Author Topic: 91 k100RS vs 93 K100LT forks  (Read 4398 times)

Offline Filmcamera

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91 k100RS vs 93 K100LT forks
« on: November 13, 2018, 04:46:23 PM »
The forks on my bike are old and pitted and pretty beaten up so when I got the chance to by the forks from a 93 K1100LT the other day very cheaply I went for it.


First I checked on Max BMW and the part numbers are the same so I thought I would be good to go.


I was wrong...


When I went to fit them today it seems the forks are slightly different.  Not hugely so, the LT forks are maybe 5mm shorter than the RS ones.  I also have the pre 8/91 forks with a valve in the bottom on my RS compared to the LT forks which have a regular screw.


Does anyone know the spring rate differences etc between the two forks?


I am thinking of going ahead and installing the LT forks despite the small differences but if they are WAY softer or WAY stiffer I might not bother.


Any insight would be greatly appreciated.
  • San Jose, Costa Rica
  • 1991 K100RS 16v ABS1, 2018 Triumph Street Triple RS 765
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Offline The Mighty Gryphon

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Re: 91 k100RS vs 93 K100LT forks
« Reply #1 on: November 13, 2018, 04:56:43 PM »
Owning both an RT model of the K75 and a K100RS sister ship I can suggest that the LT forks may have more travel and softer springs.  I haven't checked the specs on travel, I'll leave that to you. 

I do know that my RT has more brake dive than my RS, but, very surprisingly, the RS has a smoother, plusher ride over the crap paving in the communist republic of New York.  I have no idea why this is so.
  • In my garage in Marilla, NY
  • '91K100RS White/Blue
Current:
'91 K100RS16V "Moby Brick Too"

Past:
'94 K75RT "Ilsa, She Wolf of the SS"
'92 K100RS16V "Moby Brick" (RIP, deceased in a vehicular assault)
'94 K75S Special Edition Dakar Yellow "Cheetos"
'89 K100RS Special Edition "Special Ed"

Offline Laitch

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Re: 91 k100RS vs 93 K100LT forks
« Reply #2 on: November 13, 2018, 06:27:57 PM »
When I went to fit them today it seems the forks are slightly different.  Not hugely so, the LT forks are maybe 5mm shorter than the RS ones.  I also have the pre 8/91 forks with a valve in the bottom on my RS compared to the LT forks which have a regular screw.
Please post images of each and give us the part number
  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles
I wept because I had no radials until I met a man who had no splines.
https://tinyurl.com/RillRider

Offline Filmcamera

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Re: 91 k100RS vs 93 K100LT forks
« Reply #3 on: November 13, 2018, 06:47:01 PM »
I will try and post pictures tomorrow. The part numbers are identical.  The entire fork is  31 42 2 312 911.


Actually looking at the diagrams again more closely it says from 8/91 for noe fork and to 8/91 for another one and the previous part number is 31 42 2 310 795.


That is actually encouraging it seems that from 8/91 the RS had the same forks as the 1100 LT. My bike was manufactured in 9/90 so has slightly different forks as shown by the locking screw at the bottom of the fork which was 31 42 2 310 804 to 8/91 and wa a valve type which became  31 42 2 332 056 post 8/91.

If the forks were the same in both the K110LT and the K100RS after 8/91 logic would seem to indicate t I should be able to just do a swap with no major ill effects.  I doubt the whole front end is much different since it seems only the forks are different, the triple trees etc. all stay the same, even the fork slider inner tube part number stays the same after the absorber part number changes (31 42 2 310 799).


  • San Jose, Costa Rica
  • 1991 K100RS 16v ABS1, 2018 Triumph Street Triple RS 765
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Offline The Mighty Gryphon

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Re: 91 k100RS vs 93 K100LT forks
« Reply #4 on: November 13, 2018, 09:09:52 PM »
It looks like the travel is the same at 5.31 inches.  Curb weight is a lot higher for the LT, 639 pounds vs 518 pounds for the K100RS.  This means the spring rate for the LT is probably going to be about 15-20% higher. 

If you change forks, you might want to consider not swapping the springs.
  • In my garage in Marilla, NY
  • '91K100RS White/Blue
Current:
'91 K100RS16V "Moby Brick Too"

Past:
'94 K75RT "Ilsa, She Wolf of the SS"
'92 K100RS16V "Moby Brick" (RIP, deceased in a vehicular assault)
'94 K75S Special Edition Dakar Yellow "Cheetos"
'89 K100RS Special Edition "Special Ed"

Offline Filmcamera

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Re: 91 k100RS vs 93 K100LT forks
« Reply #5 on: November 14, 2018, 10:54:08 AM »
Here are a couple of pictures of the forks


The one on the right is the current fork and the one on the left from the LT.  I measured and the one from the LT is 7mm shorter.








The spring inside the shock absorber has the same part number for both which would seem to indicate it is the same spring so same spring rate etc.



  • San Jose, Costa Rica
  • 1991 K100RS 16v ABS1, 2018 Triumph Street Triple RS 765
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Offline The Mighty Gryphon

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Re: 91 k100RS vs 93 K100LT forks
« Reply #6 on: November 14, 2018, 11:42:16 AM »
I avoid doing anything beyond draining and refilling the forks on my bikes.  I find them to be confusing beyond my ability to figure them out. 

From the stuff I have glanced at, I would suspect that while the externals are the same there is something that is different in the guts.  How that translates to how they work or the difference in length is beyond me.
  • In my garage in Marilla, NY
  • '91K100RS White/Blue
Current:
'91 K100RS16V "Moby Brick Too"

Past:
'94 K75RT "Ilsa, She Wolf of the SS"
'92 K100RS16V "Moby Brick" (RIP, deceased in a vehicular assault)
'94 K75S Special Edition Dakar Yellow "Cheetos"
'89 K100RS Special Edition "Special Ed"

Offline Laitch

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Re: 91 k100RS vs 93 K100LT forks
« Reply #7 on: November 14, 2018, 12:29:48 PM »
Thanks for providing the photos and verification. If both pairs are fully operational, mount whichever is preferred. I could not allow a 7mm length difference to pilot an obsession; however, I believe equipment purchased from others should be thoroughly examined before hard usage. If I were intending to mount a newly-purchased used fork assembly, I'd be compelled to disassemble it, inspect the components, compare their layout to diagrams, resolve differences that arise during that comparison, clean the parts and reassemble them.

Now, whoever likes to do that, please raise your hand. :giggles
  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles
I wept because I had no radials until I met a man who had no splines.
https://tinyurl.com/RillRider

Offline The Mighty Gryphon

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Re: 91 k100RS vs 93 K100LT forks
« Reply #8 on: November 14, 2018, 12:54:32 PM »
I agree with Laitch about the disassembly.  Anyone who has been around K bike forums for any length of time knows of several, at least, instances where owners have gone into their forks and found parts broken, assembled incorrectly or missing, not to mention having two different fork legs on the same bike.
  • In my garage in Marilla, NY
  • '91K100RS White/Blue
Current:
'91 K100RS16V "Moby Brick Too"

Past:
'94 K75RT "Ilsa, She Wolf of the SS"
'92 K100RS16V "Moby Brick" (RIP, deceased in a vehicular assault)
'94 K75S Special Edition Dakar Yellow "Cheetos"
'89 K100RS Special Edition "Special Ed"

Offline Laitch

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Re: 91 k100RS vs 93 K100LT forks
« Reply #9 on: November 14, 2018, 07:02:21 PM »
The one on the right is the current fork and the one on the left from the LT.  I measured and the one from the LT is 7mm shorter.
The K100RS4V should have a 17" front wheel and and 18" rear wheel. The K1100LT should have an 18" front wheel and a 17" rear wheel. The differences in fork tube length might be to accommodate the differences in wheel size and their effects on handling.
  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles
I wept because I had no radials until I met a man who had no splines.
https://tinyurl.com/RillRider

Offline Filmcamera

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Re: 91 k100RS vs 93 K100LT forks
« Reply #10 on: November 14, 2018, 07:40:35 PM »
Yeah that could be for sure.  Good idea about dismantling the forks to see wat is going on inside, I have been a tad shy about doing that to my forks but now that I have a donor pair I will it give it a go.


For now all I have done is remove the current forks and change the oil - which was WAY low - 200 ml in the left and 300 ml in the right when it should be 400 ml in each.  I suspect that alone shold have a major effect on how it handles. 


I have never taken a fork leg apart before. All I did before was change the oil and the seals, I never actually opened up the guts to get to the spring. Any tips or tricks?
  • San Jose, Costa Rica
  • 1991 K100RS 16v ABS1, 2018 Triumph Street Triple RS 765
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Offline Laitch

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Re: 91 k100RS vs 93 K100LT forks
« Reply #11 on: November 14, 2018, 10:54:56 PM »
I have never taken a fork leg apart before. All I did before was change the oil and the seals, I never actually opened up the guts to get to the spring. Any tips or tricks?
How did you remove the seal?
  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles
I wept because I had no radials until I met a man who had no splines.
https://tinyurl.com/RillRider

Offline Filmcamera

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Re: 91 k100RS vs 93 K100LT forks
« Reply #12 on: November 15, 2018, 06:46:13 AM »
What I have not done is get to the spring itself inside the shock absorber tube.


I have removed the shock absorber from the fork tube do get to the seals and replace those.
  • San Jose, Costa Rica
  • 1991 K100RS 16v ABS1, 2018 Triumph Street Triple RS 765
Poserbricker

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