Robert Taylor
1st July 2011, 21:51
In years past everything was a lot less critical about how many components could have their fasteners cranked up really tight to preclude them coming loose. When referring to clip on type handlebar clamps this wasnt a huge issue with older style ''right side up'' forks, as the top of the forks were well above sliding surfaces and the wall thickness of the tubes was such that it could have been sourced from the Harland and Wolff shipyards in Belfast.
Modern sportbikes with upside down forks are a different kettle of fish and some of the very recent models are EXTREMELY sensitive to the torque applied to handlebar clamps and the fitting precision of the handlebar clamp.
Why is this? The inner fork tube of course slides up and down inside the top outer tubes. At full closed distance the top face of the inner tube strikes the underside of the top fork cap. The handlebar clamp is assembled around the outside of the outer fork tube in this very vicinity. Inside the forks are two ''glide'' bushings. There is always a fixed glide bushing immediately next to the fork seal area at the lower end of the outer tubes. In many designs there is a second fixed bushing that is maybe 80 to 100mm below the top of the forks, importantly below the clamping area of the handlebar clamps.
BUT, more and more recent designs have followed MX fork practice in having a glide bushing that is fitted to the inner tube so that it moves with the inner tube. This has one merit because it is assembled into a recess that is barrel ground so that it can rock a little, reducing fork stiction under the very real deflective loads that occur. BUT, that same glide bushing at full closed distance is moving into the very area of outer tube that is clamped by the handlebar clamp.
OEM handlebar clamps are extremely well toleranced so as to minimise squeezing the top of the forks out of round, as long as the manufacturers torque setting is not exceeded, but preferably less than that setting. Its common practice on many of these handlebars that they will have some alignment pegs that will only allow the forks to arc through a very narrow radius, thats a good safety feature if the clamps work loose. Alternatively there may be a lockbolt into the underside of the top triple clamp. Some manufacturers also place circlips around the outer tube not far below the underside of the handlebar clamp, as a further backup to stop the alignment pegs disengaging should the handlebar work loose But racers being racers they often like to change the handlebar position by a fair amount so they grind off the alignment / limiter pegs. And then they ham fistedly crank on the lockbolts because they are ( understandably ) paranoid about the handlebars coming loose. Many pit helpers who are otherwise not mechanics will also likely not have this issue on their radar screens
Modern sportbike forks are very light and part of that lightness comes from increasingly thinner wall top outer tubes. If you crank on those lockbolts excessively you WILL distort / squeeze that fork tube out of round and that will cause a LOT of stiction as the sliding glide bush moves into that area. The scary thing here is that it may be totally imperceptible in the pits but when on track and undergoing deflective loads the forks basically stick / temporarily lock up at close to or at full closed distance
In other threads Ive been scathing on occassion of a number of aftermarket parts that are churned out on otherwise idle CNC equipment. Much of this stuff looks very nice and works very well but at times you think more credence is paid to making something that looks nice but doesnt work very well e.g the industry cancer of ''lowering links'' No less so for aftermarket handlebar clamps. This afflicted Craig Shirriffs terribly until in our busy stupour we worked out what was happening. He fitted a set of clamps from a company ( no Im NOT naming them ) that otherwise make some very nice stuff. There was more depth in the clamps and two lockbolts per clamp. So what was happening was the inner tube and glide bushing was engaging into the ''squeezed area'' sooner. With two lockbolts how can you also attain even torque and therefore consistent pressure through the full depth of the clamp, especially as the material thickness was somewhat less rigid than the stock clamps. Allied with that the bore tolerance was not as accurate as oem and some internal taper relief on the inside of the split may have reduced localised pinching of the forks.
But fundamentally with all handlebar clamps ( including oem ) the bore is machined and then there is a split machined and a lockbolt ( or multiple lockbolts ) added to squeeze it tight on the outer tube. BUT YOU ARE SQUEEZING THE BORE OF THE CLAMP OUT OF ROUND. Engineering bastardised by accountants, really familiar.
For those in disbelief of this try loosening a top fork cap with the top triple clamp pinch bolt still torqued up, then retry with that same bolt first loosened...........
In Craigs case we came across the issue whilst changing fork springs and preparing for an oil level change. Undo the top caps and let the forks fully compress to the bottom so the bike is sitting fully nose down, Set oil level, change the springs and then get the boys to lift the bike so that the stand could be placed underneath it. To our horror the forks stayed fully compressed and this was the very first time we had experienced such a scenario. I immediately knew what it was and told Craig to chuck the aftermarket clamps as far away as possible, and then fit oem. Immediately he went from struggling to competitive, followed by a maiden win at Manfield.
Bikes that have this sliding bushing design include all those equipped with Showa BPF forks I.e year 10 onwards ZX6, year 11 ZX10, CBR1000 08 on ( non BPF ) , year 10 on GSXR1000 and year 11 GSXR600.
With a couple of year 11 ZX10 forks that we have had a[part this week it appears that Showa may have recognised this issue and bandaided it by moving the glide bushing downwards so that it doesnt get so far into the handlebar clamped area, if at all. I say bandaided because it then means that at full open ( extension ) distance there is less disposition between the two load bearing surfaces, making for a weaker and ''stickier'' / flimsier fork. Compared to a true thoroughbred racing fork all of these modern sportbike forks are very flimsy, this sadly is as good as it gets with what we are allowed to race with. Race bred? Yeah right.....
My advice, stay well away from most if not all aftermarket handlebar clamps or at least be VERY attentive to the issue.
Modern sportbikes with upside down forks are a different kettle of fish and some of the very recent models are EXTREMELY sensitive to the torque applied to handlebar clamps and the fitting precision of the handlebar clamp.
Why is this? The inner fork tube of course slides up and down inside the top outer tubes. At full closed distance the top face of the inner tube strikes the underside of the top fork cap. The handlebar clamp is assembled around the outside of the outer fork tube in this very vicinity. Inside the forks are two ''glide'' bushings. There is always a fixed glide bushing immediately next to the fork seal area at the lower end of the outer tubes. In many designs there is a second fixed bushing that is maybe 80 to 100mm below the top of the forks, importantly below the clamping area of the handlebar clamps.
BUT, more and more recent designs have followed MX fork practice in having a glide bushing that is fitted to the inner tube so that it moves with the inner tube. This has one merit because it is assembled into a recess that is barrel ground so that it can rock a little, reducing fork stiction under the very real deflective loads that occur. BUT, that same glide bushing at full closed distance is moving into the very area of outer tube that is clamped by the handlebar clamp.
OEM handlebar clamps are extremely well toleranced so as to minimise squeezing the top of the forks out of round, as long as the manufacturers torque setting is not exceeded, but preferably less than that setting. Its common practice on many of these handlebars that they will have some alignment pegs that will only allow the forks to arc through a very narrow radius, thats a good safety feature if the clamps work loose. Alternatively there may be a lockbolt into the underside of the top triple clamp. Some manufacturers also place circlips around the outer tube not far below the underside of the handlebar clamp, as a further backup to stop the alignment pegs disengaging should the handlebar work loose But racers being racers they often like to change the handlebar position by a fair amount so they grind off the alignment / limiter pegs. And then they ham fistedly crank on the lockbolts because they are ( understandably ) paranoid about the handlebars coming loose. Many pit helpers who are otherwise not mechanics will also likely not have this issue on their radar screens
Modern sportbike forks are very light and part of that lightness comes from increasingly thinner wall top outer tubes. If you crank on those lockbolts excessively you WILL distort / squeeze that fork tube out of round and that will cause a LOT of stiction as the sliding glide bush moves into that area. The scary thing here is that it may be totally imperceptible in the pits but when on track and undergoing deflective loads the forks basically stick / temporarily lock up at close to or at full closed distance
In other threads Ive been scathing on occassion of a number of aftermarket parts that are churned out on otherwise idle CNC equipment. Much of this stuff looks very nice and works very well but at times you think more credence is paid to making something that looks nice but doesnt work very well e.g the industry cancer of ''lowering links'' No less so for aftermarket handlebar clamps. This afflicted Craig Shirriffs terribly until in our busy stupour we worked out what was happening. He fitted a set of clamps from a company ( no Im NOT naming them ) that otherwise make some very nice stuff. There was more depth in the clamps and two lockbolts per clamp. So what was happening was the inner tube and glide bushing was engaging into the ''squeezed area'' sooner. With two lockbolts how can you also attain even torque and therefore consistent pressure through the full depth of the clamp, especially as the material thickness was somewhat less rigid than the stock clamps. Allied with that the bore tolerance was not as accurate as oem and some internal taper relief on the inside of the split may have reduced localised pinching of the forks.
But fundamentally with all handlebar clamps ( including oem ) the bore is machined and then there is a split machined and a lockbolt ( or multiple lockbolts ) added to squeeze it tight on the outer tube. BUT YOU ARE SQUEEZING THE BORE OF THE CLAMP OUT OF ROUND. Engineering bastardised by accountants, really familiar.
For those in disbelief of this try loosening a top fork cap with the top triple clamp pinch bolt still torqued up, then retry with that same bolt first loosened...........
In Craigs case we came across the issue whilst changing fork springs and preparing for an oil level change. Undo the top caps and let the forks fully compress to the bottom so the bike is sitting fully nose down, Set oil level, change the springs and then get the boys to lift the bike so that the stand could be placed underneath it. To our horror the forks stayed fully compressed and this was the very first time we had experienced such a scenario. I immediately knew what it was and told Craig to chuck the aftermarket clamps as far away as possible, and then fit oem. Immediately he went from struggling to competitive, followed by a maiden win at Manfield.
Bikes that have this sliding bushing design include all those equipped with Showa BPF forks I.e year 10 onwards ZX6, year 11 ZX10, CBR1000 08 on ( non BPF ) , year 10 on GSXR1000 and year 11 GSXR600.
With a couple of year 11 ZX10 forks that we have had a[part this week it appears that Showa may have recognised this issue and bandaided it by moving the glide bushing downwards so that it doesnt get so far into the handlebar clamped area, if at all. I say bandaided because it then means that at full open ( extension ) distance there is less disposition between the two load bearing surfaces, making for a weaker and ''stickier'' / flimsier fork. Compared to a true thoroughbred racing fork all of these modern sportbike forks are very flimsy, this sadly is as good as it gets with what we are allowed to race with. Race bred? Yeah right.....
My advice, stay well away from most if not all aftermarket handlebar clamps or at least be VERY attentive to the issue.