Upgrade to rear RSR - spring selection help
 

Hi Guys

I am in the process of upgraging the rear suspension to the TRG upside down RSR coil overs.

I currently have 22 and 26 mm torsion bars. I want to keep the 22 mm fronts with the Bilstein sport shocks, but would like to know what woulde be the maximum spring rate for the rear to maintain a reasonable balance between over & understeer.

The 21/26 bar setup with Bilstein HD was a good balance. But the car is becomming a track dog, and want to be competitive.

I have done a search and found different comparrisons between torsion bar size and coil over spring rate, and would like to get this clarified. People talk about springs between 200 and 900, and these are for the rear.

Thanks

bump

I trust this torsion bar to spring rate conversion:
http://instant-g.com/Data/911CoilConv.html

The springs you want will depend on lots of factors- weight distribution, tires, power, preference, etc, so that's tough to answer quickly. Having tried both I've found I like the "softer rear" setups more.
One caution- I think it's more important to get the front to rear shock valving correct, as you can't balance that out with a swaybar. You DON'T want the rear shocks much stiffer than the front (relatively), so make sure the front valving is in line with RSR spec if that's what you have on the rear- I can't remember how much softer the bilsein sport valving is, and it depends if it's standard RSR valving or custom.

For a better answer for actual spring selection, do you have adjustable sway bars? What track width is the car- narrow, medium or wide body? Weigh distribution and motor? LSD? Roughly without this info I'd say 250 lb rear springs, and you'd do better to have the front shocks re-valved to RSR spec to match the rear (or have all 4 valved to a newer, better valving spec- maybe Smart Racing's GT40 or something).

My setup will be as follows:
2500lb 82 SC. Now used mainly as a track car, but you cannot leave something that good in the garage, so it is also a daily driver.
I will have 23 hollow torsions bars up front with Bilstein sports and raised spindles with bump steer sorted out. Elephant racing bushes and monoballs are also on the cards, with triangulated strut brace.
Rear will have the recommended RSR reinforcements. I will be fitting the 935 swing blade.
There is a bolt in roll cage.
GT LSD fitted with 50/80 lockup.
I will race with Dunlop 210x16 up front and 235x16 at the rear.
I like the car set up realy low, with the top of the tyres in line the the bottom of the fender lip, when on the 205 and 225x45x16 SO3's.
I have been racing this year, quite competitively with a 265 fwhp and 21 & 26 torsions with Bilstein sports up front and HD's on the rear.
I am realy interested in what you guys are running.
By the way, I also like to setup my toy very neutral. I like to throttle steer.
Thanks - hope this helps

If it were me, I'd do 250s rear, which will give you tons of traction out of the corners letting you get on the power very early, but leads to a very "911" driving experience with a light front end. Some prefer 300s, which probably work better if you still have some weight in front of the front axle (battery, bumpers). This leads to a car that drives more like regular cars, which is probably more confidence inspiring for many, but can't put power down out of the slow stuff as well in my experience. Springs are cheap, and it would be easy to try both for a couple events and see what you like.

There are a number of other threads you can find that list the torsion bar sizes or springs most of the guys here on the board run. When you find those you can use the conversion to see what that would be in spring rate. Remember that the track width effects the wheel rate/ spring rate, so the widebody cars are going to feel different on the same springs.

250 is very soft. I have seen the charts attempting to compare spring rates of coils versus torion bars and am not sold on them. I run a 350 in the FRONT and have run 400/450/500 in the rear of my 2450lb car. It "feels" to much different than a 23/33 car I drove. It is also not that much more stiff then the 22/28 combo I used to run.


There are two comparison charts I have seen. One by instant-g and the other by Wil Ferch (I think) and they have very different numbers......


Cheers

Yes, 250 is very soft compared to what a lot of people run- as I said I prefer the "soft rear" setup, though I agree that's a soft overall setup for a track car. I'm no expert, but I have tested something like 10 spring configs on one of my cars, and my other car is set up with a stiff rear setup with torsion bars. For a light front end, narrow body car I found like the rear softer than the front spring-rate wise. I would say the whole thing is softer than I'd really want, and I'd go a bit over 50% stiffer all the way around if I could (can't get front torsion bars that big, so that would need coilovers their too...).

It's hard to compare the torsion bar and coil spring cars because there is so much less sticktion in the coilover cars. They "feel" much softer for an equal spring rate.

FYI, Steve (instant-g) is a prof teaching suspension design, and his data jives with another pro I know, which is why I tend to trust it.

Thanks Guys,
I have spoken to the guys out this side who run closer to 700 springs. A friend of mine is prepared to swop the 700 springs for something softer if i need them.

I will have TRG set the shock rates to be suitable for between 600 and 700 springs.

Thanks again

petevb and others......

See what I say here about spring rates-->

http://forums.pelicanparts.com/showthread.php?s=&threadid=206875&highlight=Ferch+ Torsion+rate

In my write up, I desribe "how" I came up with the equivalent spring rates for torsion bar 911's. With the post you show ( which I believe is from the Timmins web site)...no mention is made of how the rates were calculated.

Caveat Emptor....

FYI...

Wil

Thanks Wil, I could not find the link. I have no idea who is right. I only know how my car "feels" and that is the equivilant of the butt dyno! :) Not too empirical!

However, anyone who lives near me or sees me at the track is welcome to take it for a spin and compare to their set up. I am ugrading a car in the next couple of weeks to 22/30 and two of us can drive them back to back for comparisons.

The nice thing about coils is they are cheap and easy to change and there is a pretty good market for used ones.....

Cheers

No question that coils are easier to understand ( change) as well as easier to set up for ride height, etc..... a core reason Porsche went that way as early as 1964 ( 904) for their racing cars and as late as 1989 for the 964 and later road cars.

Wil

Make no mistake, coils are for people who need stiff spring rates and are willing to fiddle. You also have to be willing to dig into your wallet a little to experiment with different set ups. I love them, but hardly recommend them. You can set up and excellent torsion bar suspension easily.

Back to the original question. My limited experience make me think you will be happy with your spring choice in 600-700 range. If you are like me and prefer more rear spring rate, you will love it.

Cheers

Will- your calcs are pretty good. Somehow I think I know Timmins came up with his by moving the suspension to a known position (measured at the shock, I believe) and measuring the load on the spring with a corner balance scale. This takes into account all of the variables (ie all the other pieces flexing in the suspension besides the spring). Because all of the variables are measured it's my prefered method, but of course this doesn't mean it's right (or right for a particular car).

Back to the initial question, by all means give those 700 lb springs a try. Having tried everything from 300 to over 1000 lb springs, I will say that you will get the car to "feel" good, but that you will be slower against the clock. That's what I found- and by the way I also like to set my car up very neutral so that I can drive it with the rear. BTW, that's in the ballpark to what the Racer's Group normally suggests (a tad stiffer at the rear, softer in the front), though it conflicts with some other well known tuners (including Porsche).

One thing I did not mention that i own a full service workshop. Mainly dealing with other makes, buit a few porsches.

I have no problem with trying different springs. I just wanted to have a guideline for you guys. I didn't want to start with a 250 spring and start way off. I also wanted the shocks valved for the correct range of springs.

I have a lot af data at all our local circuits, ie lap times and more recent DL1 data at two of our circuits.

This will help with setup and eliminate the "feels good" and may be slower.

Once I get the car back on the road/track i will report back.

Thanks again

Sounds like you’re on a path to get there.

One thing to keep in mind while you’re playing around:
Both Wil's and IGs chart agree on the rear spring to torsion bar rate, so there is no argument there. So with 700 lb rear springs you're talking about the torsion bar equivalent of 22 front, 37.4 rear(!), so if you don’t want to start “way off”... I've never heard of anyone running anything close to this, and it is no where close to the 21/26 balance you said you liked before. For comparison 250 rear springs would be 22/29 rear, and 300 lb would be 22/30+ rear. These are "normal" setups, and if you want stiffer (which I think you do) just go stiffer in the front too. Just trying to save you some time, but I'll leave you to it now.

Hi petevb

I will be running 23 mm torsions in front.

When I ran the 22/26 setup I had the rear anti roll as stiff as possible, and had to reinforce the mounts, as they kept on breaking off.

I am hoping to be able to run much less anti roll with the 23 and 600 - 700 setup.

If the worst comes to the worst, I will swop out the springs for something softer. No harm no foul.

This is also a good excuse for weekly visits to the local circuit. We have open practice every wednesday afternoon. Now beat that!!

Thanks again

FYI. hollow 23s work like 22s. Unless they are really larger than 23 mm.