Using Ride Height or Bars to Tune Balance

[johnc]

Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: markortizauto@windstream.net. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.

 

USE RIDE HEIGHT TO TUNE CORNERING BALANCE, OR ANTI-ROLL BARS?

 

I have a FF1600 (i.e. no ground effects) and the person who runs the car and I differ on how it should be set up:

 

1. He first gets the handling roughly right by raising the rear ride height and then fine tunes the handling with the rear ARB. This assumes the ARB is already of the required size,

2. I think the car should be kept flat so the CofG is as low as possible and only use the ARB

3. He says that having the car flat will require an harder rear ARB (which is true) and this in a turn lifts the rear inside wheel more

4. My response is that if the handling is the same the weight transfer must be the same. This means the rear roll stiffness is the same so the inside wheel will be lifted by the same amount.

 

So who’s correct? Point 4 isn’t quite true because a lower CofG will affect the handling but the general principle still applies. I could try this out in the workshop but this would be a lot of unnecessary effort if the answer is already well known.

 

Basically, the questioner is correct here. The tires don't know if the load transfer comes from geometry or springs or anti-roll bars.

 

Raising the rear ride height affects handling mainly by raising the rear roll center. This adds load transfer, just like stiffening the rear bar does.

 

In general, the car will go fastest with both ends as low as possible. How low we can go will generally be determined by the need to avoid bottoming, with perhaps some additional influence from aerodynamic considerations. Both of these factors may call for slightly more ground clearance at the rear than at the front.

 

 

 

It is common for race cars, especially rear-engined ones, to have lower natural frequencies in ride in the rear than in the front. That will mean the rear suspension deflects more at the bottom of a dip than the front suspension.

 

To get good air flow under the car, it generally helps to have the under-car space open up a bit toward the rear. This will generally reduce both lift and drag.

 

But the idea is still to run both ends of the car as low as these constraints will allow. It is not a good idea to balance the handling by running either end higher than necessary.

[tube80z]

I really like Ortiz but I think his answer was way too short. There's a huge difference in the speed of the weight transfer between geometric/linkage changes versus those done with spring/shock/ARB changes. The tires will know the difference.

 

CAry

[260DET]

This is the bit I like, its very applicable to our Z cars.

 

"Raising the rear ride height affects handling mainly by raising the rear roll center. This adds load transfer, just like stiffening the rear bar does."

 

While both achieve the same end the practical result is different in that rear ARB's tend to cause lifting of the inside rear wheel, not a good thing. But is there a speed of weight transfer involved here of the sort Cary mentioned? If so, what are the implications?