K75 final drive ratio change

[snowcat]

I have the opportunity to change my 88K75C 3.20:1/drum 18" rear to disk with a 17" wheel and a 2.82:1 final drive off of a K100RS.
1. Is this a fairly easy swap?
2.Will I need a different RH footrest plate for the mastercylinder?Everything else is included.
3.How will the Ratio/wheel size change affect the bike?
4.Will there be a noticeable difference at all?
I'm looking to upgrade my rims in the future so I'll need to convert to disk eventually and since I need tires,suspension and a spline lube anyway......
thanks
Dave
88K75C

[roy]

I have the opportunity to change my 88K75C 3.20:1/drum 18" rear to disk with a 17" wheel and a 2.82:1 final drive off of a K100RS.
1. Is this a fairly easy swap?
2.Will I need a different RH footrest plate for the mastercylinder?Everything else is included.
3.How will the Ratio/wheel size change affect the bike?
4.Will there be a noticeable difference at all?
I'm looking to upgrade my rims in the future so I'll need to convert to disk eventually and since I need tires,suspension and a spline lube anyway......
thanks
Dave
88K75C

Dave, I would be concerned that the change in tire size may screw-up your speedometer. Since these are electronic pulse pick ups I am not sure if the calibration would be the same. example 100 pulses = 1 MPH

There was an article in the IBMWR tech pages on changing these ratios but I do not remember it covering going from drum to disk brakes.

3a) If the rear of bike is lowered to much it will probably effect how it handles and corners.

I am sure there are people on this forum with more knowledge than me so lets hope they answer.

[deilenberger]

I have the opportunity to change my 88K75C 3.20:1/drum 18" rear to disk with a 17" wheel and a 2.82:1 final drive off of a K100RS.
1. Is this a fairly easy swap?
2.Will I need a different RH footrest plate for the mastercylinder?Everything else is included.
3.How will the Ratio/wheel size change affect the bike?
4.Will there be a noticeable difference at all?
I'm looking to upgrade my rims in the future so I'll need to convert to disk eventually and since I need tires,suspension and a spline lube anyway......
thanks
Dave
88K75C

Dave - it's certainly doable.

I'll answer by the numbers:

1. Depends on your wrench skills. It involves not only R&R of the rear drive, but also installation of the complete brake system off the donor bike. If you get the system intact and not open to air - piece of cake. If it has been open to air - then it's more involved since at a minimum you have to bleed it.

2. Shouldn't. The same footrest plate was used for both models. You will need a new brake pedal besides the master cylinder (and various springs..) Not sure on the brake light switch since that changed in design and position on the K's.

3. Without knowing the rolling diameter of the tire on the 17" rim, hard to say. You're going down in ratio - which would normally result in lower RPM's for a given speed, but the smaller wheel/tire will tend to counteract that.

4. Dunno. Haven't done this, and haven't even ridden a drum brake equipped K75, so I have no feel for how they are compared to a disk brake model.

Best,

[snowcat]

The final K100RS final drive is an 86 and the early style so I would also need the drive shaft.
Now I'm having 2nd thoughts.
Other than fit is there a difference?
Why did they change the design?
Dave
88K75C

[deilenberger]

The final K100RS final drive is an 86 and the early style so I would also need the drive shaft.
Now I'm having 2nd thoughts.
Other than fit is there a difference?
Why did they change the design?
Dave
88K75C

Dunno why they changed the tooth count. Both wear if not kept lubed. Aside from the tooth count - they are identical.

[PGlaves]

I did a similar swap and it was simple. The brake system is a straight bolt-over.

But - and this is a big but. I went to the RT 2.91:1 final drive and it was maybe borderline too tall - that is a 10% change. I like it at interstate speeds but not running two-lane back roads. I now have a 3.20:1 in the bike. I'm planning a 49 state ride in May with lots of interstate routing - so the 2.91 is going back in. I would not consider a 2.82. You won't like it!

[snowcat]

Thanks for the info,
I found a 3.20:1 rear drive with good splines(got lucky)on E-Bay so I'll Keep the same ratio.Only now with a 17" rim.
great homepage by the way and good luck with your trip
Dave
88K75C

[PGlaves]

Thanks!

Don't forget the little return spring on the pedal that goes forward from the pedal to between the two centerstand springs.