oddball Posted August 11, 2011 Report Share Posted August 11, 2011 All, After my son riding his Rookie 80 (2007) for the first time in anger this week, it as apparant that the forks are too stiff for him to get hardly any movement from them when transfering his weight about on the bike - yes all 5 stone of it! so apart from feeding him up, I need to make the forks more suitable for him. I have stripped one side down to the point of removing the top cap, pulled back the plastic sleeve and undone the locknut - removed the topcap and spring, and drained the fluid According to the handbook it should have 140cm3 of fluid, I have drained 225ml from one side.I 'think' - according to google that 1cm3 =1ml. If this is correct, could this be the problem that the forks are overfilled?? Or would another solution be to cut the preload spacer / plastic tube down some to give the first part of the suspension more give? - if so how much? it measures 92mm long as it is - made of black plastic. Never done bike suspension before so thought i'd ask rather than cock it up Thanks in advance for any help / advise Quote Link to comment Share on other sites More sharing options...
fastducs Posted August 11, 2011 Report Share Posted August 11, 2011 I don't have a Rookie 80, so I can't comment on that particular bike, but I've had to adjust suspension for several other bikes for my son. Have you already ruled out excessive stiction (e.g. bad seals or bushings) as well as fork misalignment? If the forks are twisted in the triple clamps, or if they are not parallel between the axle and the triples, then they will bind for sure. The triple clamps themselves can be tweaked, or twisted relative to each other. The front fender also acts as a fork brace, so if it's not installed with proper amount of shimming, it will pull (or push) the forks out of parallel. Same goes for improperly tightened axle. There is a bunch of fork alignment info on the web. Here is one example: http://w6rec.com/duane/bmw/forktool/index.htm Too much fork oil will make the last part of fork travel overly stiff. But from your post it sounds like your son is having trouble with the first part of travel, which is mostly a matter of spring rate. If you cut down the spacer, you will be reducing preload, which may help somewhat but it may not be enough compensation for a too-stiff spring rate. Basically there is no substitute for correct spring rate, and preload spacer is only there to do minor adjustments (primarily to set sag correctly). The first thing you'll need to do is to measure the spring rate. If you have a suspension shop, then they can easily measure it for you. Or you can measure it yourself using a ruler and a digital weight scale. Spring rate is measured in kg/mm so place the spring on the scale, compress it by 100mm and then divide scale reading by 100. There is also a formula floating around, but I don't know how accurate it is, since it assumes all steel springs have the same spring constant. K= (W^4 x G)/ (8 x N x D^3) Where: K = Spring rate in pounds per inch W = Diameter of the spring wire in inches G = 12,000,000 for steel springs (a constant) N = Number of active coils (number of coils that are free to move + 1/2 coil) D = Diameter of the coils in inches (I believe D is measured to the center of the spring wire. So, take the overall diameter of the coil spring and subtract the diameter of the wire (W) to get D.) Quote Link to comment Share on other sites More sharing options...
mcman56 Posted August 11, 2011 Report Share Posted August 11, 2011 I agree with fastducs. But, if you decide you do need a softer spring rate, you could replace the plastic spacer with a spring. You would need to find one of an appropriate size. Some bikes come stock with a two spring set up. Some shocks are like that too. I believe the formula for two springs is: 1/Ktotal = 1/K1 + 1/K2 The number itself may not mean much to you but would give some gage for comparison....like spring X is 10% stiffer than spring Y. Quote Link to comment Share on other sites More sharing options...
jse Posted August 12, 2011 Report Share Posted August 12, 2011 I agree with fastducs. But, if you decide you do need a softer spring rate, you could replace the plastic spacer with a spring. You would need to find one of an appropriate size. Some bikes come stock with a two spring set up. Some shocks are like that too. I believe the formula for two springs is: 1/Ktotal = 1/K1 + 1/K2 The number itself may not mean much to you but would give some gage for comparison....like spring X is 10% stiffer than spring Y. We used to do that with the old Bultacos at times. Engine valve springs work well. Any engine repair shop will usually have a bunch you can have for free. Jon Quote Link to comment Share on other sites More sharing options...
oddball Posted August 13, 2011 Author Report Share Posted August 13, 2011 Thanks for help I have gone down to 2.5 sae fluid and reduced the preload by 14mm, just left enough to stop the spring rattling. On re-assembly I checked that shock alignment was ok, also very carfull fitting the brace and axle to ensure a nice smooth action. The result : excellent The bike now has about 30mm sag / Droop when he's on it at standstill, and he can actually compress the forks about another inch or so with his own weight - just need to work on technique now and keep feeding him pies Quote Link to comment Share on other sites More sharing options...
jse Posted August 13, 2011 Report Share Posted August 13, 2011 just need to work on technique now and keep feeding him pies That's always worked for me! I get more traction than anybody. Jon Quote Link to comment Share on other sites More sharing options...
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