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Chassis Setup Procedure |
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| This is
the process I use when setting up the car after a rebuild. It’s held me in good
stead over the years. I will admit, though, that it’s a bit
“nit-picky”. I will also grant you that most SRF racers will not go to
this trouble ... but I’ve got to say that I think results speak for themselves. |
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| Step | Procedure | |
| 1 | Set all left and right side links (LCA, Strut, and Toe Link) to the same overall
length (from mounting point to mounting point). When measuring the links all that
matters is that you can accurately repeat the measurements from part to part. The
method I use is to measure from the center of one rod end (putting a bolt of the correct
size through the rod end to clarify the center point) to the other rod end
centerpoint -- measuring in a straight line. In addition, you want to set your spring perches. Unless you really know better, set the left side equal to the right side. (Obviously, the front and rear perch heights will be different.) |
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| 2 | Without changing the link lengths, assemble the car and put it on a level surface (preferably with scales under each wheel). If you are scaling the car, be sure you disconnect the sway bars (one link at each of the front and rear ends). Ideally, you'd also set the shocks at full soft now (easy with the Penske’s). | |
| 3 | Check ride height (I do this per the GCR’s -- at axle centerline, body on, no driver). Be sure you’ve got water, oil and about 2 gallons of fuel in the car. If the ride heights vary by more than 1/8 inch or so side to side, check the following (in order of likelihood): | |
| a) | Is the platform the car is on really level? | |
| b) | Which one of the upper rocker arms is bent? | |
| c) | Which spring has sagged? (I've never seen this) | |
| If the problem is “C” (a sagged spring), you need to stop and replace it. If the problem is “A”, fix it. If the problem is “B”, you can either adjust around it (by raising the appropriate perch) or get a new rocker arm. I’ve done both, but I definitely prefer to have straight rocker arms. | ||
| 4 | Set the rake. Assuming you have a “straight” car left to right (per step #3 above), you should now adjust both spring perches on each end to get the rake you’d like. I generally try to keep the front end as low as legally possible and then raise the rear end only to get the rake I want. Be sure you adjust each ends’ spring perches equally left and right. | |
| 5 | Now set the corner weights. You’re going to do this by adjusting the spring perches and, yes, it will wreck the ride heights you so lovingly set in the steps above. Weight is more important than ride height however. If your car is square, you won’t be far off. Don’t forget -- weight transfers diagonally and that typically to make a weight change, you will change all 4 spring perches at once. Shameless plug -- S/R Chassis-Man models all variations of SRF spring combinations and reliably predicts weight by corner to closer than 5 pounds. | |
| 5a | If your camber is way off from where you want it to be, there is a little “tail chasing” we’ve got to do now. The camber settings affect the weight. If the tire is straight up, the corner weight on that corner is going to be high. If the camber is excessive, the weight will be low. Now is the time to “rough-in” the camber and caster settings and iterate step 5 above. When you’ve got the camber and caster within a degree or so, proceed with step 5b below. | |
| 5b | When you’re done with the weight setting, reconnect the sway bar links and “zero” them out. By twisting the links you will be able to feel when the sway bars have no load on them. This is what I mean by “zero-ing them out”. You probably should also reset the shocks to their normal rebound setting at this point. | |
| 6 | Place the car on ride height blocks. This means disconnecting the shocks and dropping the frame onto blocks which are set at the running ride height of the car. Note that the running ride height will be lower than the static ride height you set in step 4. I generally lower the car by 1/4 inch. This lower height simulates fuel, aerodynamic down force and, much to my regret, the necessary evil of a driver. | |
| 7 | Lock the steering in a centered position. I’ve used two different methods over the years and both work. My favorite is a steel strap bolted between the steering shaft (using an open hole in the shaft) and the upper main frame rail. I’ve also made blocks that fit between the steering rack rod ends and the rack body, but they’re a lot harder to use. Regardless, you must lock the steering so that it doesn’t move during other adjustments. Any steering movement will affect your measurements and drive you nuts trying to figure out why you can’t get consistent readings. | |
| 8 | Mount the toe strings on the chassis and set initial toe to 0 both front and rear. | |
| 9 | Now carefully measure the caster and camber. It should be pretty close to equal side to side. If it isn’t either you didn’t set the links up with equal length or something is bent on the car. Check your UCA brackets for bending...I’ve had this a couple of times. | |
| 10 | Set your desired caster and camber. Do caster first. Note that you’ve got to hold your toe at 0 when measuring caster and camber because they both interact with toe. Shameless plug -- with S/R Chassis-Man you can do this in one pass, saving all kinds of hours of chasing your tail and cursing. | |
| 11 | After you have the car setup at your desired caster and camber, you want to set bump steer. My preferred tool for measuring bump steer is the Longacre single dial indicator system. Works well. The adjustment process is different front and rear as described below: | |
| Front: | Here you will adjust the thickness of the shims between the steering rack and the steering rack mount ears welded to the frame. In general, the less the caster, the greater the thickness of shim required. | |
| Rear: | The way to adjust
the rear bump steer is to adjust the height of the either or both the inboard and outboard
toe link rod ends. You can do this by adjusting the height of the inboard mount
(either via the existing shims, the big hammer method or re-welding the mounts).
Many a rear toe link bracket has cracked off after having been subjected to the big hammer
method of rear bump steer adjustment. I don’t recommend it. You can adjust the height the of the outboard end by adjusting the caster. In general, the left rear bump steer “comes in” for most cars with about -1 degree of negative caster (top of the hub forward). The right side is usually a bigger problem. On many cars I’ve seen, the right inboard toe link bracket needed to be about 1/4 inch lower on the frame and then -1 degree of caster worked pretty well there too. |
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| 12 | Re-measure everything one last time and set your final toe. | |
| Rev. 11/21/98 | ||
| Feedback | ||
| I welcome your comments or ideas on this information. Feel free to drop me
a line via “Comments for the Bro’s”. Barry |
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