May 15, 2020 at 3:20 pm #33428
Rank: 250cc Rider
Hi all, I’ve fitted some modern forks to my 1990 XTZ750 Yamaha. As it’s a 200kg bike the standard springs are too soft at about 0.47 (CRF450R) The springs are the same dimension as those fitted to some KTM’s like the 950 or 990. I’m wondering on the offchance if any owners of large KTM’s have had their fork springs upgraded and would have originals lying around. I’m after something with a rate above 0.6. Spring dimension is dia 43mm x 485mm long but obviously length can vary a little. Racetech don’t do this heavy rate and another supplier in the states would be about $400 landed so keen to try my luck here first. Thanks for your consideration. NickMay 20, 2020 at 10:26 pm #34392
Rank: 1200cc Rider
- Bike: BMW R80 G/S, R100GS & M72 Sidecar, Gilera Runner ST200
If the springs are too long, you may be able to cut them to raise the spring rate. There are calculators out there that can tell you by how much this would stiffen things. You need to ensure that the spring won’t become coil bound. Provided that this cannot occur, then you can use spacers to get back any ‘missing’ length.
I have done this on Gus, an R100GS running YZ250 forks. Raised the spring rate from 0.41 kg/mm to 0.48kg/mm. Gus is 200+kgs & ran 250mm of fork travel. The conversion write up is here
I also did a similar operation on my R80 mono sidecar to stiffen the forks.
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