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Many of us are "upgrading" their front suspensions with more recent front forks but is it all good or not ?
There is in fact a simple equation that links the different elements that define the bike's geometry.
R: front wheel radius
D: offset between fork legs and fork axis ( measured directly on top yoke for std front forks )
Alpha: rake angle of the fork ( measured to horizontal axis) expressed in degrees
T: trail ( distance between the point of contact of the front wheel and the intersection of the fork axle with the ground).
The equation is:
T=R/tan(alpha)-D/cos(90-alpha).
Let's take an example with an 1100 Kat
Alpha= 60.5°
D= 50 mm
R= 320 mm
T= 320/1.767-50/.87=123.63 mm
Now let's fit a GSXR 1100 RSU fork with 17" wheels while keeping the same ride height by lengthening the front fork legs.
R=300 mm
D= 32 mm
T= 300/1.767-32/.87=133 mm
So here we have considerably increased the trail despite a smaller wheel because of the decreased offset of the GSXR fork.
To correct this we can lower the front to increase the rake angle.
Let's take one degree increase corresponding to roughly a 26 mm lowering for a bike with 1515 mm span between the wheels.
Alpha= 61.5°
T=300/1.842-32/.879=126.5 mm
We are now very close to the bike's original geometry.
To conclude, I would say that it's quite safe to change to more modern forks BUT you will typically loose some nimbleness due to the increased trail unless you lower the front end that in turn will make you loose ground clearance
Life is not so easy...
There is in fact a simple equation that links the different elements that define the bike's geometry.
R: front wheel radius
D: offset between fork legs and fork axis ( measured directly on top yoke for std front forks )
Alpha: rake angle of the fork ( measured to horizontal axis) expressed in degrees
T: trail ( distance between the point of contact of the front wheel and the intersection of the fork axle with the ground).
The equation is:
T=R/tan(alpha)-D/cos(90-alpha).
Let's take an example with an 1100 Kat
Alpha= 60.5°
D= 50 mm
R= 320 mm
T= 320/1.767-50/.87=123.63 mm
Now let's fit a GSXR 1100 RSU fork with 17" wheels while keeping the same ride height by lengthening the front fork legs.
R=300 mm
D= 32 mm
T= 300/1.767-32/.87=133 mm
So here we have considerably increased the trail despite a smaller wheel because of the decreased offset of the GSXR fork.
To correct this we can lower the front to increase the rake angle.
Let's take one degree increase corresponding to roughly a 26 mm lowering for a bike with 1515 mm span between the wheels.
Alpha= 61.5°
T=300/1.842-32/.879=126.5 mm
We are now very close to the bike's original geometry.
To conclude, I would say that it's quite safe to change to more modern forks BUT you will typically loose some nimbleness due to the increased trail unless you lower the front end that in turn will make you loose ground clearance
Life is not so easy...
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