Heads up - this post explores bike geometry in pretty significant detail. If you don’t know anything about bike geometry but want to follow along, this website is a great primer.
Last year I bought a high-end, boutique trail bike - used, of course. It was a 2017 Intense Spider, and it had been modified pretty significantly by its previous owner. The bike came with a 150 mm fork, which replaced the 140 mm fork that came stock. The shock had also been replaced with a larger, high-volume shock, which required replacing the upper rocker link to keep it from hitting the top tube.
I was suspicious of how the changes affected the bike’s handling, so I decided to measure its geometry. I set up the bike behind my building and put my camera on a tripod. I used a long lens to minimize parallax error and imported the resulting image into Fusion 360. I measured the wheelbase IRL and scaled the image so it matched the measurement. Here are the results:
There is a bit of error here, but I think this method produced better results than could be expected from, e.g., a measuring tape.
For some context, I decided to compare these numbers against the measurements for similar bikes: an unmodified Spider, the 2017 Recluse (with which my bike should have been more comparable, after mods), and the Santa Cruz Bronson and Norco Sight from 2017 and 2022 (both 27.5” trail bikes with 140-150 mm travel). I used formatting to highlight differences in the parameters I cared most about:
As expected, the updated fork, shock, and link slackened the bike a bit, reducing the head tube angle and increasing the front center. Reach has also been reduced by quite a bit, though to be honest I don’t mind that at all. Seat tube angle has been reduced a bit as well, though I’m not sure I would notice 0.8 degrees on the climbs - especially since I can theoretically just move the saddle forward a bit to compensate.
All of these changes point to a more stable bike, except one - that bottom bracket height is now considerable higher than it was to begin with. That’s easy enough to fix with some offset bushings, which effectively shorten the extended length of the shock. To figure out how much I would need, I once again used Fusion 360, this time to simulate the effects of varying amounts of offset:
And once again, I compiled the results into a table:
From this chart, it seems that 1 offset bushing with 3 mm of offset would bring the bottom bracket height into the same realm as the offerings from Norco and Santa Cruz. 2 offset bushings might further improve stability, but at the expense of more pedal strikes. Notably, the offset bushes should also act to reduce reach, seat tube angle, and head tube angle, though the don’t affect front center or wheelbase much. Based on this table, I decided to install 1 offset bushing.
One thing I haven’t yet figured out is how all these changes have affected suspension dynamics. That seems like a lot more effort. But I’m already this far down the rabbit hole, and now I’m curious, so maybe look out for a blog post on that soon…