This weekend I took a shot at cutting my front springs as an inexpensive route to stiffening up the very soft front suspension. I set up a test rig with a digital bathroom scale to make sure I wasn't too far off on my calculations. The racetech.com site is very helpful because it will tell you a suggested target spring rate. It told me I needed a .90kg/mm spring for my weight on the street. I decided to shoot a little lower and test ride for a while, and then cut more off later if really needed. Once you cut it you can't put it back.
The stock spring is linear. This is an older design than the progressive spring more commonly used today, but it makes it much easier to accurately calculate the cut needed to get a desired spring rate. Both ends of the spring are also identical, which is just one less thing to get confused. This is as good as it gets for cutting springs. In stock form it is 17 inches long. The racetech site listed the stock spring rate to be .585 kg/mm and I measured a similar value on my test rig.
From another post the formula to calculate the spring rate is:
the original spring length/ new spring length X Original spring rate = New spring rate
I ended up with a new spring length of 12 inches, that is cutting off 5 inches of spring. That kind of scared me at first
(sounded pretty drastic). That should give me about a .83 kg/mm new spring rate which was about where I wanted for starters. I verified it on the test rig after cutting and it was about what it should be.
Couple more things. Fog's tip about using a 2 claw puller to take out the fork plug is super! I can see struggling with that thing trying to get that little snap ring out. It just takes 30 secs and it's out.
16mm washers fit just right in the fork tube. I replaced the stock washer with one of these because it was much thicker and stronger for distributing the loads from the smaller surface area of the cut end of the spring to the spacer. They also make an easy way to increase the preload or length of the spacer if you need a little fine tuning. About a 1/16 inch per washer.
I used 3/4" black pipe (used for gas piping - metal, not ABS plastic) for my spacer. It has a thick enough wall so is plenty strong for the long spacer I needed. I combined all spacers into one (and preload) so ended up being 8 13/16 inches long! The outside diameter was a little smaller than I ideally would have liked but that is what fit and was readily available. $2.50 per spacer from HD or Lowes. Type K 1" copper pipe has a better outside diameter and a pretty thick wall but would cost much more. I found an old piece I may use for any adjustments I finally make.
I set my oil level at 5 3/4" to start with and will add if needed. I used 10W oil (actually Mobil 1 Synthetic ATF). The local cycle shop didn't have any good stuff, and I had this on hand. It is very temperature stable (as seen in big improvements in my vehicles cold weather shifting) and was better than the stuff at the cycle shop(also basically ATF). I expect I may be changing it out eventually as I tune but has worked very well so far.
I preloaded the spring 3/4" which I realize now is probably too low. dad had reccommended a preload between 3/4" to 7/8" for a .90 kg/mm spring and I realize now I should have gone for 7/8", maybe an 1" with my lighter .83 kg/mm (dad can correct me here) but it was late and I was tired. Fortunately, a couple 16 mm washers can fix that easy, so it will be OK.
I have not measured the sag yet so I really have no results to report technically. I managed to sneak in a quick ride yesterday and it was MUCH improved. The fork used to dive just coming off the centerstand before, now it's nice and stable. I didn't try any panic stops, but even the overall feel is so much improved. Like I noticed when I changed the rear spring, ironically, large bumps or drops were more comfortable than stock because the suspension travel was so much reduced. I did tend to feel small road imperfections more but not at all harsh. Another nice improvement was in stability in turbulence on the interstate. Now I can feel the air buffeting me from all sides but the bike stayed nice and stable. It used to get a squirmy feel in that situation as I increased my speed and I'd have to slow back down to feel safe.
Anyways, more tuning is needed but even so far the road feel of the bike is greatly improved between the proper rear spring, the cut fronts and 1 5/8" rise from dogbones. I'm hoping to go on my first "road trip" so to speak next week and I am much more confident with the improved feel.
Many Thanks to dad, Rich and Fog and everyone else who helped so much along the way! This forum rocks!
Thanks
The stock spring is linear. This is an older design than the progressive spring more commonly used today, but it makes it much easier to accurately calculate the cut needed to get a desired spring rate. Both ends of the spring are also identical, which is just one less thing to get confused. This is as good as it gets for cutting springs. In stock form it is 17 inches long. The racetech site listed the stock spring rate to be .585 kg/mm and I measured a similar value on my test rig.
From another post the formula to calculate the spring rate is:
the original spring length/ new spring length X Original spring rate = New spring rate
I ended up with a new spring length of 12 inches, that is cutting off 5 inches of spring. That kind of scared me at first
(sounded pretty drastic). That should give me about a .83 kg/mm new spring rate which was about where I wanted for starters. I verified it on the test rig after cutting and it was about what it should be.Couple more things. Fog's tip about using a 2 claw puller to take out the fork plug is super! I can see struggling with that thing trying to get that little snap ring out. It just takes 30 secs and it's out.
16mm washers fit just right in the fork tube. I replaced the stock washer with one of these because it was much thicker and stronger for distributing the loads from the smaller surface area of the cut end of the spring to the spacer. They also make an easy way to increase the preload or length of the spacer if you need a little fine tuning. About a 1/16 inch per washer.
I used 3/4" black pipe (used for gas piping - metal, not ABS plastic) for my spacer. It has a thick enough wall so is plenty strong for the long spacer I needed. I combined all spacers into one (and preload) so ended up being 8 13/16 inches long! The outside diameter was a little smaller than I ideally would have liked but that is what fit and was readily available. $2.50 per spacer from HD or Lowes. Type K 1" copper pipe has a better outside diameter and a pretty thick wall but would cost much more. I found an old piece I may use for any adjustments I finally make.
I set my oil level at 5 3/4" to start with and will add if needed. I used 10W oil (actually Mobil 1 Synthetic ATF). The local cycle shop didn't have any good stuff, and I had this on hand. It is very temperature stable (as seen in big improvements in my vehicles cold weather shifting) and was better than the stuff at the cycle shop(also basically ATF). I expect I may be changing it out eventually as I tune but has worked very well so far.
I preloaded the spring 3/4" which I realize now is probably too low. dad had reccommended a preload between 3/4" to 7/8" for a .90 kg/mm spring and I realize now I should have gone for 7/8", maybe an 1" with my lighter .83 kg/mm (dad can correct me here) but it was late and I was tired. Fortunately, a couple 16 mm washers can fix that easy, so it will be OK.
I have not measured the sag yet so I really have no results to report technically. I managed to sneak in a quick ride yesterday and it was MUCH improved. The fork used to dive just coming off the centerstand before, now it's nice and stable. I didn't try any panic stops, but even the overall feel is so much improved. Like I noticed when I changed the rear spring, ironically, large bumps or drops were more comfortable than stock because the suspension travel was so much reduced. I did tend to feel small road imperfections more but not at all harsh. Another nice improvement was in stability in turbulence on the interstate. Now I can feel the air buffeting me from all sides but the bike stayed nice and stable. It used to get a squirmy feel in that situation as I increased my speed and I'd have to slow back down to feel safe.
Anyways, more tuning is needed but even so far the road feel of the bike is greatly improved between the proper rear spring, the cut fronts and 1 5/8" rise from dogbones. I'm hoping to go on my first "road trip" so to speak next week and I am much more confident with the improved feel.
Many Thanks to dad, Rich and Fog and everyone else who helped so much along the way! This forum rocks!
Thanks
