As there has been a lot of talk regarding the bump stops and how they may affect lowering the A4, we thought it would be helpful to share a few of our thoughts and measurements.
Purpose: The bump stop functions as a helper spring, adding spring rate to the coils in order to prevent the shock from bottoming. It will also limit overall suspension travel. Each bump stop features a progressive spring rate, usually with a soft initial engagement that will get firmer as the bump stop compresses further.
Trim the bump stops? This is a frequently used term when lowering a vehicle, and is usually the subject of debate. When you lower a vehicle, the shock rod length becomes shorter, thus decreasing the distance until the bump stops engage. To keep this distance the same as stock, many users will opt to trim the bump stops.
B9 A4 Rear Standard (8W0512131F) Bump Stop Length: 4.90” / 125mm
B9 A4 Front Standard (8K0412131E) Bump Stop Length: 3.189” / 81mm
B9 A4 Front Sport (8K0412131F) Bump Stop Length: 2.834” / 72mm
B9 A4 eMMOTION Front Bump Stop: 2.150” / 55mm
How do we know how much to trim?
By taking the motion ratio (a factor of how much the wheel travels in relation to the coil/strut), we can get a pretty good idea of how much should be trimmed for proper functionality.
Front Motion Ratio: 0.75
Rear Motion Ratio: 0.80
This means that for every inch the wheel travels, the shock/strut will travel by approximately the above values. So in an ideal world, the front bump stops would be 0.75” shorter for every inch the car is lowered. In most cases, this isn’t possible due to shock travel and/or wheel interference, so a compromise is made.
On a B9 A4 (standard suspension) test vehicle, which was lowered 1.6” front and rear, we measured the shock rod lengths at:
This means that on the standard suspension (lowered 1.6") without any bump stop modification, you will be sitting on the bump stops with the car at ride height. In terms of feel, this will lead to a firmer feel with a more notable bounce as the dampers try to handle the increased energy. We have found the best results on the non-sport suspension by trimming 1.2" in the rear, combined with our front bump stops. This allows enough travel for the shock rod, while still limiting rubbing with aggressive wheel setups (some offsets could rub with this amount of travel). This number will change based on how much the car is lowered on stock shocks/struts, but the motion ratios can serve as a good reference point for calculating how much to trim. We always recommend starting with less, and trimming more if needed. We prefer to trim from the top of the bump stop, as this is the firmest rate portion. Hope this helps answer some questions!