Renn 951

Good point, Smokey. I've got 500# springs on the front, so I'm probably getting less weight transfer to the front and therefore more braking force out of my rear brakes than some people.

Mike Buck

John,
This isn't a problem to mess around with. I learned this the hard way at SP the other weekend. My car is now sitting at IMA with multiple dents and dings in the chassis rails. If I didn't have the welded cage, it'd be a lot worse off. Rear locking brakes are no fun. That little wiggle and chirp turns into a spin, huge plumes of tire smoke, and a bent car at 120mph+

Put those street pads in the rear ASAP and go from there.

Renn 951

Mike:

I saw your post on the Potomac site and I'm really sorry to hear about the extra damage -- that's really bad luck.

If I play around some more with the track pads this weekend it will be in a large, empty shopping center parking lot. The one time coming home the other night I really stomped them and the rear end got a little "happy" I had a nice drainage ditch off the road to the right and I was conviced that was not the place to be playing around!

Which proportioning valve do you have, the stock one or did you put one of the "upgrades" in?

Mike Buck

AFAIK I have the stock proportioning valve.

There is a chance I could have my car back for VIR next weekend. If not, its nice to know I have access to a back-up car

westcoastprshe

I was under the impression that spring rates do not affect weight transfer, they simple affect the geometry during braking. The weight will be shifted around the same no matter what angle the car is sitting at on its wheels.

Renn 951

It's probably more of a second order effect. Whether you have a 500# spring or a 200# spring, it doesn't directly change the load acting on the spring, only the amount of compression. However, by changing the geometry and getting more nose dive with the lower rate spring, a secondary effect is a slight forward shifting of the center of gravity of the car, and that in turn would add a little more load to the front. Maybe not a huge amount, but probably enough to have some noticeable effect.

smokey

When the car tilts forward as the front springs compress, the centre of gravity of the car rises relative to the front of the car, and creates an additional forward moment with the front tires as the pivot point, further unweighting the rear. After reading this thread, I'm leaving in me 33 valve.

Robby

The following is a LONG reply....

Smokey AND Renn- the effect of stiffer front springs- I had always thought that w/stiffer springs, you would have less weight transfer, BUT, Danno told me the same thing as what you guys are saying, BUT- imagine this- IF a car is at rest- you can corner balance by raising & lowering specific corners- the higher the spring, the less weight it's holding- SO, you want more weight on the right front, you RAISE the left rear slightly, etc. I know we're talking about a moving car & weight RE-distribution, as opposed to distribution, but.... IF you go from a 100lb spring to a 200lb spring, you would need to add an EXTRA 1000lbs of pressure, to compress the spring one inch- SO, IF you increase the front, &, you're braking w/same force, & the front is NOT diving as much (I KNOW stiffer srpings allow the front to compress LESS than softer ones)- then, how is the same amount of weight being transferred? I mean, if you have DBL stiff srpings it take DBL weight to compress them a given amount, SO, if your car is diving less under threshold braking, then, it would seem to me, that the stiffer spring is allowing LESS weight to transfer-

IF you ALSO increase the REAR rate- wouldn't it take MORE weight transfer to UNcoil the rears & allow the rears to lift (unweight) than w/the softer springs? I would think there would ALMOST be a stronger, almost "pulling" effect at the rear, then before, under intense braking....

OKAY- now, let's assume you ARE transferring the same amount of weight forward w/a stiffer set of front springs. One of you said you would be "loading" up the front tires more w/a softer spring- I've read that loading up the fronts first, by gently braking for an instant, BEFORE hitting them hard & threshold braking, will allow the fronts to load up more, &, keep their level of grip HIGHER, than if one had just slammed on the brakes FULL-FORCE, immediately- that it BENEFITS you w/shorter stops- anyone think this is true? I've always thought the opposite, BUT, I can see where this might be true- I've ALSO heard of some people saying that you are SUPPOSED to turn the car, slightly, quickly, the OPPOSITE direction, just barely, of each turn you are about to enter- like, if going into a LEFT-hand turn, you would whip the wheel RIGHT, just BARELY, for just an instant, THEN, turn it back HARD left, to whatever degree you need to corner the car- that THIS loads up the OUTside tire & gives more traction- is this really true? I've never really played around w/this- I'm wondering what experienced track guy's would say(?).

SOOOO- what you were saying then, about loading the fronts by using softer springs? Would this HELP, OR, HURT? I've always thought that keeping ALL 4 wheels in play, as much as possible, would be the best way to stop short- SO, by THIS line of thinking, I would need to keep the weight at the rear as much as possible. OR- like over/understeer- you stiffen up the end you want to slide more- if you want more OVERSTEER, you stiffen the rear, OR, soften the front- I've always thought this to be weird- I understand it IS true, BUT, it's weird that when you stiffen up the suspension, overall, you get MORE grip, BUT, stiffening one end REDUCES that end's traction. By this line of thought, you would NOT want to stiffen the front springs for better braking

So, for shorter stopping distances, would you be better off by using STIFFER fronts, OR, SOFTER fronts?

Sorry this got so long- this is a very intersting topic & I was just wondering how this would really work...

As for the initial question- like David said- S4's are Turbo S calipers & Big Blacks are GTS's... YES, the 33/5 bias valve works very well- even w/STOCK normal 951 calipers- it was originally used on 928's, like the S4, GT, & GTS, etc- considering that 951's have basically the same weight dist & same brakes, etc, then, it would make sense that they would have the same bias valve. The next up is a 44/5 & then a 55/5- these are generally considered TOO much & are used on 911's w/their ~40/60 weight dist- you would NOT want to run one of them on 951 w/out ABS, AND, ABS is REALLY not designed to be a bias valve anyway- There IS one person on this list who has used a 44/5- he had BR's & stock rears- I think he ALSO had a much grippier front pad too, as well as ABS, etc.

Also, I never knew anything about adj bias valves being tougher to install, OR, being any more expensive. Windward had adj ones for CHEAPER than the OEM- of course, I wouldn't buy anything from them, for several reasons- BAD information- for instance- they told me that I could NOT install a 33/5 bias valve on my 88 turbo S- that it would be fine on an 89(?)- my mech got it on in less than 5 minutes- we already had the wheels off & were doing brake work, BUT STILL....


Thanks everyone...

Renn 951

Robby:

You sure ask a lot of questions for a guy from Knoxville!

You may find some interesting reading at this link: Physics of Racing

In general, I prefer to keep the inputs to the car as smooth as possible. Smooth braking -- squeezing the brakes instead of slamming them, and smooth steering instead of flicking the car around. I get in less trouble that way!

Robby

Ah hell, I just like writing lot's of online BOOKS! I guess you YANKEE VA guy's just don't like READING them- is that b/c of a lack of ABILITY or... I'm just kidding... from one backwoods state scapegoat to another....

BTW- where are you? Near Norfolk? I bought MY Turbo S there in October '99- talked to a guy w/a red 951 at NN airport- kind of funny- bad memories, considering how much trouble I've had w/this car....


I normally like to keep my inputs smooth as well- EXCEPT under braking- I stab the pedal as fast & hard as possible when trying to get the most from them- BUT, I DO have ABS... Not SURE this is the best way to get shorter stopping distances or not, but, like I said- Don Schroeder was one hell of a driver- he was the senior tech editor for C&D until his death in '99 or so- in a highly modded RENNTECH Mercedes- blew up at high speed >170 IIRC- they didn't know WHAT went wrong. Anyway, HE said that the short 150-0 stops vs the much longer stops of the Vipers, was due to the ABS of the shorter stopping cars vs NO ABS in Vipers- I'm SURE he could threshold w/the best of them if driving w/OUT ABS, so, when HE said that ABS allowed much shorter stops from such high speeds, I believe him- I can dig the article up if anyone wants- can't scan it, but, would be glad to get it & quote from it....

anyway, I'll check out your link in a few minutes- thanks a lot...

Renn 951

Robby:

No, I guess I am a true "Yankee" VA guy being from NORTHERN Virginia -- Alexandria, to be specific. Of course, that was the hometown of R. E. Lee, but in the view of many down-state politicians we might as well be New Yorkers!

I remember the story of what happened to Don Schoeder -- very tragic. But no matter how good a threshold braker someone is, it can be hard to beat the ABS computer consistently. In my case, I don't have ABS on my 951.

I think you'll find a lot of answers to the questions you asked in the "Physics of Racing" link. I really need to sit down and read thru all the material myself someday; to date I have just kind of browsed thru it from time to time. You asked some really good questions, and there is a ton of material in there that should help shed light on what is really happening with all the dynamic forces acting on a car on a race track. Enjoy!

FSAEracer03

"AFAIK I have the stock proportioning valve.

There is a chance I could have my car back for VIR next weekend. If not, its nice to know I have access to a back-up car "

Hey Mike... I saw your car at the shop yesterday, and I just thought I'd say "hi!" I'm sure I'll get to meet you in person soon enough.

"You may find some interesting reading at this link: Physics of Racing"

Great link! Reminds me of the "...To Win" series by Carrol Smith. By the way, nice meeting you at the shop. You and Mike should come to the local meet up in Frederick later this month (or ealry June... still being figured out). I moved most of the discussion to the cross-post in the 993 board due to the amount of locals there. Keep tabs on it and let me know if you guyts can make it!

Danno

I don't remember where I saw it, but someone calculated two weight-transfers during braking. One was due to the deceleration and that is based upon the wheelbase and the COG height. The other was due to the forward tilt of the COG. The ratio was somthing like 95/5%. So by limiting dive under braking, you're only affecting that last 5%.

One thing I've noticed is that with super-stiff springs in front, the reduced dive gives less feedback on the actual deceleration rate. I find it's tougher to gauge exactly how much braking-force you're actually generating. Locking up the brakes was much easier to do because the modulation was tougher.

Lotus did some tests back in the mid-80's with their active-suspension Elan and could actually program the system to have zero dive under braking or even negative dive (nose goes up). In both cases, the drivers were so disoriented by the different feedback sensations that they couldn't achieve maximum-braking forces.

One thing I'd like to see is a braking comparison with our cars with and without ABS with all other factors being identical like brake-pads, tires, tire-pressures, wheels, etc.

BTW - the only thing you need to fit S4 brakes onto '87+ cars are the rotors, calipers and caliper-adaptor blocks.

Robby

thanks for the input guys- even you yankee guys are okay by me...


Danno- EXCELLENT info- I COMPLETELY agree! You're absolutely right about feedback w/really stiff springs- by that, I would think that slightly stiffer than stock springs, we SHOULD be stopping a HAIR better- combine that w/stickier, wider, lower profile tires, SS lines, better fluid, bettter pads, &, last but not least, a better prop valve like the 33/5 to make USE of what we're keeping more toward the rear, &, I would think we would have SUBSTANTIALLY shorter stopping distances- assuming EXCELLENT STREET tires, I would think we should be able to get to ~110ft, from ~125ft of a normal turbo S- I REALLY DO NOT think that BR's & larger rotors/pads up front would actually DECREASE distances really- MAYBE another foot- especially if using lighter 2-pc rotors, BUT- at any rate, we're really only talking about a couple of feet by using stiffer springs up front, IF THAT... IMO.... LAST- the wheelbase was a GREAT point- I've ALWAYS believed that this was PART of the reason why the Bimmers are stopping so muchn shorter than us- they all have much longer WB's- an E36 M3 is just a couple of inches longer than a 951, BUT, the WB is ~8" longer IIRC- Porsche's have short WB's- TOO SHORT in all honesty, BUT, we learn over teh years & the 951 is getting old, unfortunately- BUT, we do good w/what we've got I guess- optimize things a bit & we can keep up about hte best of them...

J Chen

Hi Guys,
How can you identify if the
prop valve is 5/33 version ?

Thanks