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Old 05-10-2012, 09:34 AM   #15
BMWzenrider OP
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Joined: May 2003
Location: CheeseHead Land...
Oddometer: 1,130
Originally Posted by SantaRosa View Post
Is there a height difference? Is it possible to use this method to gain or lose height?
It all depends upon the tire you select to mount on the new 15" rim.
With a 15" car rim there is a wide range of tire sizes which will fit in the space between the swingarm and exhaust can.
How that will affect ride height is that it will raise/lower the rear axle by 1/2 of the difference in tire diameter. And you will probably notice about 1/2 of THAT difference at the seat/pegs. So going with a tire that is 1" smaller diameter would result in a change at the seat of around 1/4".
If you also go with a smaller section tire up front or do something else to lower the front ride height, then the entire change in height would be seen at the seat.

Be aware that changing the rear tire size too much without also altering the ride height at the front will change your steering head angle and trail figures. Which might affect how the bike handles.

The stock rear tire on the R1200GS is a 150/70R17.
According to my Metzeler catalog the Tourance in that size has a new tire diameter of 650mm (25.6" dia.)
The more dirt oriented Metzeler Karoo II in that size is listed at 667mm (26.3" dia.), so it will raise the rear axle by .35" (8.5mm).

The tire that most of the GS riders who convert to a car tire setup use is the 165/80R15.
Depending upon manufacturer, that tire is listed as being 25.0-25.5" in diameter when new.

On my sidecar rig I run a smaller diamter rear tire in order to drop the overall gearing of the drivetrain.
It would be equivelant to changing the gearing in the final drive unit, but much less expensive to try...

My current tire choices for my bike is a 165/60R15 for the summer. Diameter 22.8"
And a 145/65R15 for the winter, with a diameter of 22.4".

There is also a 155/60R15 which is slightly smaller at 22.3" diameter if you really want to get maximum drop and lower gearing in your bike.
That tire would put the rear axle about 1.65" (42mm) lower than stock. But remember by dropping the rear axle that far without altering the front ride height it will also result in the head angle changing by around 1.6-degrees (reduced steering angle, and increased trail).

{ tan(angle) = rise/run = (change in axle height)/(wheelbase) }

Wasn't that simple...
I'm not weird; I'm gifted.
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