Track Width Question
I've enjoyed your articles for years, and have read and re-read your chassis book many times. It always gets me thinking. In your book, the only reference I see to track width is a suggestion to have your right-side tires in line with each other. Does this also apply to a road race car?
In other words, are you recommending equal track widths front and rear? In some GT cars, slightly wider tires are run on the rear. Again, should track widths remain equal? In one of Carrol Smiths' books, Tune To Win, he seems to recommend a wider front track, for less understeer.
My car was built many years ago with a rear track that's about 2 inches wider than the front. I suspect it was done because the builder used a junkyard 9-inch Ford rearend with disc brakes from a Lincoln (about 65 inches rotor to rotor). The builder has passed so I can't ask him!
I use this car on road courses and once in a while in a hill climb. The only easy way to widen the front track would be with wheel spacers or offsetting the wheels. I don't think this is a good idea because the scrub radius is more than 2 inches now, due to the splindles he used. The car has never been very stable under heavy braking.
In your opinion, would I have a car that turned better if I ran a rear track width equal to or less than the front? If you've covered this in the magazine let me know and I'll look for a back issue. Hope to hear from you soon.
- George Harrelson
Track width differences do make some difference, although not as much as you might think. In the rear of a straight axle car, or one with the solid rear axle, the car rides on the top of the springs and doesn't know how wide the track is.
There is a slight amount of unsprung component load transfer change due to the wider track in the rear, but not enough to really affect the handling. Because on circle tracks most of the load ends up on the right-side tires in the turns, it's important for those tires to track inline. It's less important where the left-side tires end up as far as alignment goes. So, track width is less important for circle track racing.
If you take that thinking to road racing, if the track widths are different, then for every turn, right or left, the outside tracks are not inline with the rear tire and end up either inside or outside the front tire. So, for that type of racing you might want equal track widths.
I would try to shorten the axle tubes if that's possible rather than use different offset wheels at the front for the exact reason you stated, the center of the wheel ends up too far outside the scrub line. That does affect the driver's feel when braking while the car is turning or driving over bumps such as curbs at the apex of the turns.
If your problem is a tight car that won't turn well, try looking at the front moment center design. I've worked with a few road racing cars and we always design the front geometry first. If that's not right, the car won't turn well.
I really enjoyed the small bar big spring article. And I got the gist of it. But you guys left out what general range of shock valvings you were using with this setup. Thanks for the great magazine and articles.
- Jeff Partington
For shocks, we basically had what used to be referred to as 6s on the front and 4s on the rear. If you run adjustable shocks, you can tune from there. We usually run less rebound on the left rear to help entry. For hard entry tracks you might run a slightly stiffer shock on the right front or a 5 left front and 6 right front.
If you have comments or questions about this or anything racing related, send them to my email address: Bob.Bolles@sorc.com, or mail can be sent to Circle Track, Senior Tech Editor, 9036 Brittany Way, Tampa, FL 33619.