View Full Version : what is ideal front roll center- tr4

06-02-2006, 02:16 PM
i need help,
what would the ideal roll center be for the front on a solid axle, fast road, 1962 triumph tr4.front springs 450#,rear 160#,6" ground clearance, 5/8" rear anti roll bar, 1" front anti roll bar-both adjustable, quaiff diff, 2200 lbs. close to stock weight distribution (5% less in front than stock). 1.5-2 deg front camber, 1 deg rear camber. /ubbthreads/images/graemlins/confused.gif /ubbthreads/images/graemlins/confused.gif

06-04-2006, 04:05 AM
What do you mean by roll center?

6" sounds high.....It depends on bump though.

Do you have cornerweights?

06-04-2006, 03:41 PM
no corner weights.car is being set up as fast road/rally car so ground clearance is correct. 5" will be the lowest static ground clearance but will start at 6".

06-05-2006, 01:28 PM
A car has two roll centers, one front and one rear.
The roll center is the precise geometric point around which the chassis rolls. Roll centers may be adjusted by making suspension changes to raise or lower the roll center, or even to move it left, right, forward or rearward for specific tracks and purposes.
A NASCAR racer has the roll centers offset from the chassis centerline for the round tracks but they put it in the center for Watkins Glen (so I'm told, anyway).

Generally speaking, the rear roll center of a normal live axle car, (like your TR4 or my Spridget) is about in the center of the diff. On my car, this makes it about 9.5" off the ground (the radius of the tires). A simple way to drop the rear roll center is to use lower profile tires (or maybe smaller diameter wheels and tires).

And there's not a heck of a lot else that you can do without some major re-engineering.

The roll center of the front suspension should be about the same height as the rear. Otherwise, the car may handle in an weird fashion.

So, since the rear roll center is essentially fixed (unless you use some tricks), you'll need to get the front roll center at about the same height.....which it approximately already is from the factory. If you change camber, lower the car or do other front end mods, you should try to get the roll center back to about where it was. This is tough unless you have very adjustable suspension. I'm sure mine is off from front to rear. Many racers move the pickup point of the A-arm on the chassis to get this worked out.

There are a number of software programs that will help you get things sorted out, but you'll need to make a good number of very accurate measuments on your car first.
Here's a short article that talks about the basics of all of this:

And about those "tricks" for getting the rear roll center lower....
You could fit a Watts linkage or a Mumford link (as seen on Mallock race cars).
Here a link to the Mallock design:

[edit] I just noticed that you mentioned 1 degree rear camber on your car. Not possible on a live axle car.

06-05-2006, 06:25 PM
" I just noticed that you mentioned 1 degree rear camber on your car. Not possible on a live axle car."
actually it is possible but not easy.the axle can take a small amount of misalignment where the shaft enters the diff. the tubes are bent with some heat and pressure. i thought the front roll center depends on the geometry of the suspension but the roll center height requirement depends on the front center of gravity,the weight transfer,and that this is not necessarily the same as the rear (especially on a tr4 which is heavy in front with a high center of gravity because of that big lump of motor. i have read that the circle track guys set the front roll center lower than the rear.
any help apreciated.thanks

06-05-2006, 08:38 PM
There was a well-known case where one of the NASCAR racers built a negative camber live axle (Elliot, I think). That involved some very trick machining that undoubtedly cost a small fortune. NASCAR outlawed it straight away.
Any bending of the rear axle that caused a useful amount of negative camber would kill the rear end pretty quickly. You'd have better luck shaving the tires so that they had a "negative camber" contact patch (another NASCAR trick).
The center of gravity is important for good handling, but it's not really related to the roll center.
Honestly, one of the most important things to get right on a sports car (or any car that has been lowered for better handling) is bump-steer.

06-06-2006, 12:21 PM
Typically, with IFS and a live rear axle, the front roll centre would be lower than the rear, so the roll axis slopes back to front. Handling tends to be a bit more predictable that way.
How much depends on the vehicle, and how much you can adjust it anyway. Not being a TR4 expert, I can't say, but if it's adjustable easily, start at 3" and see.