Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/125847
• II IIGI • .... IIGI o g Z ;... g (J) W ..J .... o > o alV ::IV cO a::al ..... ~ i Q ~ -e- r- Q) ....-< O>~ I- « • ~;; - o .:!~ (J c o~E I- U w.~ en ~ 0) :: «~Oal w ~..J Q) ........ .>,. - .. :l ..0 Cl.) ~ q: ~ .... ~ ..0 When a motorcycle goes up , it usually comes down. The landing w ill be less memorable if you've mounted the correct rate shock springs. s= Boing, Boing, Boing / By John Huetter When you're setting up the suspension on your machine, whether for competition, street riding, or trailing, do you really know what rate springs on your rear shocks is going to give the most controlled, co mfortab le ride? Yo u d o ? Good, you saved yourself fro m hav ing to read th is. Please tum t he page. Bu t, if you don't really know or have a sneaking suspicion that something might be wrong since you keep bottoming your shocks and shortening your spine whenever you ride, then dig on the fo llowing sim ple ma th em ati cs which , for want of being creative, we term "\Vitham's Theorem. " Tim Witham is no t exactly a fr ousy, gray-haired, bespectacled math prof at Slippery Ro ck Teachers' College or anything like that. He builds springs; springs mainly for motorcycles; valve springs and shock springs. T o be brutally honest, he builds S&W springs, which most serious competitors agree are the best proprietary boingers around. (Webco distributes them and that's whe re we stole the fo llowing handy formula.) What spring should you be using on your mo'sickle? Well, if you're competing on dirt, then a constant wind is the only way to go . Period. The compression action will always be the same and it takes some of the surprises out of racing, or even fast cowtrailing. What rate? Well, Janene Turton uses 40-60 on he r little Bul and most Girlings from Britain come with 110 lb . springs. What will work for you is probably _ somew here in between there. Witham's Theorem, can be most simply ex pressed by t he formula R=~. Go t that? OK , go figure it out. Ha! Gotcha! You'll need to know what the symbols represent first. R is the final spring rate you should have when M is th e su m of 65% of your motorcycle's weigh t and 50% of rider weight, (Be honest, now . Nobody needs to know abo ut your shrinking k idney belt.) Y is the swingarm length from pivot point to rear axle eye divided by the distance between the swingarm pivo t point and the shock mounting eye on the swingarm. Fo r example, you haul out the bathroom scales an d det erm in e that your m o ty sickle, set up the way you want to ride it and, we assume , that's with some gas or pre-mix in the tank, weighs 240 lbs . You, you svelte devil, make the needle hover around 160 with some riding gear on. Swingarm length to the rear axle is, generously, 20 inches and the distance back to the shock mounting point is 18 inches. Now, just plug all the numbers in and whip out your handy slide rule or ele ctronic digital ca Icu la to r.. .OK, a pad and pencil if you want to get "home workshop" about it and you discover that ; R =( .65 X 240) + (.50 X 160) X 20/18 4 and therefore (Lem me see ...times five, carry the two, divide by the square root... ), it turns that out R =65.5, or 66 Ib./in.. and t hat's the spring rate to use on that motorcycle with you on 'it. It seems to make sense. The formula also seems to work in determining wh at the minimum reasonable spring rate is for the swingarm modification wherein the shocks are moved forward . Take t he same theoretical bike and rider weights (240 lbs. and 160 Ibs.) and say he's m oved his shocks forward six in ches. Quick slipsticking magically reveals that a spring rate of 85 Ib./in. is ind ica ted . T he closest rate commonly available is 88 lbs .j ln. so that would be th e way to go. Generally speaking, the softest spring you can ride on without bottoming is the right one since this will allow the shock to work the way it's su p posed to . Now if your shocks aren't too swell to start with, Web co would probably recommend that you get some of their swell S&W dampers to go with the S&W springs you've just calcula ted that you need. And on that co mm erical message, no co m m en t. e · • ~ Q! ~ i ~ ~ I ~ ~ i i q: en Ia:: 0 N Q.~O ~~O _ N O ~ x CO «0"": I-:z:; a:l= __ cnZ~..c: ~Wal(J . a:l ::J';; ... ° °c :ECa::« 9w ~ ~ I i q: ~ ~ s ~ en«N a::;90 0'- U') C~ 1-«0) 0", . ~ ... ", cU a:: '" Wen"",. c I-Q.W'" cnOo-E wO~ ::I 3: U ~ a:l ~ ~ 31