In the midst of a yet another Cafe build, I love my LT but my legs were too long for the fairings,
 
I have a rough understanding of bike Geometry and seen loads of K bike cafes with frame modifications, different forks.... but not seen much on how that change effected the ride.
 
Selfishly some guidance on my situation would be appreciated.... Which also may help others looking to do the same maybe not because of height.
 
I'm 6'5'' and would like the cafe stye/look whist still retaining some comfort (and saving my back!!!). I have been playing with the ergonomics and have done the following which feels pretty comfortable but before I alter anything permanently wanted to see what others thought or experience after doing the same. Last thing I want to do is build a beautiful bike (IMO) and it ride like a death trap.
 
I have dropped the triple clamp/tree raising the forks and placed clip Ons above the top triple tree (about a 5cm drop). 
I intend to run two new steel tubes from under the tank to the tail and raise the tail by about an inch-inch and a half this will result in needing to re-attach the rear suspension - I could potentially drop the tail height to maintain a closer match to the original geometry (Need to check how this will impact the travel distance of the rear wheel and drive train and clearance. Equally this then helps lower my centre of gravity.
 
 

The geometry of a motorcycle concerns the chassis, suspensions, wheels and direction specs.
You're worrying about the ergonomics ( and its look?) , that's not the same thing...

First: the K100 very unique feature is that it was design to eliminate most of efforts when riding it.
For this, the center of gravity has to be on a line going from the bottom of the steering axle to the point where the rear wheel meets the ground.
Some people accused the K100 to have a soft frame, but it's this particularity that gives the impression that the bike is moving in curves. It's not, at all.
By removing (or adding) weight, you modify the center of gravity and it will generate instability or will reduce handling.

2nd: by lowering the front of the bike, you modify the center of gravity, plus you reduce the trail angle. This will generate high unstability, but it's not the only problem.
A low center of gravity is not the Graal on motorcyles. It's not a car.
All things being equal,the lower it is, the more you have to give angle to your bike in a curve. I'm these days testing various seat heights on my K75GS, I promise that the effects of seating 3 cm more or less are spectacular.
So the lower = the more angle. Too bad...as you have dropped the fork, the more angle you take, the more unstable is your bike with its reduced trail angle. And, as a bonus, the closer is the engine case from the ground...
The worst is when you ad large wheels (usuall not aligned at the rear!) that claim more angle to steer, all things being equal, than thin tyres.


About the rear frame modifications you are expecting, remember that the shaft transmission is much more reluctant to modifications of geometry than a chain transmission. With a chain, you can do almost anything as long as it is in line and without too much tension, it is OK. With a shaft, you have interferences between the shaft rotation and the suspension. It was very sensitive on old airheads, less on the K100, but it remains an issue when riding - and braking- fast.
If you want to modify the position of the upper anchor of the rear shock, it will have an effect on the whole suspension behavior. And I don't event think about issues of stiffness of the new rear frame...

All the geometry of the K100 is designed to be a bike ridden from behind the tank. Not from the tank. To be ridden with the body in line, not to be ridden hanging besides the saddle. To be piloted by the eyes, not the arms.

Do what you like with ergonomics and look, but don't alter geometry.

As JBT says, don't touch the geometry. That's a serious black art.

https://www.amazon.co.uk/Motorcycle-Tuning-Chassis-John-Robinson/dp/075061840X

This was one of our bibles back in the Eighties/Nineties when we were desparately trying to make Japanese rocketships actually handle. It mostly came down to extending the swingarm for stability and adding a steering brace and a steering damper to get the front under control, or, if you were rich, modifying the yoke to take a better front end. What most bikers don't realise is that a tank slapper (or that nasty feeling of instability you sometimes get) actually starts at the swingarm. Dropping the forks through the yokes will theoretically quicken up the steering but it's more likely to give you a nasty dose of over steer. Same if raising the rear shock mount as it effectively tucks the front end in.

The other bible was this one:

https://www.amazon.co.uk/Motor-Cycle-Chassis-Design-Practice/dp/085045560X

It's no longer available and s/h copies are fetching stoopid prices. No, you can't have my copy

Having said all that, have a squinty at this if you want a different swingarm:

https://www.k100-forum.com/t19143-hybrid-project-in-colorado#219809

Thanks JBT & Dai for the detail.

Going to look for clip ons with risers (better to play it safe) and reset the forks to standard. There are so many K's with other branded forks which I would assume are shorter and thus assumed it wasnt an issue but I heed your advise.

I have messed around with foot position and dont like the idea of further back rearsets, I find the stock position fairly good. That said with my size I could do with the tank being slightly higher (so my 15m legs can fit into the correct space on the tank) and my seat taller too.

If I were to run a new line as can be seen in the photos but leaving the stock frame as is and welding to the existing tail would that be a concern? That would enable the leg space I need both in tank and  seat comfort. and that way not messing with the geometry. 

I would quite like to remove the existing seat frame (see red in photo) (keeping the triangle clear) (under the new poles but I guessing your advise would be Dont!!) That way the geometry isnt changed and should be structually sound... and solid.

The_Tick wrote:. There are so many K's with other branded forks (...)

On the internet, not on road. Altered K's with short big upside down fork are the worst thing you can do to a K. Reduced ground clearance, trail angle, trail (due to modified triple clamp), suspension travel, and the worst is that usually, the turning radius is very, very low. Modern triple trees are way too thick and flattened to fit a K100 frame and tank.

I have messed around with foot position and dont like the idea of further back rearsets, I find the stock position fairly good. That said with my size I could do with the tank being slightly higher (so my 15m legs can fit into the correct space on the tank) and my seat taller too.

You can easily change the stock footpeg plates for another. K100RS1 and K1 plates provide rearset footpegs, but they are higher than stock. You can also mount cheap chinese footpeg lowering kits. I bought some for aprox 25€.
Or expensive footpegs lowering kits for K, made in Germany, at MV MotorradTechnik for 10 times more. I did also: it's perfect.

If I were to run a new line as can be seen in the photos but leaving the stock frame as is and welding to the existing tail would that be a concern? That would enable the leg space I need both in tank and  seat comfort. and that way not messing with the geometry. 

It's OK, but why welding a new tail where you just need a higher seat? Foam, ham electric knife and upholstery are easier to handle.

I would quite like to remove the existing seat frame (see red in photo) (keeping the triangle clear) (under the new poles but I guessing your advise would be Dont!!) That way the geometry isnt changed and should be structually sound... and solid.

It is not the seat frame you want to remove: it's the frame. This part is the link between the rear suspension and the gearbox mounting on the frame. It's not optionnal.
Some make a new tail as you intend to do it - and they're right on a visual point of view, as the frame shape of the K is a PITE when it's naked. But this is engineering, not plumbing. If you know which metal to use and how to dimension, cut and weld it, do it. If not...don't. Or ask someone who knows.

Dai wrote:tank slapper

Even factory K bikes aren't totally immune to tank slappers. I've never heard of anyone else having a tank slapper on a K but I had one once on a K75RT. It very rapidly went lock to lock about ten times and scared the bejeesus out of me. I have no idea what caused it (going straight and hard braked the front a little in a heavy rain with the rear loaded up for touring) but it must have been a perfect storm of bad kinetics that caused it. When it happened I let off the throttle because I had no control and forward momentum carried me out of it.

About ten minutes later my heartbeat went back to normal.

What you propose doing to the frame has been done as JBT says. To get the rear suspension back to stock, you'll then see that a lot of 'builders' (sorry - most of them don't have a fckn clue what they are doing, pretty results or not) weld a bracket partway up the frame tube. On standard frame (amost any frame that's twin-sided or single-sided, but not the centre-mounted monoshock types) you'll see that the top shock mount is  welded to the frame in an angle. Think of the force lines through the rear shock: 

- with the standard mount the shock is punching into the frame and the force is being absorbed by the frame
- with a bracket welded partway up the frame tube the shock will be trying to rip the bracket off the frame.

If you want to go for the top tubes as you propose (and it does solve the tank/seat line issue which is a bugbear of mine) I think you need to look into a laydown shock which punches into the frame just above the gearbox mounting and absorbs the forces back along the top link.

https://www.meister-engineering.com/bmw-k100/2021/2/18/meister-engineering-launch-new-lay-down-shock-conversion-bmw-k100

Be warned: you'll need to take out a new mortgage.

duck wrote:

Dai wrote:tank slapper

Even factory K bikes aren't totally immune to tank slappers. I've never heard of anyone else having a tank slapper on a K but I had one once on a K75RT.

Strange... I've ridden K75RT for years, with an administrative immunity and, modesty left apart, good riding skills. Mountain roads, highways, city streets...even  on race track, with completely unreasonnable cruise speed about 160 kmh. It was another era. I think that in this job, we had applied the worst stress a K75RT ever endured.  Crash bar and side cases cover were changed every 3 months because of continuous scraping...without falling.
I never had any tank slapper, even on hard mode, during car chases for example.
But as you say, it must have been a unique combination of bad kinetics...