5X Racing Mini Cooper Bump Stop Kit

The factory designed in understeer by using a long and rather firm front bump stop and a softer rear stop. Plus, the rear gains camber faster than the front: 0.4 deg per inch of compression (front) vs. 0.66 deg per inch of compression (rear). All these factors plus the rear trailing arm design that provides toe-in under compression leads to terminal understeer BUT with a propensity for unforgiving lift-throttle oversteer mid-corner with an unweighted rear-end.

The 60mm OE front bump stops (~200 lb/in rate) touch after 20mm of wheel travel in compression. The rear 56mm bump stop touch after 40mm of wheel travel. It's clear the car was meant to corner on the front bump stops! To optimize this situation for stock or lowering springs (especially for Stock class autocross), we consider the ratio of the motion ratios (0.89 / 0.65 front/rear). For reasonable ride and also better handling, we suggest keeping the rear length at 56mm, and using a shorter/softer density front stop. Theoretically, that would be ~41mm front and 56mm rear. We supply a 47mm strut front (100 lb/in) which is a legal length and there is 13mm of room to add additional bump stop spacers to adjust the car's balance. Especially if you decide to also add a larger rear sway bar, having additional front bump stop packers on hand is a good idea to further tune the car's balance. For stock class, we suggest a 47mm front stop (~100 lb/in rate) and a 200 lb/in rear bump stop (our 58mm white). The rear stops need to be no longer than 56mm (what we measured as OE length) to maintain SCCA-legality with stock-length shocks. I suggest cutting 2-3mm from the thicker end of the bump stop, so the rear spring rate does not ramp too abruptly. The 'nose' of the bump stop helps make the engagement gentle and seamless. For the front bump stops, you may need to add the grey spacers shown in the image below if there is too much oversteer. The grey spacers are larger ID to accommodate a shaft vs. the shock-type black spacers for the rear. With this combination of FCM bump stops and tuning flexibility with the front and rear spacers, your stock class Mini will have improved front grip and be very tunable while you take advantage of a larger, adjustable rear bar. The car's grip will improve as the suspension stays balanced through the compression travel range, inspiring further confidence which will also lead to faster lap times.

Partnering with Fat Cat Motorsports allowed us access to their proven engineering and techniques on how to custom tune your suspension by way of bump stops. When the average person thinks about suspension, they think springs, shocks, and bushings. Bumpstops are the "cushion" between the shock body and the shock mount, without them the metal shock body would come in contact with the metal shock mount during full shock compression causing damage, and a terrible ride! The factories take bumpstops into consideration, however they tune them to be used with factory shocks and springs and often use inferior materials like rubber (which is unforgiving when compressed) or lower quality MCU (micro-cell urethanes) which can't handle the thermal loads as the higher quality MCU bump stops we use. When you start customizing your suspension by replacing the shocks and springs, the factory bumpstops now become mis-matched to the rest of your suspension. Most people will try and reuse the factory bumpstops, but this will not yield optimal handling characteristics because of the now lower and shorter suspension travel plus lower bump stop quality. When you upgrade your springs and shocks to lower your car or if you simply want to improve the handling and durability of your stock suspension, bumpstops can be the "missing link" in your suspension that you never thought of!