mikey_audiogeek

I'm running 400/850 on Bilstein Escort with no rear bar (M030 bar at front). Very deliberate choice, works better than you might expect. Best thing you can do with a semi-trailing arm rear suspension is to ditch the sway bar and go up on spring rate. This has the same effect as lowering the roll centre.

Oh yeah, forgot to mention - this is a street car. One thing I learned early on is that Bilsteins ride harsh with too soft a spring rate. They actually ride better as you
increase the spring rate! (up to a point)

I've done 15,000km with no regrets... better watch out for bushing failure though (thanks Van for photos)

Cheers,
Mike

Van

Mike, that's about what I run - and I'm happy with it.

I'm actually going to increase my front spring rate a little for my next race (because I have the springs) and see what I think. Now that I have the front splitter, and have more front downforce, I think I'll need a slightly stiffer spring to compensate.

I don't know if I mentioned earlier, but Bilstein Motorsports can weld spherical ball joints of for you when they rebuild the shocks.

mikey_audiogeek

Hi Van, thanks for the advice, will keep it in mind. Bilstein have a very good shop about 5hrs away.

My kit is actually the H&R RSS setup for the 944, but it's based on the Bilstein Escort shocks with H&R springs as selected by them. I was a bit surprised at the spring rate when I got 'em but experience (and recent posts clarifying wheel rate calcs) wins out.

Cheers,
Mike

drift a 944

My 951 street car has Bilsteins with hypercoil 350 front and 550 rear, no torsion bars. Rides perfect, I love it.

I'm curious though, since I am not the original purchaser of the kit, and apparently Bilstein offers a limited lifetime warranty, would they still warranty and rebuild a bushing failure incident even though I am not the original purchaser?

Van

Don't know... In the motorsports world, rarely is anything warrantied... You learn to expect failure.

mikey_audiogeek

They might send you a new $2 bush...

THK make a metric spherical bearing that may be suitable for the existing eye, part no. NB 14T. It's 14mm bore and 34mm OD. I'll let y'all know what I find.

Cheers,
Mike

333pg333

Wow, I can't believe how different some people run their springs compared to others. Amazing really. Mike I've never heard anything near the spread you've got but I still can't get mine how I like them so I can't argue if you're having success. Then again it could be just a Kiwi thing. lol.

I posted this up today in the 951 forums. This is an excerpt from some communication I had with Karl Poeltl of Racers Edge who knows a thing or two about these cars. I was getting his opinion while I waited for my Custom KW 2 way race setup to arrive. I bought some of his goods of course. This is what he had to say. It was in context re some other figures another person (Bob) came up with.
For the record my springs are 110Nm front and 50Nm rear in concert with my 25.5mm T-bars.

"The info on Paragons site is stuff they got from me a few years ago. In any case, the 47% is a number that I have calculated by taking measurements. I am now pulling out one of Porsche's own Motorsport sheets that shows all the rates of their Turbo and "Cup" cars back when they ran competitively.
Porsche lists the 25.5 mm bar as 31 N/mm which is 177 lbs/in. They also then give the variable rate coilover helper spring rates at 34-65 N/mm which is
194 lb/in - 371 lb/in. They then give the total Rate at the wheel(T-bar plus
coilover) as 45.4 - 58.5. Back out the rate at the wheel due to torsion bar which they list as 31 and you have 14.4 - 27.5 at the wheel due to the coil over. So take your pick, 14.4 / 34 is approx 42% or 27.5/65 is 42%.

Bob is right about them being inboard but his numbers are off. Actually they are correct I think in that the motion ratio is about 65%. But when calculating wheel rates from spring rates it is the motion ratio squared that is uses. So 0.65 ^2 is , guess what,... 42.25% which is the number that Porsche's own sheet claims as I outlined above. So your torsion bar is 177 at the wheel, and your helper spring rate is a 285 which is 119.7 lb/in at the wheel. So working backwards 177 plus 119.7 is 296.7 pounds per inch at the wheel. Divide this by .42 and that is you equivalent coilover, or 706 lb/in coilover (initially I had 661 lb/in which is attributable to my measurement error - I had 47% and Porsche lists it at 42%).

So it may be a bit stiffer in the rear than I might run but with the ability to tune sway bars etc... you should be Fine. Your setup is actually much stiffer in the front than the Porsche cup setup which ran progressive front springs (200 - 371 lb/in) with the rear setup I described above which is not that far from what you have( yours is 296 at the wheel and theirs was 259 -
334 lbs/in at the wheel).

Hope this make sense, but believe me, what is above is 100% correct. I can fax you the Porsche motorsport sheet if you think it will help."

Van

Indeed.

Potomac-Greg

There are a number of factors that affect performance balance. You can start with tires. Some people have staggered setups which will justify/require a stiffer rear to balance the handling. Then there are bushings -- softer will be more compliant. Then sway bars. Lastly springs and shocks/struts.

Most importantly, I think there are some folks that are 100% street use, and probably do not test the limits such that balance really makes a difference.

mikey_audiogeek

Hi folks, great discussion. Thanks to the good info posted on motion ratio and wheel rates, are we best talking about wheel rates now? Especially since our cars are nominally 50/50 weight distribution.

The way I figure it, my setup calculates as follows:

front wheel rate = 400 x (0.91)^2 = 331lb/in.
rear wheel rate = 850 x (0.63)^2 = 337lb/in.

I think we could call this an "even" setup. by the way, I tune roll stiffness with front RC height.

Greg is right - context is everything... one factor is I only have about half the horsepower of some people here!

Potomac-Greg

Is the 0.63 calculated based on the torsion bar geometry? I would assume that that fraction would be higher if measuring effective spring rate at the coilover geometry. So, if you spring rate is based solely on coilover, it would be lower.

Van

That 0.63 is a measure of the shock... if you raise the rear wheel up 1", the shock compresses 0.63". So, a coil over compresses 63% of the wheel travel.

shiners780

That matches real close to my findings when I recently lowered my car.

The rear was lowered 15/16" measured from a point on the outer surface of the wheel relative to a point on the quarter panel. That change required the spring on the coilover to be lowered by 8.8/16", which is 59% of the wheel movement (real close to the 63% stated). Had I been able to measure the wheel movement from the centerline of the wheel, it would probably be spot on 63%.

Cool.

mikey_audiogeek

Hi Greg, not completely sure what you mean by this, but Van is correct.

Anyone prepared to go up on rear spring rate now?


http://www.rqriley.com/suspensn.htm

Van

This site has some good videos and explanations on suspension frequency: http://www.racing-car-technology.com.au/