BMW K bikes (Bricks)

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mister_anger

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hey,

I am starting my K100RS 8V project. I am looking to replace the front end but I am wondering about the fork length.
A lot of people are converting to a GSXR or R1 front fork but these forks are a lot shorter then the stock ones.
I can't find anything about installing fork extensions on the GSXR or R1 forks. Does anybody have any info on the effect of installing these shorter forks?

the thing is that I have a triumph sprint ST 1050 rebuild fork in stock but if I install this then I'll be lowering the front end by 6 cm. Wondering if that is a problem.
I could lower this 6 cm by installing a step down top yoke. but I kinda like the aggresive stand of a lower front end.

thanks, M

    

chris846

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Steering geometry is a big topic, but for starters...

The thing you don't want to introduce is unstable steering - tankslappers. The main preventative is trail, rather than rake (have a read of Tony Foale's experiments with rake & trail on a modified boxer).

By lowering the front you will reduce rake and, usually, trail. However, you might not be reducing trail if you use the GSXR yokes. This is because the GSXR yokes have less offset than the standard BM ones (offset is the amount by which the fork centreline is set ahead of the steering axis - find some K100 yokes and GSXR yokes on ebay and compare them visually).

Offset is the main way to alter trail. More offset means less trail. Later GSXRs & R1s are heading towards the 25 degree region for rake. However, their yokes are starting to look 'flat' i.e very little offset. This flatness maintains a safe amount of trail.

So....if you are using forks and yokes from a 26 ish degree rake bike (google you donor bike and find out its specs), then you can be reasonably confident that the matching yokes will provide the right sort of trail to go with this.

First step then is for you to calculate what your new rake figure is likely to be after you've dropped the front of a K by 6cm or so. If this shows you're heading towards 26 degrees, then things might work out for you with your fork/yoke swap.

This is only the start of the story. It'll keep you safe that's all.

    

mister_anger

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I checked the triumph sprint and it has a rake of 24.5 and a trail of 80 mm. A gsxr has a lower rake but larger trail. I can always have a new top yoke and triple made to lower the offset. I will have to check when it is installed. Or I can have the rear lowered. Thanks for the reply.

    

mister_anger

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Forgot something. The forks of a gsxr are 30 mm shorter then those of the sprint.

    

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