Author Topic: Final Drive Question (Read 11376 times)

gag1025 · « **on:** May 04, 2016, 12:38:41 AM »

A friend of mine sent me this statement saying and I quote "I've heard the k100 rear gears will raise your gearing 6-8%. ( K 75 is under geared, my opinion)". Anyone care to comment on this? My question to you if this is true is this. Is it worth the cost of buying a final drive unit for a 6-8% increase. Please comment.

TimTyler · « **Reply #1 on:** May 04, 2016, 12:57:14 AM »

Have a look here: http://www.largiader.com/articles/gearing/

Laitch · « **Reply #2 on:** May 04, 2016, 01:58:43 AM »

Quote from: gag1025 on May 04, 2016, 12:38:41 AM

A friend of mine sent me this statement saying and I quote "I've heard the k100 rear gears will raise your gearing 6-8%. ( K 75 is under geared, my opinion)". Anyone care to comment on this?

Does your friend mean "raise your rpm" or "raise your speed"?

gag1025 · « **Reply #3 on:** May 04, 2016, 01:43:01 PM »

Thanks Tim. Laitch, if I understand him, what he is saying is that using the K100 final drive will allow the bike to run at lower RPM's at a certain speed in 5th gear (say 60 mph) than the K75 final drive because of the difference in gearing ratios between the two final drives.

jakgieger · « **Reply #4 on:** May 04, 2016, 06:55:06 PM »

Original sales brochure from 1984 lists final drives, k100@2.91, k100rs@2.81, and k100rt@2.91. Sorry, no listing for k75.

johnny · « **Reply #5 on:** May 04, 2016, 08:21:28 PM »

you could just put a k1200 trans in there and have 6th gear...

gag1025 · « **Reply #6 on:** May 04, 2016, 08:48:05 PM »

Now that is a thought. Is it a direct exchange?

Brad-Man · « **Reply #7 on:** May 04, 2016, 09:38:12 PM »

Too much work...and not much benefit - only slightly taller 6th gear than the 5th gear you have...

Going to a taller final would mean more clutch slipping from start.

K75's are so smooth - why bother?

rbm · « **Reply #8 on:** May 04, 2016, 09:42:15 PM »

Bell housing mounting hole locations are shared between the classic K's and the early K1200's, meaning you can direct swap from the bell housing back. However, you'll have some engineering puzzles to solve - adapting the clutch spline / input shaft spacing, figuring out the mods to do to adapt the K-bike frame, adapting the hydraulic clutch to a cable pull system, and starter. But it will eventually work.

gag1025 · « **Reply #9 on:** May 04, 2016, 10:22:36 PM »

I have no desire to make this switch but always like to explore the options. Plus it makes for interesting discussion. Again my question is how would a K100 Final Drive swap decrease the RPM's in 5th gear at 60 mph.

Laitch · « **Reply #10 on:** May 04, 2016, 11:10:29 PM »

Quote from: gag1025 on May 04, 2016, 10:22:36 PM

Again my question is how would a K100 Final Drive swap decrease the RPM's in 5th gear at 60 mph.

I think the table at the link Tim supplied answers that and a few other questions, gag.

gag1025 · « **Reply #11 on:** May 05, 2016, 07:27:12 AM »

Laitch, the first table does not mention the K100 plus the fact that being new to BMW bikes (this is the first BMW I have ever owned) I have no idea which transmission I have other than it is a 5 speed. The second table says that the K75 has a final drive gear ratio of 3.36 and the K100 has a final drive gear ratio of 2.91. So, to me the first table does not give me the information I need because under the K75 listing the ratios vary from 2.54 to 3.36. Please excuse my ignorance so you tell me what your interpretation is.

Laitch · « **Reply #12 on:** May 05, 2016, 08:10:58 AM »

Quote from: gag1025 on May 05, 2016, 07:27:12 AM

. . . you tell me what your interpretation is.

I think the "etc." in the K75 etc. column includes the K100—its ratios are in that column along with the K75's. All I did was select the gear ratio I wanted to compare in that column then consulted the graphs that followed for comparison. The ratio's relationship to speed/rpm can be selected from the hyperlinks below the table.

To know which ratios to choose for comparison, go to this site to view the difference between the gear ratios of the two bikes to make choiced.

Anyway, I enjoyed exploring the table's data output. Of course, my bike's future well-being isn't at stake if I've misinterpreted something because I'm also satisfied by its current setup.

jakgieger · « **Reply #13 on:** May 05, 2016, 08:22:28 AM »

Thanks for that link Laitch. I had not seen it before...

rbm · « **Reply #14 on:** May 05, 2016, 10:43:18 AM »

If you were not aware, the gear ratio of the final drive is stamped on the drive itself - on the flat portion where the vent is located. That way you can compare tables independently of the specific model because your bike may have been modified during its life.

jakgieger · « **Reply #15 on:** May 05, 2016, 02:25:49 PM »

:dunno2: :dunno

gag1025 · « **Reply #16 on:** May 05, 2016, 05:24:26 PM »

Thanks guys for all the information. being a new BMW rider I was not aware of the transmission numbers. I appreciate your comments and especially the information.

jjs1234 · « **Reply #17 on:** May 09, 2016, 04:07:43 PM »

I bought a r1100rt (I think its rt) rim that is 18in. The normal one I think is 17in.

That extra inch might make a slight difference in RPM and Speed, also I know for a fact that a smidgen of more rubber will be in-contact with the pavement.

jakgieger · « **Reply #18 on:** May 26, 2016, 06:53:18 PM »

gag1025, don't know if you have figured out your question yet...today I found the attached chart in my 1989 owners manual, and learned one of the "special" differences. A "non" special RS will have a faster top speed

gag1025 · « **Reply #19 on:** May 26, 2016, 09:10:03 PM »

jackgeiger, I have not. Actually I have had so much going on with all my projects, work, hobbies, etc. I have not taken the time to do this. This will be my winter project. Thanks for the chart. I will make myself a copy of it and puit it in my Clymer or Haynes book. I appreciate the information.

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Author Topic: Final Drive Question (Read 11376 times)