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Discussion Starter #1
Greetings from RedHook NY.
Hi to all members, I'm new here and hope to be around for a while and contribute as much as possible. I'm in possession of a 1991 K75S and would like to lower the front end some but can't find formulas or procedures on how to go about it.
The reason I found this forum and subscribed was because of a Google image from member Wydglyd on his K100 build and upon reading his intro he mentions lowering his forks by 45mm, but no mention of process.
So my inquiry, would greatly appreciate any input regarding lowering BMW forks.

TIA, Pp.
 

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do you want to reduce the travel, or shorten the total length and keep the same travel, or a combination of both? do you know when the front wheel/guard hits the engine or lower triple?

neither is really a good idea handling wise, altho springing the front with reasonable springs will reduce the travel it uses anyway.

has it got a 17" front wheel?
 

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Redhook eh....Go walk over to Moto Bogortaro and ask Peter if he thinks this is a good idea. He's at 97 union street.

In the mean time go to the bottom of the page and use the calculator:
https://www.rbracing-rsr.com/rakeandtrail.html

then loosen the trees and slide the front forks up in the trees until the bike is lowered to where you want it and retake the measurements and re enter them into the calculator. You want to make sure you don't have a dangerously small amount of trail. What dangerous? well it varies based on factors but if you are under 90mm you probably don't want to ride the bike.

to put this in perspective, most sport bikes like the yamaha r1 have between 91 and 103mm of trail and FLH Harley has 157mm of trail. Trail is your measurement for stability. Too little and your front wheel is like the front wheel on a shopping cart (shopping carts have negative trail), too much and the bike won't turn at speed or will run wide in turns.
 

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Discussion Starter #7
Thanks guys for the responses, up front sporting 18 rim with 100/90 Pilot.

Will check out Peter on Union St.

Have decided to move clippons to very top of tubes after I slide them up just enough to get the bars on.

Thanks for the input.
Pp.
 

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the only dimensions that are needed are rake, wheel diameter and triple clamp offset. problem with something like a k75, with the soft fork springs (and no compression damping), is that the rake will probably change when you sit on it and the front sags more than the rear, and again when you're out on the road cafe rrrrrracin' and you hit the front brakes hard before tipping it in and the front dives like crazy and the rear probably lifts. for every inch you move the front up or down, you'll change the steering head angle by about 1 degree. a 1 degree change in steering head angle will change the trail by about 8mm. so you could get a 1" change in trail from cruising to braking hard.

those things allegedly have 7" of fork travel and 4 1/2 at the rear, so the front's going to be all over the place. reducing the fork travel and stiffening it up probably wouldn't be a bad thing.
 

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Discussion Starter #10
"Brad black" you took the thought right out of my head.
I was thinking about stiffening the front springs and maybe using a different grade of fork oil in order to keep the front forks from diving so much.

Pp.
 

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they're damper rod forks, they don't do low speed compression.

you can put emulators in them, or even adapt some cartridges in. the sports valves have both rebound and comp adj, but the prices scares a lot of the not so serious off.

7" of travel is nuts. well sprung 4" should be fine.
 

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Discussion Starter #13
Found leaky tube while rearranging clippons and headlight and removing accordion tubes.
Ordering seals and springs and some other assorted parts, in the meantime will tackle exhaust and electrical.

If anyone has any resources for simplifying wiring please let me know.

TIA.
Pp.
 

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Found this @ Dennis kirk site, a simple weekend upgrade that should improve ridability.
https://www.denniskirk.com/progressive-suspension/fork-springs-35-50-spring-rate-lbs-in-11-1126.p58967.prd/58967.sku

Pp.
they're more accurately a dual rate spring. to be truly progressive the gap between the various coils has to increase incrementally. the rate they give is 0.63 - 0.89 kg/mm, which is too soft to on the soft side. that's what the std "too soft" ducati monster and ss forks springs are, and the k75 will be heavier on the front.

the main issue i have with "progressive" springs is that they generally start off being too soft and giving a heap of sag (unless you preload the shit out of them), then they get too hard at the end. especially if the oil height is a bad spec and gives a lot of air spring effect.
 

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Discussion Starter #15
Thanks Brad Black for the insight, now I know why so many go for Honda and Yamaha forks in order to improve on this design.
In the meantime replacement forks aren't on the menu or budget so will have to do my best with what I have.

One other thing I noticed is that there is no dust shield on the lower legs after removing gaiters ,was hoping to get rid of them, don't like that BMX look they give.

Any input will be greatly appreciated.

Pp.
 

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Go back and re-read Brad Black's posts. Shorten the travel an inch or 2, fit the progressive springs and use slightly heavier fork oil set to the correct level. Put in emulators if you want to spend the money.
 
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