Upgrades 23. Springs

I've always been underwhelmed by the look of the stock springs at the front of the car, they look a little bit cocktail stick like to me. Once I got to see a set of race springs on a 620 in the flesh I decided I wanted to achieve that look, bigger, fatter and more colour coded! Before making this change I have racked up 1000 miles in the car and from this I can work out if I want to stiffen up the front at the same time. The default spring rate is 170lb/in with a 1.9" x 7.5" spring. A lot of people go straight to a 250lb/in (2.25" x 7") spring, which is what Caterham offer, but I think this may be too stiff for me so I've opted to go with a 200lb/in (2.25" x 7") spring, which sits in the middle of the two offerings nicely.

For the record, the standard rears that my kit was shipped with is 140lb – 250 lb/in (part no 74076, which is probably standard on most rears anyway). I'd love to know what Caterham actually supply as their standard "Track suspension" upgrade from the configurator, I'm pretty sure it is just the two fronts upgraded to 250lb/in given the £200 price tag, which would cover the 2 x 250lb front race shocks price.

Next was a colour choice, I really wanted to go white, but decided that this would just end up being a nightmare to keep clean as my white 7 badge at the front is bad enough to keep clean. So I opted for red, which matches the colour choice of the 420 Cup.

As well as buying the new springs (Which I got from Faulkners Springs), I needed to buy new upper spring retainers to fit the 2.25" spring, which were sourced from Caterham and cost more than the new springs themselves.

I bought the set in red, and then after a bit of a play with some white insulation tape, decided that red was the right choice and that I didn't need an extra powder coat to change them to white.

Having built the car myself, removing the damper and replacing it was trivial, I jacked the car up and then rested it on some axle stands, and then removed both front wheels (I could have done this without removing the wheels, but they would have just got in the way, and the extra weight would have been awkward when re-aligning the damper and bolts again). Removal of the damper was easy enough, it involved pushing the aluminium skin around to get the top bolt out, but nothing that worried me after having done it at the build phase. Once the dampers were out I measured the distances that the existing springs covered (pre loaded), so I could set the new springs to the same pre loaded rate.

I also took the time to take some photos of the original part numbers (before removing stickers) so if I never needed to revert them or source originals, I knew what pieces my car came with (for some reason my detailed pick list didn't contain the damper product codes)

So on with the damper strip and rebuild. Once the dampers were taken off the car I measured various parts of the spring to calculate the current preload, and the extension of the current spring. The existing springs are slightly longer, but less force, so I wanted to make sure that when I put the new ones back on, I didn't mess up the current setup too much. Then it was a case of using the spring compressors (I used Sealey Motorbike ones) to compress the existing spring, and then the upper retainer just slides off. I needed to slide the rubber bung down slightly but this was fairly easy. Once the upper retainer was off, I loosened the spring compressor so the spring was no longer loaded, and removed the spring.

The Caterham Bilstein dampers come with adjustable ride height/platforms for both 1.9" and 2.25" springs. So in order to accommodate the larger spring, I needed to flip the platform over. Both grey rings simply unscrew off the top, then you can remove them, flip them and put them back on the other way around. I then set them so the platform gave a similar distance from top to bottom to that of the original spring setup (I tweak this later to be more exact).

Then the new spring is placed on the damper and compressed, then you can put the new upper retainer in place. Once you are at this point you can then start to de compress the existing spring to tighten the damper back up. At this point I then worked out what distance the upper and bottom retainer needed to be to give the same preload as the original spring, I've only done this to keep the front springs setup in a similar fashion to the originals, the ride height will be slightly changed because the spring is stiffer, however I'm booked into Northampton Motorsport for a full flat floor platform setup in a few weeks time so this will be changed by them anyway.

So after lots of measuring with the calipers and tape measures, the original spring was preloaded at 176mm (top to bottom), the spring length is 190mm, rate is 170 lb/in (29.82 N/mm).

Compression => (190-176) = 14mm
Force => 14 * 29.82 = 417.48N
Effective mass under gravity => 42.5Kg

The new spring is 179mm long, with a spring rate of 200 lb-in (35.09N/mm), so to work out the preload compression to match it:

417.48 / 35.09 = 11.89mm (Compression)
Preload length = 179 - 12 = 167mm

So with this in mind, I needed to make the distance from the top to the bottom 167mm for the same preload as the original spring. I'm sure I've probably gone a bit far on all this but I wanted to make sure what ever I changed was at least mostly correct in my mind. Of Course the spring is stronger, and therefore the ride height will likely be higher anyway now. It is going to be quite interesting to see what Northampton Motorsport change when flat flooring it. I'll post up about that once it has been done.