Sunday, October 6, 2019

Two Bikes Redux

About 1 year ago, I wrote a post about how the specs of my KTM 690 Enduro and my Kawasaki KLR650 were almost identical, but they felt very different when riding.  In fact, they required a different riding style to remain confident and have some flow to riding corner to corner.


After 3 seasons, I have 19k miles on the KTM and I have learned how to ride it with confidence and flow.  It is still a very different style than the KLR.  In fact, I took the KLR out for a ride this summer and just couldn't make it work.  My muscle memory has so completely adapted to the KTM that I can't ride the KLR the way I used to.

But this time, I may have the answer to the differences between bikes.  It's said that lateral flex in a motorcycle is a key part of the suspension during cornering.  For example, if you are leaned over at 45 degrees, the force from a bump is split between the direction the suspension works and lateral force trying to bend the forks and swing arm.  So flex from the forks, frame, and swing arm are the only "suspension" for the lateral loads.  In addition, it is thought that the forks stop working because the side load increases the friction in the fork and it "sticks".

This article discusses the effects of lateral flex on Motogp bikes and the effect of tire design on rider feedback and performance.

https://www.motorsportmagazine.com/opinion/motogp/why-motogp-has-gone-soft

I don't claim that either of my bikes nor the rider are anything like a motogp bike, but lateral stiffness may explain the difference in feel between the KTM and the KLR.  For example, the forks on the KLR are 38mm diameter.  The KTM has 48 mm diameter forks.  The KLR has a frame that is not triangulated with only a single down tube and top tube from the steering head.  The KTM has a triangulated trellis frame that  wraps around the engine and uses the engine structurally.  Finally, the swing arm of the KLR is fabricated small rectangular tube while the KTM is cast with large sections, especially the cross brace.  My guess is that the KTM chassis is an order of magnitude stiffer than the KLR.

Finally, a big difference and one which may explain the difference in feel.

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