Shortening front forks.
in [Tech]
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From: Greg.Procter on 24 Oct 2009 22:45 On Sun, 25 Oct 2009 15:00:40 +1300, little man upon the stair <macmiled(a)gmail.com> wrote: > On Oct 24, 6:31�pm, "Greg.Procter" <proc...(a)ihug.co.nz> wrote: > >> 27" rims are generally safe! > > Modern sportbikes have tires with an outer diameter of 22 to 25 > inches, anything smaller will lose traction on all but billiard-smooth > pavement and anything larger than 25 inch diameter will out-track > badly when the rider tries to turn into a corner. > >> - Bigger wheels don't fall into big pot-holes as badly as small ones. >> - Smaller wheels give better acceleration. ;-) >> - Smaller wheels have less gyroscopic effect. (rider/bike stability) >> most other effects relate more to suspension than size. > > I got heavy into this business several years ago and was the OP that > started this thread. > > http://groups.google.com/group/alt.motorcycle.sportbike/browse_thread/thread/47927af34e5a9560/6476145991efd6f5?hl=en&lnk=gst&q=%22good+vibrations%22#6476145991efd6f5 > > I got into the vibration problem in the first message and went on to a > study of spring preload and how to adjust sag to avoid hobby-horsing > over bumps at the desired cruising speed. > It strikes me that you are concerned with how to get the best performance from existing suspension systems, rather than how to build the best/ most practical suspension. (practical vs theory) That's fair enough. One's theoretical motorcycle and rider must cope with both raised bumps and dips in the surface. My theory says the suspension needs to be set mid-point in it's travel with the bike normally laden at rest. The standard of the road surface will dictate whether one wants that rest point to be above or below the half way point. The length of movement, vehicle speed and spring rate determines the spring design. Preload is something to be considered when attempting to improve an existing suspension. With zero unsprung weight (unachievable) one would need (almost) no spring preload. One needs the wheel to remain in direct, weighted contact with the road surface, but the unsprung to sprung weight ratio in conjunction with the change in spring pressure with the extension due to that movement affects the rate at which the suspension will move. (also of course the rate at which the bike and rider move vertically) Coil springs are effectively torsion springs which have (almost) no rebounce. (naturally damped) Of course, the total suspension design etc means that dampers are required. I remember riding on an old concrete slab road where each slab was slightly tilted. I watched the front wheel rise and fall over each slab and settle a tiny fraction higher each time (somewhat higher speed than legally acceptable) Eventually the suspension hit the top stop and I was forced to back off. Watching the suspension rebound was also interesting!
From: little man upon the stair on 25 Oct 2009 10:19 On Oct 24, 7:15 pm, "Greg.Procter" <proc...(a)ihug.co.nz> wrote: > That would be a matter of suspension rather than tyre diameter. (down to a > point where the radius is greater than about 1/3rd the depth of ruts, > holes and corrugations) Back in the late 1960's, most of the motorcycles known as "scramblers" and "enduros" which were available to the off-road enthusiasts in America had 19-inch front wheels. We were amazed by the improvement in ride and precise steering offered by the 21-inch front wheels that European motocrossers favored. The outside diameter of the tire on the 21-inch wheel really wasn't larger than the outside diameter of the 19-inch wheel, but Honda caught on to the possibility of larger diameter wheels and equipped one model with 23- inch wheels. That only lasted one or two model years on their XR models in the 1970's... > "Hobby-horsing"? Don't parents give their kids rocking horse down in NZ? When your front wheel hits a bump, it compresses the spring and compression damping and the ratio of sprung to unsprung weight play a role in limiting the upward movement of the handlebars. But the handlebars are connected to the chassis and a certain moment is transferred through the springs and rear dampers to the rear wheel *before it hits the same bump that deflected the front wheel. Then the rear wheel hit the same bump a fraction of a second later and the process is reversed as the bump is transmitted back through the chassis to the front wheel and this chassis rocking is called "hobby- horsing". > Have you considered damping rates??? Hydraulic dampers cannot work if there is very little motion of the fork or shock. In order to eliminate hobby-horsing on California freeways, one has to carefully adjust front spring sag so it's about twice what you'd use on the rear springs. ..
From: little man upon the stair on 25 Oct 2009 10:25 On Oct 24, 7:15 pm, "Greg.Procter" <proc...(a)ihug.co.nz> wrote: > On Sun, 25 Oct 2009 15:00:40 +1300, little man upon the stair > and anything larger than 25 inch diameter will out-track > badly when the rider tries to turn into a corner. > > Is there something magic about 25"? No, it's not magic, the optimum tire size and weight distributions of modern sport bikes have been arrived at by trial and error and the perceptions by experienced riders as to what feels right to them. We saw sportbike manufacturers adopt 16-inch front wheels in the early 1980's to speed up steering response. But motorcycles equipped with 16- inch front wheels would not recover from a front tire washout, the riders could expect to crash. The 17-inch front wheel has been universally adopted as the standard for sportbikes. But some professional GP riders like 16.5-inch wheels for the quicker steering.
From: little man upon the stair on 25 Oct 2009 10:53 On Oct 24, 7:45 pm, "Greg.Procter" <proc...(a)ihug.co.nz> wrote: > It strikes me that you are concerned with how to get the best performance > from existing suspension systems, rather than how to build the best/ most > practical suspension. (practical vs theory) That's fair enough. Most sportriders have no interest in designing a motorcycle, they just want to get their machine's suspension system adjusted so they can ride with their friends. When they read the recommendations of the motorcycle magazines, which are made by expert riders after riding around a race track that they know very well, the recommendations will be for a race track set up which will not be very comfortable on California's rough freeways, where each pavement section is set at a slightly different angle than the next. Even in cases where the freeway was paved in one continuous pour of concrete, which is brushed to enhance traction before it dries, the pavement will have height irregularities. When attempting to ride on freeways so paved with a race track suspension set up, I encountered eyeball jiggling so bad that I couldn't focus on distant objects. When I backed off spring preload to the point where half the travel was used up in sag, suddenly turning into a tight corner resulted in severe out tracking as weight transferred to the front tire. > One's theoretical motorcycle and rider must cope with both raised bumps > and dips in the surface. My theory says the suspension needs to be set > mid-point in it's travel with the bike normally laden at rest. Race track set ups usually recommend that front spring sag be somewhere between 25% and 33% of the total travel. I wound up with about 40% sag as the best compromise. Also, the Bridgestone BT-020 sport touring tire that I was using was about one inch larger in diameter due to a taller sidewall. I changed to a tire which more closely resembled the favored sporty sport tire profile and diameter to eliminate the out tracking, which was always noticeable at any speed. > I remember riding on an old concrete slab road where each slab was slightly > tilted. I watched the front wheel rise and fall over each slab and settle > a tiny fraction higher each time (somewhat higher speed than legally > acceptable) > Eventually the suspension hit the top stop and I was forced to back off. > Watching the suspension rebound was also interesting! Your front suspension was packing down because the fork oil was too thick. Damper rod type forks are very sensitive to fork oil weight, that's why cartridge forks with their stacks of flexible washers to act as a variable orifice were invented. And the Gold Valve Emulators offered by Race Tech are a much cheaper alternative to buying a whole new cartridge fork to replace a damper rod fork. Of couse the ultimate fork would be an Ohlins, with damping that's adjustable at both high stroking speeds and low stroking speeds. Nevertheless, a hydraulic damping system cannot damp motion very well if the suspension units aren't moving quite a bit. Then traction reverts back to the tire contact patch and the damping caused molecular friction (aka "hysteresis") in the rubber has to do all the work of absorbing those tiny bumps that may only be 1/4 of an inch high. And larger diameter tires will do a much better job than a small diameter tire in such cases. But, consider a go kart chassis which has no suspension at all. The go kart driver expects to be turning constantly and he *wants* his rear tires to be "loose" so he can turn rapidly. Mini-bikes often have tires that are about the size of go-kart tires and some of the smaller scooters have tires that aren't much bigger, so they will feel "loose" at rather low speeds, compared to the speeds that I ride my sportbike at on the highway.
From: The Older Gentleman on 25 Oct 2009 11:27
little man upon the stair <macmiled(a)gmail.com> wrote: > On Oct 24, 12:34 am, totallydeadmail...(a)yahoo.co.uk (The Importunate > Sniveller) wrote: > > > Do you have a minimum size that you consider to be safe? If so, what? We > > need to know. > > Who's this "we" you're referring to? Are you in bed with Spacker > again? I thought that was your 'thing'. No, come on, this being a tech forum, we really need to know what 'moderate sized wheels' are. What diameter? Because obviously we don't want to be unsafe, do we? -- BMW K1100LT Ducati 750SS Honda CB400F Triumph Street Triple Suzuki TS250ER (currently Beaving) Damn, back to five bikes! Try Googling before asking a damn silly question. chateau dot murray at idnet dot com |