Tag Archives: suspension geometry

Still thinking, remain in cover until the all clear has been given

I’m still reading my new book about suspension geometry and contemplating the information, but it hasn’t resulted in any changes to the Sprint-T. The front is still a tube with fore and aft location by parallel 4-bar and laterally by a bent Panhard rod to clear frame members, rear is still fore and aft by dual trailing links and laterally by a Watt’s link and a torque arm for control of reaction torque from the engine. And torque reaction links for the brakes. All rotational reaction forces are isolated from the suspension to prevent locking up the suspension from either engine torque or brake torque, allowing the suspension to move freely at all times. The front springs are mounted as close to the ends of the axle as will clear moving parts, and the rear springs are mounted to the trailing links originally designed for swingarm suspensions of much heavier vehicles on rough tracks. This allows easier tuning of suspension stiffness by changing the motion ratio for the springs as there is quite a bit of motion ratio to play with.

To clarify that, the bottom of the spring mount moves less than the wheel in both single wheel and double wheel bumps. This means I can get smaller changes in wheel rate than steps in spring rate which gives me finer control over wheel rate. Also the total travel of the shock is less than the travel of the wheel which reduces the amount of force per inch of wheel travel. There is a formula that gives us the effective wheel rate when the spring rate is known (mr2*Spring Rate) or the square of the motion ratio times the spring rate. On the rear suspension for the Sprint-T there are two motion ratios, the motion ratio for a single-wheel bump or body roll and the motion ratio for a two-wheel bump which is just the motion ratio of the swingarm. The single wheel bump is the swingarm ratio times (the distance from the far wheel to where the swing arm attaches to the axle housing divided by the track).

Got a new book

I bought Advanced Race Car Chassis Technology HP1562 for the Kindle a few weeks ago and I’ve been absorbing it a little at a time. It’s a complex subject, but from what I’ve been able to read so far, if I get it right I don’t need to make the frame all that torsionally rigid. If I’m reading this right there’s no twisting load on the frame if I have the suspension geometry right and tuned the spring rates. If my sums are close there will be about 300 pounds ± on the springs of the front corners, and about 350 ± on each rear. That’s not a good sprung/unsprung ratio, but that’s what I’m stuck with.

The front axle weighs about 65 pounds plus the hubs and brakes, because it’s a honkin’ huge chunk of iron steel with tons of thick brackets welded to it, and the spindles are likewise big chunks of forged steel. The rear axle is right at 100 with the diff and and axles (drive) but without the hubs and brakes, which are almost the same weight for both ends. The rears are a touch lighter than the fronts because the rear disks are 11.75″ and the fronts are 12.19″. All of that is unsprung (bad) weight, axles, drive axles, hubs, brakes, the whole shootin’ match all bad weight. Well none of it is good but unsprung weight is super bad compared to sprung weight. Like excess unsprung weight causes tires to lose contact with the ground over a bumpy road even if the bumps aren’t that big, bad. And a lightweight car makes unsprung weight even worse because sprung weight pushes the unsprung into the road and not having enough sprung weight means the unsprung weight is just kinda floating out in space. It only contacts the ground intermittently because the two weights, sprung and unsprung, are just oscillating independently. I’m getting close to that condition with the front of the Sprint-T. If I understand the theory right the only thing you can do with a car that has sprung:unsprung ratio approaching unity is make the dampers stiffer to prevent oscillation from setting in, assuming there’s nothing to do to make the unsprung weight less. Right now the only option I have is to use “drillium” in the front axle, that is to remove weight by drilling the front axle full of holes. Since this axle was designed for a car almost twice as heavy as the Sprint-T this is actually an option.

As far as the geometries of both ends are concerned, the front end will not have any adjustability beyond caster and toe, the roll center height will be fixed to prevent the front axle moving under load and changing the steering angle. But the rear axle will be located by a Watt’s link with the pivot located by a nut on a threaded bolt that adjusts up and down to change the balance of the car for different conditions, like understeering for highway travel, neutral for road courses, and slightly oversteering for autocross. That can be done by turning the bolt to move the rear roll center up and down.

Oh, here’s the link to the build video for the GenIII Hemi™ giveaway. Lotsa power and torque from a nearly 600 pound engine.