Torsion bars and helper spring set up

[Ollies930]

I am probably looking at this the wrong way (which is why I am posting this), but if one is using a torsion bar with coil over helper spring, would the amount of preload on either make a difference? What I mean by that is, if the coil spring is compressed (arbitrarily) 1" and the torsion bar is twisted 3", would that be the same as the torsion bar being twisted 2" and the coil spring compressed 3", as long as the ride height is the same. Meaning, that if the ride height is not even with the torsion bars only and one were to adjust the coil springs tension with adjustable collars to even out the ride height, would the overall spring rate still require the same amount of weight to compress either side an additional 1"?
In my head I believe that they overall rate would be the same. Am I wrong?

[911tracker85]

great post. I am also considering this. got a set of front reinforced RS raised spindle struts and a matched set of f/r bilstein revalved shocks with collars for coils.

I have read such an arrangement can be difficult to get is set up correctly.

I see a lot of folks do coil overs on a stock tub, but it was not built/reinforced for that. you will get quite a bit of chassis flex. great for a race car with full, integrated cage tied into the tub.

part of my thinking is using light progressive coils. the idea is to have the initial suspension not too harsh by not having BIG torsion bars alone. during hard driving the progressive coils come into play to quickly firm it up.

I suspect there would be a difference in the set up you describe. the spring rates on the T bar and coils would be different.

looking forward to thoughts from the gallery.

[flightlead404]

Originally Posted by Ollies930

I am probably looking at this the wrong way (which is why I am posting this), but if one is using a torsion bar with coil over helper spring, would the amount of preload on either make a difference? What I mean by that is, if the coil spring is compressed (arbitrarily) 1" and the torsion bar is twisted 3", would that be the same as the torsion bar being twisted 2" and the coil spring compressed 3", as long as the ride height is the same. Meaning, that if the ride height is not even with the torsion bars only and one were to adjust the coil springs tension with adjustable collars to even out the ride height, would the overall spring rate still require the same amount of weight to compress either side an additional 1"?
In my head I believe that they overall rate would be the same. Am I wrong?

remember the basic spring formula f=kx with k being the spring constant for that spring.

So, for a given spring the force for displacement is directly related to the distance displaced.

The coil and the torsion bar will necessarily have different spring constants (rates). So, setting the ride height by adjusting the displacement of one spring on another will cause the force required for further displacement to be different depending on bias (spring or torsion) is chosen.

[Ollies930]

Bob,
with progressive springs you would absolutely have to set the torsion bars up first and then adjust the progressive coil overs to set ride height, as the spring rate will change as you compress the coil over.
BTW, where did you get the shocks revalved and the threaded bits added? Looking very nice!

My thinking was, with straight rate springs, for every additional inch a suspension is compressed, it would take the same amount of force applied to the torsion bars and the coil springs on either side. I.E. if the coil spring were 150 lbs/in and the torsion bar were the equivalent of 350 lbs/in, then 500 lbs of force would compress the suspension an additional 1in, no matter how wound up either spring already is.

[flightlead404]

Originally Posted by Ollies930

Bob,
with progressive springs you would absolutely have to set the torsion bars up first and then adjust the progressive coil overs to set ride height, as the spring rate will change as you compress the coil over.
BTW, where did you get the shocks revalved and the threaded bits added? Looking very nice!

My thinking was, with straight rate springs, for every additional inch a suspension is compressed, it would take the same amount of force applied to the torsion bars and the coil springs on either side. I.E. if the coil spring were 150 lbs/in and the torsion bar were the equivalent of 350 lbs/in, then 500 lbs of force would compress the suspension an additional 1in, no matter how wound up either spring already is.

Thanks, a much better way of saying what I was trying to say

[gumba]

I set the ride height with the torsion bars, with no load on the helper springs. The collar just puts enough pressure on the spring to keep it from rattling. This is just to deal with turbo squat.

[full quack]

If your end game is simply the looks of the car, then by all means continue on with your current path.

However, if proper, predictable and balanced handling is your goal, you are going to have to step back and re-think your plan of attack.

These cars only respond well if the suspension is set-up as a matched overall package, spring/torsion rates for your driving style, shocks/strut inserts valved for your spring rate and intended driving surface street or track or street/track combination, tire and wheel choice, and sway bars size and adjustment further refines the overall package. Last but not least, they need to be properly corner balanced as part of the overall alignment process.

One important reality to be aware of, is that there is no perfect world, you can have ultimate handling, compliant ride, high speed stability, or low speed flickability{sp} or comfort....you can't have it all...pick 2, it's all you get!

I've tried the quick-n-dirty...doesn't work, I've tried the half baked approach...still no joy. There are just no shortcuts that work, they are all a big compromise one way or another in the end.

Lastly, and I hate to say it, but the corner balancing may or may not mess with your intended perfect "even ride height" that you are shooting for.

Mark

[dannobee]

Interesting discussion. A couple of caveats here though. Torsion bars have a motion ratio of 1:1 (in the basic suspension calculation lexicon, D1=D2). The "helper" coil spring's motion ratio will be different than the wheel rate because of distance from the path of the control arm outer pivot (D2 is longer than D1, and also the included angle of the coilover shock must be calculated as well), so remember to keep its motion ratio in mind.

The variances in the two would simply create a dogleg in the spring chart. This might be easier to visualize if you graph it with suspension travel on one axis and lbs force on the other axis. Your torsion bar has a linear line at say, 45 degrees. Your "helper" spring's line starts at some point along that linear line, and that secondary rate is added to the primary spring rate. How much preload on the helper spring determines its starting point on your spring chart's linear line.

If you're just trying to change ride height, have at it. But as Mark pointed out, if you're setting up a race car, remember that making one end of the car stiffer than the other will most certainly affect understeer/oversteer.

Re: corner balancing. If anything, adding coilovers to the car makes getting the cross weight and ride heights correct a LOT easier. Don't forget to disconnect one sway bar link on each end before scaling the car.

[911tracker85]

great posts.

ollie, I got them from a local Porsche master mechanic who used to build race 911s/930s. took them off one of his cars when he was reverting to stock.

mark,

you can have ultimate handling, compliant ride, high speed stability, or low speed flickability{sp} or comfort....you can't have it all...pick 2, it's all you get!

LOL I retired from the computer networking business. we had the saying "good, fast, cheap ... pick 2" you are correct you cant have it all.

my plan is to build to track spec. will not run on the track much as I am putting way to much time, work, $$$ into it to be a track car. and as I get older not sure I'll be up to the demands of such a car on track. so what if it will be a bit harsh on the street, part of the fun. and wont be a daily driver. just show and go fun.

I'll be looking in on this thread for ideas.

[dannobee]

Age has nothing to do with it. Here's one of my old competitors. (notice his age)

https://en.wikipedia.org/wiki/Hershel_McGriff

[flightlead404]

Originally Posted by full quack

Lastly, and I hate to say it, but the corner balancing may or may not mess with your intended perfect "even ride height" that you are shooting for.

Mark

This!

Years ago when I got a good corner balancing and track alignment on my first 911SC the ride heights were noticeably different left to right. At first I was upset, until I realized what I had asked for.

The handling change was an amazing improvement. I learned to ignore the difference in tire to body gap.