Factors which affect torsion bar size decision?
 

Hello all,

I currently have 22/28mm t-bars in my SC and have considered larger in the rear.

The car feels pretty balanced, but I wonder if certain factors determine whether or not I *need* larger t-bars.

I have 9" Fuchs, LSD, and Bilstein Sports.

Does having one or more of these lessen the *need* for larger t-bars? I ask this especially as to how it all relates to under/over steer.

Thanks!

-B

It all depends on what the driver is complaining about.

* Is the car bottoming?
* Does it have a chronic push or is it chronically loose? This question should be followed by the question: when? Entry/middle/exit of corner? Highspeed? Low Speed? When on the gas/brake/balanced throttle?
* Are the tires rubbing the fendors? When?
* What is your tire wear like? Tire temperatures?
* Where do you drive the car? Race? DE's? Solo? Street?

As far as I know there are no absolutes, only different ways to fix problems. I'm of the opinion that "if it aint broke -- don't fix it".

If the car is balanced to your liking during high-speed turns with this 22/28 set-up, then I wouldn't change it. If the car still exhibits a tendency to understeer or just doesn't feel neutral enough (again - high speed turns), then you might consider 29s. There are things you can do before going to 29s, though. Sway bars (change in size or adjustment) is one example.

Every car is different and every driver has different comfort levels when it comes to a car that tends towards neutrality/oversteer, so this is a tricky question.

I stress "high speed turns" because it has been my experience that even with the car relatively stiff in the rear, it will still tend towards understeer in low speed turns. High speed turns (turn 12 at Road Atlanta comes to mind) is a situation where you will see the car transition from understeer to oversteer. You don't want to exceed your "oversteer comfort level" in that turn.

FWIW, my '76 has 22/29s and it feels great. To me. That was with the stock 2.7. The 3.2 in there now may make a difference.

HTH,

Mike

In thinking about your question: "What factors affect T-Bar size decision" a little more, I don't think that I respond well.

What determines spring choice?
* Car Weight and weight distribution
* Suspension geometry
* Center of gravity height at either end of the car.
* Suspension frequency
* Expected corning loads (in G's)
* Tire and wheel size
* Roll bar selection

In Mark Donohue's book he alludes to doing some calculations and changing the springing on some of their cars. You can make up a spreadsheet to do the calculations. I tried it once and put in the data for my 911 as well as a couple of popular configurations and was surprised to see that the results came out as expected, with the cornering forces on each corner roughly proportional to the tire size.

So in a nutshell, this is what Porsche did when they selected the T-Bar sizes.

Thanks guys! I plan to drive the heck out of her at Willow with the current setup and I'll go from there. I am mainly trying to figure out if I was optimal based on those other factors. I recently installed Poly Graphite camber plate bushings and this really seemed to help with understeer since it must hold the camber better. BTW, how much does the *driver* account for in these calculations :)

Here's a question: If you were to move from a seven/225 Fuch to a nine/245 how much would you increase the t bar if the current size is 24.1 and you want to keep the same ride characteristics? Would you go to 26 or so?

The front would go from say a 19 to a 21, and from six/205 to seven/205.

The nature of this question is to relate tbar to tire size. This might be a bit too simplistic on my part...

John

Great information here

I am going to install 23/31s in my street/track car in the next few weeks and I will report back. Probably way to stiff (I will be careful) and I may need to back off. I got a great deal on this set up used so I couldnt refuse. BTW mine is a 73 lite weight with a stock 3.0 in the back. Cant wait to feel the difference.

I don't think that tires size matters in figuring spring rate except if your trying to acheive maximum cornering which is another subject. Selecting springs rates and torsion bars and properly tuning and setting them up is really for the purpose of maximising the tires contact patch with the road and therefore better handling/grip. Changing rear tire size as you speak of would be the type of situation (in my opinion) where you keep you setup and give a slight adjustment to rear torsion bar (assuming you have an adjustable one). I believe that changing engines and transmissions would effect the rear springrate more because of a change in rear weight (and overall weight for that matter).

True. But if your car isn't hitting the bump stops nor going into funny geometry zones, why change the springs (AKA: T-Bars)? They're doing their job just fine holding their corner of the car up.

The assumption within the question about changing the springs is that the current springs are somehow not fulfilling their desired function of supporting the weight of the car. I continued that assumption in my answer and assumed that the springs were in some way limiting the performance potential of the car.

and I'd add... The management of the weight which is transfered to and from the individual tires to ensure that they are all working to their maximum potential.

If it was just a question of making sure that the tires are all perpendicular with the road, car designers would dispense with sprung suspensions and design the car with a rigid suspension like a kart. And beam axles would be all the rage at both ends of the car like on a Model T.

BTW, I need to make a plug for the modification I made to my Bilstein's. I cut down the rubber bump stops and it made a lot of difference with the suspension *unspringing* when compressed. I'd recommend this change. Details can be found at the Smart Racing website.

-B

I'd be interested to see how the car feels. 23/31 on an already very light car seems like a set up for a dedicated track car. Think about getting custom revalved shocks to customize the bars to your car if you can. I have 22/30's now. I formerly had 28's in the back but kept breaking sway bar mounts. It's the front t bars that jar the car, especially on potholes.

I think one reason why you would change them even if they are "holding their corner of the car up" is to change the roll stiffness of the car. I think it is good to set the basic roll stiffness of the car with the torsion bars and then use sway bars only as a fine-tuning device.

Stickier tires impart more lateral loads which cause more roll which necessitates more roll stiffness.

Sort of an oversimplification, but increasing torsion bar size has many benefits.

Mike

Sure Mike, but now your back to the limit of adhesion idea. If you put stickier tires on, you'll be pulling more G's and your car will be leaning more which may (note that I didn't say will) put the car into a funny geometry zone or up against the bump stops.

I guess I'm just saying that we're in violent agreement.

I think you're right. :)

Mike

Why is that? . . .why not have the anti-swaybar as the primary "spring" for controlling basic roll stiffness?