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Before you lower your Bullit-Geometry 01/08

2284 Views 8 Replies 6 Participants Last post by  VIC-TIM
Continuing with our Bullit University policy of bringing you the best in Education and Entertainment, I give you today's thread.
When I was a senior in High School, I hated math, especially Calculus and Algebra. Geometry was not too bad, because I could see uses for it, and it provided answers to make my car go around corners faster. I got my first radial tires and Koni shocks when I bought a 1973 VW Super bug, bright yellow, Recaro seats, Macpherson front suspension and fully independent rear. It wasn't fast, but was a poor boy's 912.
So before you go off and lower your Bullit, here's a little Geometry to consider. See diagram below. What are?:
Bump steer: Bump steer occurs when you lower your suspension to the point that the control arm or "traverse link" and the tie rod are no longer on the same plane or angle. At this point, every time you hit a bump, especially when turning, the tie rod will tug on the steering changing your "line'. Hence the name "Bump steer." You have changed the suspension's arc but the steering's remains the same?
Ackerman: Ackerman steering is built into production cars so that when you turn a corner, the inside wheel turns more (smaller radius, tighter turn) than the outside wheel, as much as 3 degrees. Keeps tires from squealing. This also can be affected by lowering your car, (bump steer) or if you bend steering parts.
Roll center: Roll center is fairly complicated, but if you think of the center line of your car as a pivot point, when the car leans (or rolls) in a corner, the weight of the vehicle should end up the middle of the tire, distributed evenly. If you lower your car more than about 1" you change the roll center and the weight ends up out on the road somewhere.
Sway bar pre-load: Unless you have matched springs and sway bars, if you lower your car too much you will put excessive load on the sway bar so that it will be acting like another spring even if you aren't turning.
Camber: Camber is the measurement in degrees of how much your tires tip in our out looking from the top. If your tires tip out at the bottom, that's negative camber, in at the bottom, positive camber. Lowering your suspension creates more negative camber, sometimes excessively, enough to wear the insides of your tires out prematurely.
Caster: There are all sorts of fancy ways of describing Caster, but simply Caster is the difference in Camber between your wheels turning 20 degrees right to 20 degrees left. Example: I turn the right wheel 20 degrees left and measure the angle of the wheel top to bottom and it's zero degrees, then turn the same wheel 20 degrees right and measure 5 degrees. The caster is 5 degrees positive. You never want negative Caster.
Toe in/out: Usually measured in parts of an inch. Measure between front and the back of your tires horizontally. less in front than back, toe in, more in front than back, toe out.
Ok, so that's some of the geometry. Next time: What can I do without lowering the car to make it handle better and To hell with it, I want to lower my car, how do I do it correctly?? Whew!!!!!


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The drag racers hate the ind. susp. That's one thing a solid axle does well, 1/4 mile stuff.
Also it costs a lot more to manufacture ind. susp, and there's some wild geometry to keep everything where it's supposed to be. But, it does ride nicely!
So true. The majority of Mustang owner are more concerned with stop light to stop light, straight line acceleration. Or, who can do the biggest burn out. So for the majority of Mustang owners the old "stick axle" is a better choice. This site is an exception. There are many here that want to get the most "real world" driving out of their Mustang. If you really dive your Mustang, then the IRS offers a huge improvement. In my case, and I think there are a lot like me, I simply do not have the ability to push the car to the limits of the "stick axle" much less the IRS. In my humble opinion (IMHO), I thing the IRS should really be am option. Let drag racers (and chickens (me)) have the solid axle for the strength and/or lower cost. For the real drivers, they have the IRS option. I think the Bullitt and the GT350 should come stock with the IRS. Cars like the GT500, should have the solid axle for the strength.

Just my 2cents (worth less than a penny today.

Thanks Pat for all of the "classes" you have been teaching here at the Bullitt University (BU). I have learned a lot and look forward to future classes.
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