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Sometimes I get to work with people that are way smarter than me and that makes me smarter on the back end. Recently, Mark Johnson of REP Suspension threw out some adjustments/numbers for me to try on the KTM 450 SX-F and I came away with a better feeling on the track. Now this article isn’t for everyone, I understand that, but for those of you that like to tinker and feel differences on the track, I think these numbers will work for you and make you smile. It’s amazing what millimeters can do to a motocross machine and sometimes it’s overwhelming on what you can do. If you want to shut your brain off and just ride, I understand, but if you want to listen to your inner test rider, give these two measurements a try on your orange brigade. The difference in feel is fairly noticeable.
Chain Slack: Most of you just do the three finger trick (and I am guilty of that as well), but if you want a little more rear wheel traction along with a more compliant shock feel look at your chain slack measurement a little harder. It doesn’t take that much to do to ensure that you have the right chain slack measurement. Getting the chain slack at measurement between 65-68mm allows there rear end to squat without binding the swinagarm upon acceleration. (*Measure from the end of the chain slider on swingarm*) I did a back to back test with a measurement of 60mm and 66mm and the difference was crazy. With 60mm of chain slack the shock felt harsh and I could feel the rear end almost had that packing sensation. When measured at 66mm the rear end had a freer feel and didn’t give me that harsh feeling when the shock was loaded. Going too loose, past the 68mm mark, gave me a feeling of less connection from the throttle to the rear wheel! Considered my mind blown!
Axle Position: Axle position is important to the overall balance or ride attitude of the chassis. Too long can take some cornering away and too short can hurt straight line stability. So where is that sweet spot for the KTM? I have experimented with lengths before but when Mark suggested I tried 595mm, I wasn’t completely sold… Until I tried it… I was running around 608mm from the center swingarm pivot bolt to center of axle. With 595mm length I know gained a little more positive front wheel traction out of area 3 in corners and didn’t have as much squat in the rear on acceleration. This helped the ride attitude of the KTM 450 SX-F and gave me a more positive feel around three different types of tracks. Combined with a fork height of 5mm and this axle position the KTM now had a nice blend of stability and comfort that I haven’t felt before (especially on lean angle when on throttle). I use the Works Connection axle blocks or Ride Engineering axle blocks, as well as stock gearing, to achieve this measurement. I started with a new chain cut close to the desired length and allowed the natural stretch of the chain to achieve the 595mm measurement. If you’re around 2-3 mm within this measurement at any given time is close enough to achieve desired track results. When going 10mm or more off from this measurement is when you should feel changes to the KTM’s chassis character.
