I have encounted some problems while attempting to have some fork springs made for my KTM 200 enduro bike.  I was trying to increase the spring rate from 4.0 N/mm to 4.3 N/mm.

This would result in an increase in the compressed length of the spring of 35mm.

This would reduce the wheel travel by 25mm (we calculated 10mm safety buffer on the spring) and produce a potentially damaging situation with the spring potentially completely compressing.

The preload could be decreased but I am sure the relationship between the spring rate and preload will not necessarily be consistant with the ride hieght of the bike.

The engineer wanted to use an even heavier wire again, still at 4.3N/mm, reducing the wheel travel by 55mm, requiring a further reduction in pre-load, he wanted to make the spring as long as posible inside the parameters required.

I was not sure how this would have effected ride hieght.

Is there a design table for this type of calculation working out the relationship between:-

Dimensions of the spring
        Length
        Coils
        Wire Diameter
        Compressed length
        Internal Diameter

        AND

The required spring rate

        AND        

Wieght supported by the spring and the static sag at that wieght?.

Thanks

I'm not a guru but here's what the gurus have told me.

You didn't specify what year your bike is but if it's a 1998 or newer you can get front springs in a variety of rates made specifically for your bike from www.eibach.com. ; Check out their application guide.  

I suggest if they don't have it ask them for the names of other manufacturers that might or ask them if they can make one.  Also keep in mind that if you stiffen the front you should stiffen the rear or the handling may suffer.

The compressed length of the spring should not be a concern if it's made for the bike the spring should never compress on itself before the fork bottoms and therefore would not reduce your travel.

As far as spring rate vs preload I don't believe that preload changes the rate unless you are using a progressive rate spring.  The main purpose of the spring is to hold the bike up at the proper ride height so that the suspension operates optimally through the stroke.  Preloading would raise the bike but that's not necessarily a benefit.

Sure their are design tables but why redesign a spring if aftermarket springs are available.  I also suggest revalving your suspension to suit your riding style along with the new springs.

You can also contact Precision Concepts who did amazing stuff on my suspension and they also do Johnny Campbells XR650's.  Call Bob Bell at (619)593-7473, he can set you up!

Happy Trails,

Steve Hughes
steve@888teleman.com