you need to design your suspension components to accomodate that travel and use it to your advantage. AT WORST you should use tender springs to keep the springs seated.

One other possible alternative is to use much longer, but much softer, springs. Your spring rate curve will be much more controllable and you'll have more inches of spring travel.

If you INSIST on limiting suspension travle (and there's a multitude of reasons not to do it) then you may as well simply buy shocks/struts that limit the travel of the suspension and are designed to work within that range. Otherwise, you're just amking your existing shocks work within a tightly deifned range, possibly outside where they work best.

Without going into too much technical detail, your goal is to increase the spring travel, not limit suspension travel. Suspension design is a system, not a box of parts that get bolted together until they "look right".

As to the steering feel you're describing, I'm assuming you mean there's no centering and/or drag feel to the steering any more? That's not only drag in the steering mechanisms, but caster and toe as well. I'm guessing your fabricator did not attempt to maintain near-stock camber on that upper strut mount, and probably has the upper pivot far too forward. Less camber = less self-centering of the steering = less turning drag/force.

You pretty much want as much caster as you can possibly get, because it not only increases steering and centering forces, but it also gives you increased dynamic camber; the more the wheel is turned, the more negative wheel camber you get, allowing you to reduce the static camber for straightline drag and stability.

Yes, FWD cars are odd ducks, at least when compared to RWD live axle cars, but once you got it figured it out really does make sense...

[zooracer]

Ok, I spoke with the guys at Sports Car Authority, run by John Baucom, and will be taking my car in for some suspension set-up. We spoke about some adjustable collars on top of the rear springs for ride height, and for a guide for the top of the spring to keep it in place. He would just fabricate a sleeve off the collar that would extend a few inches into the inside of the spring...
We will work on the alignment settings also, along with corner weighting.
Up till now the suspension set up was done by me, and was really hit or miss. I look forward to dropping a few seconds off my lap times.
My car has the legs and the brakes to run at the front.
Look for me at the ARRC soon!
matt

[ShelbyRacer]

Originally posted by lateapex911:

Experiment at the front with your caster. Keep going backwards at the top (increasing negative caster, say from -1 degree to - 2 degrees then to -3 degrees) until it gives you the feel you are looking for. The FWD guys may have some specific figure that will work in some FWD applications, and I would think that might be a good place to start. In general, the more neg caster (-3 degrees, for example), the more self centering the steering becomes, and the better the steering feel.

Well, first, it's positive caster in that direction, and that's what you want

I'd start close to stock (any alignment shop will tell you what stock is).

The problem with too much caster is that you're also screwing with anti-dive, and the car can do weird things under braking. Bump steer is also affected. It also changes a few other things in the front-rear equation, so going a little more positive than stock is cool, but don't go far.

Try starting with 1/8 total toe out, 3 deg neg camber, and about half degree more positive caster than stock. This is all up front obviously. See how it feels, and also check tires for contact under racing conditions (use index marks, temps, and wear).

BTW- I'm w/ Greg on the no-straps point. Run less rear spring rate and more bar, and use take-ups if necessary. My rear springs always unload when the car is lifted, but I have perches that recenter them when it's dropped. Might be something to consider...


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Matt Green
"Ain't nothin' improved about Improved Touring..."

[zooracer]

I was running 1.7 degrees camber on my friends recommendation (also runs a swift).
Do you think this is too little? He had said there was no benefit from more, and I think he had a pyrometer to check.
My car is 1795lbs, and I'm running 350# front springs and 600# rear. Do you think I should go with a lighter spring and a bigger rear bar? I am running the stock 19mm rear bar right now, with no front bar.
matt

[Greg Amy]

Matt, camber should be quantified by the tire pyrometer (every session) and spring rates are different for every car. Typically, on a FWD car the front spring rates are set to keep the suspension from bottoming at the ride height you choose (which is itself based on the suspension geometry) and rear spring and bar rates are set to balance the car (on the skidpad).

On our car, we've run the gamut of high spring rates/low bars to high bars/low springs. We're evolving back to the "add a killer rear swaybar and reduce the rear spring rate" theme, but your preferences may truly vary.

[Quickshoe]

It is very common for Formula cars of all types to have droop limiters on their suspension. Most will be internal to the dampner.

Spend some time on the Apexspeed forum (FF/FC/FA/FSCCA) there are many indepth discussions from chassis engineers (Richard Pare from ICP/Citation comes to mind) they will discuss all the reasons why it could be beneficial to limit droop.

In the "to win" series you will find Carroll Smith speaking specifically to how important it is to have springs that do not become unseated. The use of tender springs gets around that problem. But I believe they have to be fully compressed at ride height per the ITCS rules (I may be confussing this with another class)? No big deal, just something you need to be aware of.

[zooracer]

I did have carol smith's books at one time, but havent seen them in a while.
I realize FWD has it's own quirks and set-up problems. From watching the ARRC I did notice the fast FWD guys usually have the inside rear coming into the air thru the corners...
Baucom and his shop should be able to help me out here. I will let ya all know what the result is...
matt

[ShelbyRacer]

Originally posted by zooracer:
I was running 1.7 degrees camber on my friends recommendation (also runs a swift).
Do you think this is too little? He had said there was no benefit from more, and I think he had a pyrometer to check.
My car is 1795lbs, and I'm running 350# front springs and 600# rear. Do you think I should go with a lighter spring and a bigger rear bar? I am running the stock 19mm rear bar right now, with no front bar.
matt

Well, you should check *your car* with a pyrometer to see. I've seen variances in camber gain on different examples of the same model car.

Your spring rates seem a little, um, different. In most FWD cars I've dealt with, we typically run a lower rear spring rate than in the front. For example, two different incarnations of the same basic car. Both about 2400 lbs. One runs 600lb front springs, 250 lb rears, no front bar, and a BIG rear bar. On the other car, we run 450 fronts, 350 rears, and no bars at all.

I'm a big fan of the stiff spring and small/no bar method, but I had a theory the other weekend that aligned with what Greg was saying. We might try a little experiment with the car I'm building, keeping the high rate/no bar attitude up front, but running a medium-low rate rear spring and a moderate bar. If my theory is correct, we should reduce the work load on the outside front tire a little bit and make the back outside tire do a little more... Just a thought at this point, but my experience is pointing me in that direction.


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Matt Green
"Ain't nothin' improved about Improved Touring..."

[joeg]

Matt--I think you have to look at "effective wheel rate" as opposed to the spring rate.

For example, many people are taken aback when I tell them thatmy car runs 650(s) in the front and 1000 in/lb springs in the back. However, because the rear springs are inboard, the effective wheel rate is about 1/2, so my fronts are more than the backs in my FWD application.

Cheers.

[lateapex911]

I was talking to a World Challenge team owner buddy of mine about the springs in his Touring race winning Acura......and my eyes popped out of my head when he told me the rates. Under 1000 up front and between 3 and 4K in the back.

But he was quick to point out that the effective rates were a bit lower...like 25%......

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Jake Gulick
CarriageHouse Motorsports
ITA 57 RX-7
New England Region
[email protected]

[Greg Amy]

In '04 we were running 750F and as much as 1300R (depending on the race track), and our motion ratio is damn near 1 to 1 ...

[zooracer]

yes, the rear spring is inboard, at least midway on the lower control arm. So, I imagine the rear may be close to the front in that respect.
I chose the spring rates based on two others who race the swift's, but admittedly neither are what you would call "pro's".
For now I will get the rear spring height adjusters installed, alignment with some caster, and corner wieght the car. After this I may start experimenting with spring rates and bar sizes.
Right now my biggest problem is lack of LSD...
But I plan to fix that soon.
matt

[ShelbyRacer]

Originally posted by joeg:
Matt--I think you have to look at "effective wheel rate" as opposed to the spring rate.

For example, many people are taken aback when I tell them thatmy car runs 650(s) in the front and 1000 in/lb springs in the back. However, because the rear springs are inboard, the effective wheel rate is about 1/2, so my fronts are more than the backs in my FWD application.

Cheers.

Yeah, I was considering that, but made a mistake on the rear setup on the Swift. I thought they were more outboard. I also forgot that a lot of the ind. rear setups use a long lever arm, thus needing lots of spring to get the same wheel rate.

I guess my live rear axle is showing, huh?



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Matt Green
"Ain't nothin' improved about Improved Touring..."