Posted by little man upon the stair on October 24, 2009, 10:00 pm
> 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/threa=
d/47927af34e5a9560/6476145991efd6f5?hl=en&lnk=gst&q=%22good+vibration=
s%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.
Posted by Greg.Procter on October 24, 2009, 10:15 pm
On Sun, 25 Oct 2009 15:00:40 +1300, little man upon the stair
>> 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
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)
A lighter wheel can obviously follow irregularities quicker than a heavier
wheel. (sprung vs unsprung weight)
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"?
>> - 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.
"Hobby-horsing"? (rebound(?))
(Sorry, possible language problem there)
Have you considered damping rates???
Regards,
Greg.P.
Posted by little man upon the stair on October 25, 2009, 10:19 am
> 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.
.
Posted by Greg.Procter on October 25, 2009, 4:31 pm
On Mon, 26 Oct 2009 03:19:06 +1300, little man upon the stair
>> 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 :-)
Posted by little man upon the stair on October 25, 2009, 10:25 am
> 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.
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