Hi all, 

New forum member here, looking to get into K bikes. I’m dreaming up a custom build that I could take to the track, and also be street legal.  I’m looking to do a K1100 as the base. Possibly buying a juiced up motor from BSK Speedworks. 

My big questions are:

1) has anyone swapped a modern super sport front end into their K bike? Thinking R6, Gixxers, CBR600rr, that kind of thing.  Any reason why that can’t work? 

2) how is the frame stiffness? Will the bike hold a line under significant lean, or would it need to be stiffened up?

Any online custom magazine will show USD front suspension on early K bikes. Try bikeexif.com, bikebound.com, pipeburn.com, returnofthecaferacer.com, rideapart.com & etc. There are companies selling the necessary stem adapter to make it work, all easily researched.

Use a K1100 frame with the V backbone as compared to the earlier frames with a single top tube. There's a thread with some specifics very recently here in the forum. It's probably scrollable from the main list without having to do a deep search.

If you're wanting serious info about track biking a K and you know about BSK, there's your source of info already. They have a blog about their exploits on the website.

Thanks! Great info and referrals. I know about some of these sites, but some others are new to me.  Didn’t know about the frame difference on the 1100, but another notch in the column of getting that as a base bike. The other reasons—Motronic instead of jettronic, wheels more compatible with modern rubber, 16 valve setup and slight displacement bump. Visual inspiration is the early German F1 team colors, which was bare metal.

K100TrackBikeProjecr wrote:
1) has anyone swapped a modern super sport front end into their K bike? Thinking R6, Gixxers, CBR600rr, that kind of thing.  Any reason why that can’t work? 

Plenty.
Modern forks are too short, so your rake will be modified, and the front of the engine will easy touch the ground. Your bike will be lowered too, so you'll have to lean more - but the engine width won't allow you to.
They are too thick, so your steering angle will be affected, but that's not a problem on track.
Their offset is not the same that the K1100 offset. Expect unexpected reactions...
And last but not least, the spring ratio and the hydraulics are not set for the weight of a K11.
The main feature of a K bike is that it's very well balanced. If you modify one element, you alter this balance. Do not lower it, do not use large tyres, the power output of the engine do not need them, large tyres will just slow you in corners and demand more lean for the same speed and corner radius.
K 16 forks are issued from the M1R fork, that was a reference on track bikes before the USD fashion. Why not keeping it? You just have to change the internals to have a fully adjustable and very efficient fork.

K100TrackBikeProjecr wrote:
2) how is the frame stiffness? Will the bike hold a line under significant lean, or would it need to be stiffened up?

Don't worry about the stiffness of the frame. Worry about the geometry & suspension settings. That's what makes you going fast, not a shiny big USD fork.

I've been riding stock (almost, just with blue lights and sirens...) K75RT and K1100LT on tracks and on open alpine roads with no speed restrictions, for years. Some tried to follow, some tried to escape. Both always failed, even with much sporty bike supposed to be more powerful with better roadholding.

As long as there's no hole in your valve case, you don't need to bother about modifying anything on your bike, it's not the bike the limit, it's your riding.

jbt wrote:

K100TrackBikeProjecr wrote:
1) has anyone swapped a modern super sport front end into their K bike? Thinking R6, Gixxers, CBR600rr, that kind of thing.  Any reason why that can’t work? 

Plenty.
Modern forks are too short, so your rake will be modified, and the front of the engine will easy touch the ground. Your bike will be lowered too, so you'll have to lean more - but the engine width won't allow you to.
They are too thick, so your steering angle will be affected, but that's not a problem on track.
Their offset is not the same that the K1100 offset. Expect unexpected reactions...
And last but not least, the spring ratio and the hydraulics are not set for the weight of a K11.
The main feature of a K bike is that it's very well balanced. If you modify one element, you alter this balance. Do not lower it, do not use large tyres, the power output of the engine do not need them, large tyres will just slow you in corners and demand more lean for the same speed and corner radius.
K 16 forks are issued from the M1R fork, that was a reference on track bikes before the USD fashion. Why not keeping it? You just have to change the internals to have a fully adjustable and very efficient fork.

Very helpful, thanks.  This is exactly the kind of thing I'm looking to learn.  Sounds like internal spring swaps on the front forks would cause the least problems.  

Looks like stock front end is 27.5 degrees of rake, and 3.9" trail.  My ideal would be the same trail, and 25 degrees of rake.  That's the setup on my KTM RC390, and it's the sweetest handling bike I've ever ridden.  Other supersports I've ridden have 24 degrees.  The s1000rr has even less!

While I don't do track time with my bikes, I have found that Race Tech cartridge emulators and fork springs worked quite well to upgrade the performance of the forks on my K75RT.  The nice thing is that they are tunable so you can dial in the best damping rate.

Since forks are essentially a spring loaded piston that goes up and down and the forks on our bikes are plenty rigid, the addition of adjustable damping and spring rate characteristics should make them usable in competition.

The biggest problem I had with my bricks was a lot of dive when decelerating.  The Race Tech single rate springs and the emulators went a long way toward making the bike more confidence inspiring in turns.  When the dive is corrected, the rest of the handling comes right into line.

Don't expect to get the same behavior on a K with the same specs that your KTM have. Wheel span is also essential to consider, and a K11 is huge, 2 cm more than your KTM. The position of the center of gravity, with and without a pilot, the front-back weight ratio, the dynamics of the suspensions are also to be included in the equation. The standard rake angle is 27,5 deg. for the K, except the K1 which has a 26,5 deg.
A 150 kg bike does not have the same specs than a 290 kg bike..
Modern bikes have a short trail and a reduced rake because they have to : their wide tyres-to-be-like-a-pilot would lead their pilots-but-not-really right in the wall at the first corner if the front end was not designed to enhance manoeuvrability. And so altering stability, so there are direction dampers.
Sure, a complete front end is easy to fit and you'll gain big brakes, large fork tubes. But you'll have to cope with a very shortened wheel travel, and a front wheel as wide as the stock rear wheel.

Copy and paste is inadequate.

Believe me: I've been part of a team who raced in Endurance championship with an old BMW R100. There was big Japanese 4 cylinders, like Godier Genoud Kawasaki, with plenty of horspower and "large" tyres (max rim width was then 4 inches) and light european bikes, less powerful, but light and easy. The fastest on the track were the Moto Guzzis, with not even the stock output of a K100 and 110 mm large rear tyre. The Guzzi team eventually retired because they were bored to win every race with no challengers.

I have plenty of Ks, particularly 2: a stock K1 and a slightly modified 1989 K100: https://www.k100-forum.com/t16398-eighties-k100-racer
This one is much more efficient and rapid than the K1, because it weights 210 kg only. It could easily pass under the 200 kg with a light fairing and a small battery.

About fork spring, progressive springs are very hype today. Yet the stock springs are progressive...
Well not really progressive; a real progressive spring would have each pitch different from the next one.
It's useful on road because you have a large variety of situations: flat tar, slight bumps, medium bumps, large bumps, and you're not always free to avoid the biggest obstacles for bullshit reasons like "there's a car on the opposite line".
Yet on track it's different: your suspension works like your throttle: on or off. Always full extend on full throttle or always full compression on full brakes. So in this situation, a linear spring is much more predictable. And if you spot a big bump on the track, you just have to change the trajectory to avoid it the next lap.
You just have to set the correct rate and preload so the fork is never 100% compressed.
(that may be challenging in endurance with a 50kg pilot and an other weighting 100 kg!)