Swingarm is 20in. pivot to axle. Shock mount was 3in. in front of axle. Shock mount has been moved to 5in in front of axle. How much of an effect will that have on the springs? What are the odds that increased preload will compensate without going stupid on the preload. Shocks were plenty firm in original position.
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Engineering Question
- Thread starter inreb
- Start date
It may not be as simple as that. The force at the new shock mount location will be higher; 20/17 vs 20/15 (~ 13%). However, the geometery of the shock to swingarm will have also changed. The shock may be more straight up and down compared to before. This will increase the capacity of the spring, while reducing travel.
Sounds like a bad idea to me.
By decreasing the angle the SPRING is on you're changing it's rate. With the spring on an angle you get a progressive rate, at 90 degrees you lose that.
How much are you trying to move the swingarm and why are you trying to move the swingarm?
Do you know about "jacking" and "squat"?
Have you seen Tony Foales site? http://www.tonyfoale.com/ If not, go there and read everything on that site and maybe buy a couple of his books. (weren't you the guy that had a steering neck modified by CMR Racing? If so talk to Denis before you do anything).
By decreasing the angle the SPRING is on you're changing it's rate. With the spring on an angle you get a progressive rate, at 90 degrees you lose that.
How much are you trying to move the swingarm and why are you trying to move the swingarm?
Do you know about "jacking" and "squat"?
Have you seen Tony Foales site? http://www.tonyfoale.com/ If not, go there and read everything on that site and maybe buy a couple of his books. (weren't you the guy that had a steering neck modified by CMR Racing? If so talk to Denis before you do anything).
Thanks. Denis is very busy, it wouldn't be right to bother him with this.
It's a Triumph Scrambler, identical (chassis wise) to the Bonneville T100 except it has 360mm shocks compared to 340mm T100 shocks. That's right, they just slapped 20mm longer shocks on to give the bike a different pose. People are putting on 370mm to 400mm shocks with no ill effects. I want to get the same ride height as a 375mm shock but with a shorter shock, hence moving the shock forward. The bike is not on the road at the moment but doing the bouncy test it seems ok. The reason I ask this question now is because I'm placing an order shortly for other items and would like to include new springs if need be.
Moving the attachment point on the swingarm forward has some other side effects because of the increased leverage that the load at the rear wheel has on the shocks. It will act like a lower spring rate, and it is a squared relationship to the leverage ratio. But as others have mentioned, the different angle of the shocks also changes the leverage ratio (and this will vary with suspension travel). Analyzing and predicting the overall effect is not simple.
Disregarding the effect of the shock angle, the new spring rate will act (15/17)^2 = 78% of the old spring rate because of the change in leverage. This is not a small change, and the damping rates will also change in that direction (but not by the same amount, because of the shock valving - the damping forces are not proportional to shock speed).
If the shocks and springs were "plenty firm" in the original position then maybe this will make them "just right"
I'd just give it a go, and ride it to see how it feels.
Disregarding the effect of the shock angle, the new spring rate will act (15/17)^2 = 78% of the old spring rate because of the change in leverage. This is not a small change, and the damping rates will also change in that direction (but not by the same amount, because of the shock valving - the damping forces are not proportional to shock speed).
If the shocks and springs were "plenty firm" in the original position then maybe this will make them "just right"
I'd just give it a go, and ride it to see how it feels.
Thanks Brian P. I went from 15m to 18mm preload on the shocks. Slid the forks up 5/8 in. put another 10mm preload on that, called the insurance, and took it for a rip. Everything feels fine. I could leave it. Definitely turns in quicker tho. When I had the shock springs out I played with the full range of damping adjustment (60 clicks) from nothing to full hard. I can find something to suit.
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