I don't understand coilover spring rates.

[949 Racing]

Bump stops are in integral ingredient to any car suspension. Engaging the bump stops on big hits is not a bad thing, it is a necessary thing. Without bump stops, you would have abrupt metal to metal contact at full bump or droop. Things would break. The key is to have tuned behavior at both full droop and full bump. With aftermarket springs or shocks, the amount of energy and space available to absorb and damp that energy changes from OEM. Meaning new bump stop shapes and materials are often needed.

What is often seen in poorly developed suspension setups is too much energy being put into stops that are not tuned to absorb it. The stop, being an elastomer, stores some of that energy then releases it too abruptly. If the shock then has inadequate low speed rebound or too much hysteresis, it bounces off the stops in an uncontrolled manner. Credit to the guys at RCE for engineering special rear bump stops with their "yellow" lowering spring kit. Sadly, most aftermarket coilover manufactures never even look at that area of the shocks performance envelope. We can sometimes just glance at a picture of a coilover kit and know the stops aren't right for the spring rate, stroke and motion ratio of the system.

In off road racing and rally cars, bottom and topping bump stops are so vital to wheel control that they are actually hydraulic and tuneable just like the shock itself. Imagine having separate bump and top bumpstops, each with two way adjustable damping.

What you are looking for is relatively transparent engagement of bump stops primarily. Meaning their contact is progressive and controlled but you know its bottoming. Bottoming in itself isn't bad, as long as its not happening constantly. In offroad bikes and cars, tuners will dial in things soft enough where it bottoms gently once or twice a lap. For pavement, we try to avoid bottoming entirely but still use all the available travel for kerb hits. An occasional light bottoming that doesn't upset the car means we are effectively using maximum travel for maximum compliance which generally results in maximum grip.

One critical factor in tuning a production car for autocross or road course is that most tracks that amateurs drive on are bumpy, often underfunded "club" tracks, not glassy smooth FIA tracks. The same suspension setup that is optimized for COTA or Laguna Seca is nearly undriveable at Willow Springs or Sebring. Parking lots are never engineered or built to be flat and smooth so trying to pull 1.6G at 70mph means compliant, not rock hard suspension tunes.

Since we don't tend to jump our 86's too much, top out stops are not so critical. It's OK to feel the shocks bottom but it should not be harsh and it should not upset the car. OEM's dial in that bump stop behavior quite well so it's good to mimic that benign character. If the car bottoms with a clack of metal, or is really harsh or bounces violently, you need to address your tuning. It could be you are simply too low for your spring rate, or you are asking too much of your system (jumping a car that's lowered 3"), need to tweak compression or rebound damping or need to try different bump stops.

When developing Xidas for any platform, we spend quite a bit of time bottoming and topping the suspension to tune its behavior in that range. A particular steep driveway into a parking lot near us is a jump we hit at 30mph both straight and angled to see how the car behaves. Bottoming is OK and expected there but it must be controlled. Another big rounded dip in the road a few blocks away, hit at 50mph with full tank and passenger, at race ride height, again to make sure its not too harsh. In those tests, we might end up changing shockpressure, nitrogen capacity, shock valving, piston design, main spring rate, helper spring rate, spring lengths, shock lengths, bump stop durometer, bump stop shape, bump stop length.

General rule is the softer the spring you run, the longer and softer your bump stops should be. Idea being you aren't slamming into them as hard. Slower, more progressive body roll and bumps in road. A pure road course setup might have very short and stiff stops. Kerb hits at high speed, more violent left-right transitions.

For a competition set up, we are looking to only engage bump stops on kerb hits and on big bumps encountered while already at max cornering G's. If you have things too low for your spring rate, you can end up parking on the stops in long turns which effectively locks the suspension out. That has the same effect as having an infinitely stiff sway bar on that end. That will greatly reduce grip and make the car jump off line with every little bump.

[jamal]

I have a hard time getting the math to work out where a 150lb/in rear spring will work. Like, it's going to compress around 5" just sitting still. How much shock travel is there, how much bump travel does that leave? Seems like you'd need a ton of preload and/or it'd mostly be the bumpstops acting as the springs.

[949 Racing]

Originally Posted by jamal

I have a hard time getting the math to work out where a 150lb/in rear spring will work. Like, it's going to compress around 5" just sitting still. How much shock travel is there, how much bump travel does that leave? Seems like you'd need a ton of preload and/or it'd mostly be the bumpstops acting as the springs.

Long spring, lots of preload, long stroke shock. Big double digressive pistons and very fine tuned shock valving make it all work.

I was actually surprised how well it worked on track. Had to steer smoother and be more patient setting up for corners but grip was high and it soaked up absolutely everything. Definitely not an optimum track setup but I think daily drivers who do that occasional 7/10ths canyon run will love it. Just buttery, planted and predictable then cushier than OEM on the highway.

Bump travel is the limit of the tires with our setup. We get this question a lot from folks that are accustomed to the shock being the limiting factor. Meaning some aftermarket or OEM/lowering spring combos run out of bump travel before all the available tire clearance is used up. In such a case the owner has to either raise the car, add spring rate, put up with too frequent bottoming or all three. Xidas are engineered to use up all available tire clearance and the struts can be raised or lowered for bump limit travel. For competition use, the suspension should always be engineered to allow big tires to travel as far up as they can go before hitting stuff. On our Miata race setups with big tires, you will see black marks on the tub up front from the tires getting every last millimeter of upward travel.

[x808drifter]

Threads like this are one of the reasons I miss being on miataturbo

[StraightOuttaCanadaEh]

Well I also don’t understand much about suspension and since getting sticky tires I’ve noticed in fast corners the car does a lot of side to side movement, almost like a busted sway bar bushing or something. After reading this thread, I went and set my coils to full stiff. Lo and behold, no movement at all now. The car is planted and confidence inspiring in the corners but bumpy as shit on the road.