heavychevy,

You appear to be very experienced on this subject, so maybe you can help my understand the generic 300lb. front to rear difference comment.

Please refer to the attached table to reference the data below.

The 996 911TT has a weight distribution of approximately 38% front and 62% rear.

Stock Setup
We all know this setup understeers a lot and has a 155lbs./in. spring rate difference front to rear (29% of total).
Spring rate: 35.4% F & 64.6% R
Wheel Rate: 54.5% F & 45.5% rear (including swaybar)
Wheel Rate: 54.9% F & 45.1% rear (without swaybar)
With 38% of the weight of the car on the front, but 54.5% of the wheel rate, it's easy to see why it understeers.

SJARVIS Summary (Bilstein PSS10, stock front bar and GT3 rear bar (next to softest setting)
This setup improves the balance very slightly, but is still very much an understeering car, with a 255lbs./in. spring rate difference front to rear (32.9% of total)
Spring rate: 33.5% F & 66.5% R
Wheel Rate: 53.2% F & 46.8% rear (including swaybar)
Wheel Rate: 52.8% F & 47.2% rear (without swaybar)
With 53.2% of the wheel rate in the front it still makes sense why it would understeer.

996TWINS Setup (Coil-overs with 685lbs./in. F and 970lbs./in. R spring rates and H&R swaybars)
I'm not sure why this would be a more balanced setup with a 285lbs./in. spring rate difference front to rear (only 17.2% of total)
Spring rate: 41.4% F & 58.6% R
Wheel Rate: 56.0% F & 44.0% rear (including swaybar)
Wheel Rate: 61.0% F & 39.0% rear (without swaybar)
With 56.0% of the wheel rate in the front, the car should understeer worse than either of the setups above.

996TWINS coil-overs would appear to be setup for a GT3 or GT3 RS which doesn't have a lot of torque to put down to the ground and would need an extremely large rear swaybar to get the needed balance.

I would expect a balanced car to have approximately 40% of the wheel rate in the front and for that to be made up of medium rate springs and extra large swaybars. This way the car would get the needed balance, but still transfer the weight to the rear to put down the power of the turbo motor.

Please give me your thoughts. What am I missing?

 

Steve, lots of good technical stuff to look at. You really did your homework.

 

Steve, I have no calculations based on wheel rates, what I do have is 450,600,674,899,1000 and 1124 lb springs in my garage or on my car that have been tried on my car in all kinds of various configurations. Yes, calculations serve a good purpose, but track testing is paramount from what I have seen.

The simple way (without delving into infinite variables) to explain why a bigger gap in spring rate causes more oversteer is that under lateral load, the front will squat more than the rear. This causes more rake front to back in a dynamic motion condition (vs static). More rake = more weight up front and less in rear. More force on contact patch in front, less in rear. More grip in front less in rear. Obviously there is a range where this applies. From my experience (and I've tried 600 lbs front and 1124 rear on my TT when it was AWD to try and rid myself of understeer) and from the vast majority of setups and shops I've talked to including Grand-Am/ALMS champion caliber crew chiefs, you do not want more than 200. Some go as far as only 100 lb split or even the same between front and rear. My mechanics have run several championship Cups in Grand-Am and also run, set up and maintain the Porsche sport driving school cars at Barber Motorsports park. They know their stuff.

I would say the discrepancy between their recommendations (and my and other experience) and your calculations are based on weight transfer rates. These rates help reduce the pendulum effect of the rear engine. If you based the rates soley on the balance of the car the weight transfers too quickly, or not uniformly (front fast, back too much slower)and the rear end flails all over the place.

Also think about the fact that the stiffer the front is, the less likely it is to squat, and the shorter time it takes to rebound from a squat. This will certainly cause more push in the car. In suspension tuning the first things you do to reduce understeer (mechanical grip, not aero grip) are from the list below:

- Reduce front ride height.
- Soften front sway
- Soften front springs
- Soften front compression/rebound

These all add more oversteer, so you would in turn do the opposite to transfer more grip to the rear end. OR do these things in turn with the rear suspension to add more grip to the rear.

What 996 Twins needs it more grip in the rear. Barring any problem with alignment or parts, the solution to add more grip to the rear is really effectively soften the rear of the car or stiffen the front with more spring rate or wheel rate.

 

Heavy, thank you for your input too.

The plan is to stiffen up the front springs. Looking at the other spring rates available, the difference between front and rear could be 172 lbs, 114 lbs, or 58 lbs. Currently the difference is 286 lbs.

With my car AWD and not being trailered to the track, what would you suggest and why?

In your statement with race teams running less than 200 lbs difference to equal spring rates, this would apply to dedicated RWD race cars and not cars driven to the track. Correct?

 

Yes, this is in reference to RWD cars, but has nothing to do with street or track only. There are plenty of street cars running the same rates as those.

In fact many many of the pro cup cars run 700/900 or there abouts. And they are running full slicks and have lots of DF.

I think you can get away with a setup a little bit different on the TT. You can get away with 300 lbs difference, but only because of the inherent power understeer that comes with AWD. So they cancel each other out to an extent.

Where I think you will continue to have problems is with R888's and 1000 lbs rear springs. Car will not put power down like that. As I've mentioned, there are a cars with way more tire (slicks) and way more DF than yours that have not run that heavy. And I'm talking about AWD Turbo's. You just don't have enough grip to compress the spring enough, and get proper weight transfer.

I would soften the rear vs hardening the front if it were me, but this combo may just be a sweet spot for the AWD. With the AWD and torque you can try some different things.

What you could also do is like I mentioned before and play with the rake a bit. Lower the rear or raise the front. That will add more grip to the rear.

 

HC, thank you again for more great info.

 

My spares are

Front 60-140

Rears 170-170

On the car

Front 80-140

Rear 190-170.

Car characteristics, oversteer which I prefer. Today I was at the track and I had very good grip. More grip I would say then a group of 2010s gt3rs I was with...

 

can i ask a non road racer question? it seems the front and rear coils are differentiated by 200 lbs when buying them. ie: 600/800 500/700 so on. is it advisable or appropriate to change just the rears? i have 600/800 eibachs on jrz coilovers to harsh for me. can i change the rears to 700 or maybe 600 with no adverse affects?

 

Depends on how you intend to drive the car. I think you would not be happy on a track or twisty roads with that setup. Car would probably push like a pig imo if same spring rates front and back. May be able to counter that with some extra rake or going full stiff in rear amd soft in front. But that would be contrary to your overall goal which is to soften the ride.

If you dont like it though its going to cost more money to do it over.

 

I will also mention that a good indy can change those springs without messing up the alignment. Soyou would save that money as well. Changing rear springs is actually quite easy diy.

 

Heavychevy,

See my comments below.

Great information. I like to include data in a discussion like this, because handling balance can be very subjective. A lot of people have no idea if their car understeers/oversteers due to the basic balance or because of their inputs. Even if they have a feel for that, they can easily mistake their balance under trail braking and on throttle to be the steady state balance of the car. As you have stated well below, the handling balance of these cars changes even more due to the rear weight bias and AWD.

Very interesting, since my assumption is a larger gap in spring rates front to rear (assuming the rear spring rate is higher than the front) will cause more oversteer. The higher rear spring rate will not allow the rear to compress as much as the front when rolling (laterally) into a corner, keeping more weight on the front tires and giving the front tires more grip. When I drove a 997 GT3 RS, it was way more neutral than my car and it had 228 front and 600 rear springs. I know the suspension geometry and track are different on the 997 GT3 RS, but that was an almost 400 pound difference and the car didn't oversteer at all. 996TWINS only has a 286 pound difference front/rear.

Of course everything changes as you begin to accelerate out of the turn, and that may be were higher rear spring rates really shine as they reduce squat keeping more weight on the front tires. Not the best for putting turbo power to the ground, but much better for reducing on throttle understeer.

I think you're really on to something with this statement. I find it strange how the front end will "pogo" in these cars. As you stated above, it is probably a "See Saw" affect where our rear tires become the fulcrum point and the engine is the counter weight. I wonder if more rebound dampening would improve this tendancy?

For your stiffer setup, I would agree, but for a softer setup like mine, the opposite may work better.
- Stiffen the rear sway
- Stiffen the rear springs
- Stiffen the rear compression/rebound

Agree

I agree, but his setup doesn't appear like it should be causing too much oversteer, that's why I think he has a different issue. I could easily see why his high rear spring rate would cause the car to oversteer on power as high spring rate won't transfer weight to the rear tires.

It could be that my spring rates are very low (260/515) and that is why they don't cause the same dynamics that occur with higher rates, but with a 255 pound variance front to rear, my car definitely understeers under steady state cornering.

Good discussion, thanks for sharing.

Later, Steve

 

As you mention in the lower section of your post, pro cup cars run these spring rates and higher, but they don't put out nearly the torque of a modified TT and they use a different swaybar setup to get the needed balance (usually adjustable blade type bars).

My belief is with our turbo cars, we need run lower spring rates (800 or less) and use the bars to better tune the balance.

Later, Steve

 

Steve,
I strongly suspect that the fronts are resting on the bumpstop(s) when going into a turn or braking (see post #43). So the above comparasion does not apply (see bold type) in my case.

 

I read it, I just forgot it.

Good luck

 

No problem. Lots of great info, specs, data, etc.