Wheel arches : sacrificial bolts

TL;DR protect the car body with weak fasteners

This modification is quite common on race Caterham cars, as their rear wheel arches are considered as mere consumables.

Replacing the galvanized steel bolts with nylon ones that will break more readily will protect the rivnuts and the skin of the car in case of contact or crash. Only the plastic bolts should snap. I've read somewhere that about a third of the fasteners are discarded, and that pilots often score the threads halfway through.

The steel bolts are standard M5x20mm with 8mm hex heads. Washers with a diameter of 3/4" (19 mm or so) are used to support the fiber arches. I thought it was to better spread the load but after inspection I come to think it's really just a matter of making with very approximate tolerances and fit.

Caterham themselves do sell a kit of nylon M5x25mm bolts (reference #BM5X25*20 "BOLT M5 REAR WING FIXING RACE APPLICATION").

It happened that I had that exact dimension already in inventory, in black nylon.

I bought a set of 5.2 / 20 mm galvanized steel washers that I quickly spray painted black to match. The new kit does really blend with the black interior side of the arches, giving an extra chance to stay unnoticed at the MOT-equivalent.

As I didn't have the patience for 18 hours of curing, the painted washers were dried for 3 hours at 70°C in a food dehydrator (don't worry it's only used in the shop). It smelled quite a lot in the room.

The change is better done with the rear wheels off. I did remove one side at a time.

Fasteners were replaced one by one, ensuring that the arches wouldn't move.

About everything with a ratchet was used but the best tools were the screwdriver with the long flexible extension, and the miniature 'hex bits' ratchet.

Note that the two bottom bolts on the front need to be shortened to 20mm. A thin metal blade did a quick job of that. The bolts were held firmly in the vise from the extra thread length I was getting rid of. As usual when shortening a thread, a metal nut was screwed in fully before cutting, allowing for a good re-threading of the end.

A corner of carpet was peeled gently from each side of the boot in order to access the bolts that are fitted 'inside-out' from there. The bottom-most ones will wait for the removal of the boot floor. The carpet was re-glued with some adhesive spray.

Note that the assembly manual shows 5 sets of inside-out bolts per side, but my 170S just has 3 in the boot area. The two bottom-most ones were actually rivnuts with bolts coming in from under the arches.

At the rear of the right side, the second bolt from the bottom is used for some electrical grounding. There's a nylstop nut to remove from behind first. As the round spade is pressed against the rivnut, the bolt and nut can be replaced by nylon ones while assuring a good grounding point.

Regarding weight, the change saved a bit over 70 grams. As I've replaced the 6 M5 screws of the air box at the same time (also M5x25mm, with tons of washers), the total weight saving for the day is a bit over 100 grams.

(assembly as show in the manual)