I know the subject of Bonneville fork springs is mentioned elsewhere but it seems to be mixed in with topics on shocks and brakes and all sorts, so here's a separate thread (until someone hijacks it). I've just fitted progressive fork springs in place of the OEM linear springs on my 2017 Street Twin. They're from TEC. I also fitted fork top adjusters. I didn't like the look of the TEC adjusters so went with YSS. Despite the freezing temperature I took the bike out today and the handling is noticeably improved. I was hoping for a firmer ride with better damping and less dive under heavy braking. And that's what I got. After 35 miles my hands were numb so I didn't get a chance to twiddle with the adjusters, so I can't comment on their affect yet. But it's definitely a worthwhile move.
I fitted the TEC progressive fork springs to both my Street Scramblers for a big improvement over standard.
This might be useful to anyone thinking of doing something similar. The TEC springs are slightly shorter than the Triumph original linear springs. If the TEC adjusters are fitted they compensate for this difference. The YSS adjusters I used do not give quite the same 'depth' of movement as the TEC ones (naively I assumed they would all be the same). To make up the difference I added two extra washers per side. These washers are shown in the Street Twin schematic (one per side). My bike now has 3 per side, instead of one, which basically makes up for the difference in spring length. Hope that makes sense.
In my view a better option is to go with linear springs based on your weight. I've done that on several motorcycles, including my 2023 ST 900. They work really well and provide consistent damping throughout the forks range. They also control front brake dive much better. I've always found progressive springs too soft at the beginning of travel and then too hard at the end.
When I get the chance to spend more time out on the bike I'll have a better idea of how this set up is working. From my short test ride the front end feels better than it did and if I can improve it even more by twiddling the adjusters then that's good enough for me. I'm certainly not going to take it apart again and try weight-based linear springs as a comparison. Although I'm sure anyone else considering this upgrade will appreciate your suggestion.
Be careful with your choice of socket for the YSS fork tops. I used a single hex but still managed to leave markings on the corners when trying to undo them only ONE WEEK later My youngest dropped the 900 in the garage and badly scratched the right leg
The difference in length is only a few mm. The washer (or spacer as Triumph call it) in the Street Twin fork leg is 1.5mm thick, so by adding 2 more it filled the 3mm gap. It just seemed the quickest and easiest method.
Nothing wrong with using washers/spacers to bring the preload to the right amount. In many years gone by I've used sockets to increase the amount of preload and it also worked fine.
Or alternatively, use a bigger socket with a cable tie insert like I've done previously Works a treat when anticipated
Yeah, good idea. Seems daft to buy a set of plastic sockets when occasionally only one will be used and tbh once I've adjusted the suspension I rarely, if ever, touch it again.
I took a freshly powder coated nut up to 90 nm using a wide cable tie inside a 4 mm too big socket, with no marking whatsoever Note; Cut the length of the cable tie to just under 6x the flat side length of the nut and allow a gap for any squash in the middle of one flat.
