Been working on my A sedan rear tire to wheel well clearance. Visually its looking pretty good, and I have about 3.5 inches between the top of the tire and wheel well. I'm thinking that might be a little tight. Looking for some input. Whats the minimum you would run?
Do you have bumpstops between the frame and axle? 3.5" is a pretty good clearance, but if you nail a big chuckhole on the freeway and bottom out, you don't want your fender to be what stops the suspension travel. Your axle should hit a bumpstop before the tire hits the fender. Since the axle might slant at an angle during something like that, the bumpstop should really hit probably at least a 1/2" before the tire hits the fender when things are level. The polyurethane bumpstops compress less than regular rubber bumpstops, so I'd use those. You can mount the bumpstops outside of the frame rails on gusseted brackets if that's easier.
It all depends on the spring and shock rates. If everything is on the firm side, 3.5" is plenty. If it is all set-up super soft, 3.5" will bottom out every bump. It all comes down to how your car is set up.
Follow rustybolts advice about using a bump stop. The simplest way to check tire clearance is to place you car on jack stands at ride height, disconnect the springs and shocks, place a floor jack under the pinion and jack the rear axle up until something hits. If your tires hit the inner fender before the axle bottoms out you have an issue. If the axle hits the frame before your tire hits the fender , look in and check the clearance between the tire and the fender. As long as it doesn't touch is not really safe enough, 1"' space will give you a safer feeling and no paint blisters and this can be controlled with a bump stop. Now this way does not take in to consideration of body roll. You should repeat the process by jacking up the axle housing at a point 1/2 way between the tire and the center and check the clearance to the inner fender well during body roll as well. Repeat this process to check clearance on the opposite side also. If you know the limits of travel you then can adjust your car by raising the spring mount to give you more clearance or in some cases a slightly stiffer spring.
One other thing I'd mention is that you want your axle to hit bumpstops before your shocks bottom out. If the shocks bottom out before the bumpstops hit, you'll most likely eventually break a shock mount off, or wreck your shocks. If you have to add bumpstops to the inside or outside of the frame, make the mounts extra sturdy with long triangular gussets to spread the load out. The bumpstops have to be able to withstand a severe beating if you wind up with not much clearance and you have spongy springs and you hit a big chuckhole. If you measure 3-1/2" clearance now, it might wind up a little less than that when you add a full tank of gas, passengers, yourself, and whatever parts you haven't finished putting on the car yet.
Is there a rule of thumb regarding bumpstop size? I have a pickup and lowered the rear. The old bumpstops are 1 5/8 inches. Do I need to use this size or can I use shorter (new polyurethane type) unit?
You can use anything that does the job. When you hit that tall rubber, it will be softer than a short poly. If you are relying on the bump stops to prevent shocks from bottoming out, you need to install them and see how far they compress to see how high or low to mount them. You are always better off to start with a tall stop and trim it down if needed than to use a stop that is too short. In any case, have a plan to adjust it once installed.
What is or are the recommended way(s) to limit shock travel? My example: the shock has 5 inch travel. The springs (leaf) to frame dimensions are: 6 1/4 suspended, 3 3/4 at ride weight. If I use a 1 inch bump stop that leaves me 2 3/4 travel in compression and 2 1/2 suspended. I have not mounted the upper shock bracket yet but would position it to handle the 2 1/2 compression motion. I would think I need to limit or restrict the down travel to 2 to 2 1/4 inches. If yes, then how. That is if this example even make sense?
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