Does your suspension correctly match your weight? Here’s how to find out – and get your preload right at the same time..
Ever wondered how the bike factories knew what stiffness of fork spring to choose? Or how stiff to make the spring in the shock? The answer is they didn’t, because you weren’t around when they were building your bike. So they guessed, based on the weight of the bike, and their best estimate of the average customer’s weight.
Well, maybe you’re lighter than that. Or heavier. Or maybe the factory accountants chose the springs. Whatever the reason, it’s worth finding out what’s in your suspension. Springs that are too stiff don’t allow the wheel to move enough – instead, the shocks pass through to you and the wheel bounces off the road. Springs that are over-soft do the opposite: there’s too little travel to deal with bumps. You can get different springs from suspension shops.
Spring stiffness, or rate, is typically measured in kg per mm of travel. A 1kg/mm spring, with 1kg on it, will compress 1mm; 2kg, 2mm; 3kg, 3mm, and so on. The right rate for you compresses a little bit under the weight of the bike (20-25mm front, 10-15mm rear), and between a quarter and a third of the full travel once you’re on board, sat normally, and wearing your normal riding gear. As most bikes have 120mm travel each end, that’s 30-40mm.
Tools For This Job : Bike ramp (preferably), straight edge, window sill, spirit level, workmate, wooden blocks, set square, dividers, pen and paper.
Cost : Nothing. But a pair of new fork springs or a shock spring start at about £100.
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| Start with the rear. Hold the bike upright and clamp the rear tyre using the ramp. Figure out a way to run a straight edge a couple of inches above the seat, exactly level, and directly above the rear wheel spindle. >> Measure the distance from the straight edge to the seat in three situations: 1. shock fully extended, 2. bike under its own weight, 3. bike with the rider in place. Make full extension your zero position, as in the picture. >> The difference between full extension and the VFR settling under its own weight is 12mm. With Ridertips team on the bike here (full gear, normal riding position) the distance is 30mm. Both values are within our range. Good spring, correct preload. |
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| If both values are too big, increase the preload. Too small, reduce it. If only one measure is within range at a time, the spring is wrong >> Now for the same three measurements at the front. Measuring the exposed length of stanchion on the VFR’s forks is tricky, so we used a pair of dividers – and found that Austin’s forks don’t move at all when he sits on the bike. >> If internal friction was reasonable, we could make the same checks as for the rear. The very handy racetech.com lists oe VFR springs as 0.7kg/mm, and recommends 0.85kg/mm. But even a simple rebuild would help things. |
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| The lack of movement is due to high internal friction, either by worn bushes or bent legs. You can measure the friction: push down and let the forks settle. Pull up, then let them settle again. On the VFR the difference is 11mm – 4-5mm would be nice... >> It’s easy to chew up slotted preload adjusters, which tend to be bigger than screwdriver blades. A piece of ally strip like this is actually a better fit. For nut-type adjusters, a bit of tissue paper prevents unsightly marking. >> If you aren’t in possession of a hydraulic ramp with a tyre clamp, you can do exactly the same job on a flat surface using two burly mates, plus a third one to do the measuring. It’s a bit less accurate, though a spirit level helps |