Goals of the Analysis

My analysis set out to compare the stiffness difference of two identical fork setups, but with differing lower tubes - WP and KYB. The end goal being to evaluate whether or not the KYB tubes have “better flex characteristics” than the WP lower tubes.

Here's an image of both lower tubes that were evaluated, lined up as they were compared. The KYB is offset at the lug area 3mm from the WP to allow the same amount of lug insertion in both tubes. The necked down portion the WP tube is certainly longer than the KYB, however it's important to note that the majority of flex does not happen at that point. It seems that this is primarily done for weight reduction. The WP tube is just a touch thicker than the KYB, which largely determines the overall stiffness. Also, the upper bushing on the WP is slightly higher than the KYB, so it does a little better job of reacting the bending moment.

Results and Conclusions

The deflection of each fork setup is plotted here at different stoke levels. The takeaway point is that you get 1.5-5.5% more flex with the KYB tubes (depending on stroke position). This for one particular load, but I also ran a sensitivity analysis to examine the response to other load levels and the system response is nearly linear with respect to load. So KYB tubes being 5% less stiff at 4000 N of load will also be 5% less stiff at 1000 N, or 100 N, or whatever load you wish within reason.

I also looked at the reaction loads at the bushings, which are accessible in post-processing the model. And they are noticeably higher on the KYB tubes than the WP, which means you will have more friction with the KYB tubes than the WP. This increased binding at the bushings may offset any change in flex "feel". I suspect the increased reaction loads are in part due to the shorter bushing to bushing distance on the KYB tubes as well as the slightly thinner KYB tube allowing more deformation than the WP.

So, what does all this mean? Flex, or in engineering terms compliance, is just the inverse of stiffness. And it's highly misunderstood within the motocross industry. Is the 3.5% increase in flex an advantage? Probably not… Just look at all of the factory KTM and Husky guys going the opposite direction running 52mm forks on the same chassis.

Flex is important, but it's not as simple as more flex = better. Flex seems to be the hot topic in the last few years and I have yet to see someone explain it correctly or how it makes things better or worse! There's plenty of talk about "improved flex characteristics" and similar. But that's a completely meaningless statement. What is meant by "improved flex characteristics"? More flex? Less flex? Flex in what direction(s)? Flex under what load conditions? Until all of those questions are answered, saying something has improved flex is of zero meaning.

From a fork lower tube standpoint, the flex direction that is affected by changing from WP to KYB (or vice versa) is in bending; vertical compliance difference is insignificant. And the bending stiffness of the fork tubes comes into play with lateral (side to side), longitudinal (front to back), and torsional (steering) forces. All of which will have an effect on suspension performance to my earlier point (negative effect for more flex).

So of the three loading directions mentioned above, longitudinal is arguably the most important as there are a lot of off-(fork)axis loads in that direction. There is no suspension action in that direction, so the flex is acting as the suspension (as an un-damped spring). The same can be said for lateral loads, but those are of a much lower magnitude. Torsional flex is rather different as it is entirely reacted by the rider (at the grips). And torsional is directly related to the amount of lateral and longitudinal flex, which are both directly related to bending stiffness.

Or in other words, by changing the lower fork tube flex, you're changing the response to lateral, longitudinal, AND torsional loading. Is that what you really want? What is the goal of changing the flex? Do you want more or less flex? And most importantly, is there another component you can change that only affects one of the loading directions and still achieves your goal (this is the right move, in my opinion)?