Might not be the right thread, but can you comment about the optimum ride height for handling?. There’s a number of aftermarket springs available, all with different ride heights, some of which seem extremely low and possibly sub-optimal or even poorly performing.
I’m also interested in track width - since I’ve got wheels with a lower offset the car seems far more refined in mid corner bumps. At what point does widening the track become sub-optimal?
The correct answer is stock and the reason is that the factory setting puts all of the suspension components in the right location for proper articulation. The lower ride height with the stock pick-up points will start to move the car away from optimal. When you lower the ride height, you change the relationship between the center of the wheel and the chassis. This means the (in the front) that the control arms and tie rods need to angle upwards at the spindle. Moving the static height and changing the link angles will cause the links (tie rod and control arm) to move at different arcs than designed. The result of that is creating a change in "bump steer". When the tie rod and control arm no longer move in the same arc due to lowering and the suspension is placed in a bump condition (compressed like you are hitting a bump) the arc of tie rod travel may cause the wheel to toe in or toe out in bump. Imagine going around a corner. The suspension compresses on one side and you are turning the wheel. You hold the wheel at a constant position to go around the corner. The tie rod moves on an arc that causes the compressed side of the suspension's spindle to toe in. This means the car is actually turning at more of a rate than you are inputting. This typically leads to a spin. If it toes out in bump, then you need to keep dialing in more steering input to maintain the corner. If the suspension is designed so the control arms and tie rods are parallel to each other and the surface, then the arc they travel in is equal in bump and jounce (up and down). The lower the vehicle is placed, the arms are no longer parallel to themselves or the surface. As the control arm and tie rod move in an arc, the radial distance stays the same, but the horizontal distance can change at a faster rate than expected. This rate of change in displacement is what alters the tie rod distance. In braking, the nose dives and the front wheels can either toe out or in, rather than stay straight. This can make the car very darty under braking.
Next up is the calculation of the roll center at each axle. This is calculated by connecting a series of lines from the spindle center, upper strut, and control arm. It is the geometry that locates where in space the vehicle will roll about. There is one for the rear and the front. Connecting the centers is the roll axis. The greater you lower the vehicle, the more you move the roll center. This can alter the roll axis as well. A very low roll center means the car actually rolls more. You don't realize it as much because the stiffer springs (assuming they are stiffer) counters the roll moment. The springs are masking a handling issue.
But racecars sit low! Yes, but they don't use the factory pick-up points. While my Mustang may be old, the physics and statics still apply. In order to lower my Mustang to get a lower CG, but keep the proper suspension geometry, I had to do the following. First was move the control arm pick-up point the same distance I was lowering the car. The subframe that mounts the control arms has the pick-up points raised in the chassis. Next was to move the steering rack. I had to use special aluminum bushings to move the rack in the chassis. Then I had to articulate the suspension while measuring toe change. The distance between the spindle knuckle and the tie rod end was then adjusted with shims until the toe change was minimal and never toe-in in bump.
Something else worth noting. The suspension bushings are typically in a neutral position at static ride height. Lowering the ride height preloads the bushing in one direction.
With track width, there are a lot of factors at play. Scrub radius, king pin inclination, caster, and wheel offset. I was starting to get into this on my Mustang before deciding the easiest solution was to use as many factory parts at the wheel end. I picked factory wheels and factory spindles, and just moved them outward using longer control arms. This meant I was using a lot of the factory designed geometry at the contact patch. At work, I was in the process of developing a new steer suspension for the front and rear of a vehicle, but moved out of engineering before we got into the design of the wheel end and the affects on handling. There were other important factors I was proving first, but can't discuss them since the patent is in process. I will be honest and say I never got a solid understanding on optimizing the offset and scrub radius. Typically, widening the track with wheels only puts a lot of stress on the bearings and increases the distance between the center of the contact patch and where the king pin inclination angle intersects the ground. This creates a moment of force that can make steering more difficult. This is why increasing caster makes steering effort harder. You change the KPI in relation to the contact patch.
Realistically, lowering less than an inch will have limited affect on the suspension geometry enough to really cause an issue. When I used the Mountune springs, the amount of lowering wasn't even enough to alter the toe to require an alignment. That suggests that there is little bump steer induced from the springs at the revised ride height. It does mean that I will potentially get into a geometry issue with less suspension travel compared to stock since the suspension is sitting statically in a bump condition. I effectively removed some of the suspension travel in bump and moved the articulation more upward in the design envelope.
![[mecry]](/images/smilies/icon_smile_mecry.gif "Me Cry [mecry]"){kind=icon}
