Shortening front forks.
in [Tech]
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From: The Older Gentleman on 25 Oct 2009 11:27 little man upon the stair <macmiled(a)gmail.com> wrote: > Modern sportbikes have tires with an outer diameter of 22 to 25 > inches, anything smaller will lose traction on all but billiard-smooth > pavement Utter, utter nonsense. -- 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
From: The Older Gentleman on 25 Oct 2009 11:38 little man upon the stair <macmiled(a)gmail.com> wrote: > 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, Ah, so we're talking about six-inch diameter wheels, now? Rather a different kettle of fish. Not 'moderate' sized wheels, which is what you first banged on about. So you're moving the goalposts. It happened quicker than I expected. -- 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
From: Greg.Procter on 25 Oct 2009 16:23 On Mon, 26 Oct 2009 03:53:43 +1300, little man upon the stair <macmiled(a)gmail.com> wrote: > 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. Of course, but one needs to understand how the system of wheels,suspension and geometry work if one wants to make real improvements. I'm in the situation of a near blank drawing board with only rake and wheel size fixed. Admittedly, the original suspension worked exceptionally well, but I'm uncomfortable with it's strength. (or apparent lack of strength for the use I intend for it) > > 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. > I'm not really sure what you mean by "out-tracking"? (perhaps the infamous separation of two cultures by language :-) >> 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. > Yes, that sounds about right. > 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. Bear in mind that that stretch of road is the only concrete slab road I know of in New Zealand. I never felt the need to change damping rates elsewhere in NZ. > > 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. I'm stuck with the original wheel diameter of 12" plus 3.50 - 19" overall diameter. That factor is fixed for authenticity. > > 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. The cart has two driven tyres on a rigid axle the ground and generally a much smoother surface than I drive on. > > 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. > As I say, my wheel diameter is fixed - I need the suspension to do the work.
From: Greg.Procter on 25 Oct 2009 16:31 On Mon, 26 Oct 2009 03:19:06 +1300, little man upon the stair <macmiled(a)gmail.com> wrote: > 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. We went with the Europeans, but I always thought the US market was poking itself in the eye demanding the smaller front wheels. > > 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? I certainly fell on my head from one of those - nowdays I think they are becoming PC incorrect ;-) > > 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". It's just the different terminology - plus of course you yanks have tended to have heavier and softer sprung m/cs which would be more prone to such antics. > >> 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. Different road conditions here :-)
From: Greg.Procter on 25 Oct 2009 16:36
On Mon, 26 Oct 2009 03:25:31 +1300, little man upon the stair <macmiled(a)gmail.com> wrote: > 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. Of course. But what you perceive may be different to my perception thereof. Also, I'm aiming at a much lighter bike that that which you might consider a "normal" sports bike. (With only one leg, I can't pick up a dropped 700lb bike. 300-400lb would be my absolute maximum) > > 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. > -- Using Opera's revolutionary e-mail client: http://www.opera.com/mail/ |