Dear MOby,

A few months ago my Sportster 48 started cohabitating with my boyfriend’s ’09 Yamaha R6. Among the many strange adjustments that interest me is why my BF seems to be so anal about his bike’s drive-chain slack, which he insists has to be 40 to 50mm. He checks it all the time. My Sportster’s belt slack is apparently supposed to be around 15mm. I never check it and don’t much care, and I feel like he sort of wants to shame me for my casual attitude. As a matter of fact, when we swap bikes I feel like his has way too much slack, sometimes feels kind of jerky, and I’m beginning to think this might extend to other parts of his personality. What gives?

Loose Cannon
Janesville, Wisconsin


Dear Cannon,

Your BF’s true jerkiness will only really reveal itself over a time period much longer than a few months. For now, let’s try to keep it to just differences in hardware and lifestyle. Your BF’s Yamaha R6 has a rear wheel that travels up and down nearly 5 inches, which makes the chain looser and tighter as the swingarm moves up and down quite a distance. In the photo above, as the bike’s rear axle moves upward those 4.7 inches, you can see the chain will get tighter because the front sprocket is some distance ahead of the big silver nut that marks the swingarm pivot. That 40-50mm of chain slack (what the Yamaha manual specifies!) is just enough to be sure the chain’s not too tight when the rear sprocket is at max distance from the front one. Too tight puts undue pressure on the countershaft (the one the front sprocket is attached to), which can lead to a host of nasty expensive internal engine problems. Too tight will also keep the suspension from being able to stroke through its full travel. It’s best, then, to err on the side of loose, but not too loose. If Yamaha says 40 to 50mm, there are very good reasons why – though closer to 40 does reduce driveline lash, and may make your boyfriend seem less jerky.

050817-ask-mo-chain-slack-harley-davidson-48-sportster

Meanwhile, your Sportster’s swingarm starts off in positive territory – you can see its swingarm rises slightly as it moves from front pulley to rear even when the bike’s at rest – and it strokes just 1.6 inches of wheel travel farther into positive territory (upward). It’s never going to need any more slack than what you’re looking at in this static photo, so 15mm is plenty. Furthermore, Kevlar-reinforced belts like yours don’t stretch nearly as much as roller chains do, so checking its tension is a thing you need to do about as often as I change my Yahoo password. Like, almost never. If only you were as low-maintenance. Come on, that’s a little man joke.

One’s really no better than the other, they’re just different, and you’re both right. Your BF’s R6 is the bike I’d want for a high-speed sortie up Angeles Crest, your Sporty is the one I’d borrow for a nice relaxing blat around town, down to the beach.

Can’t we all get along?


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  • Sayyed Bashir

    I have not adjusted the belt on my 2007 Harley Softail in 155,000 miles. Still the original belt.

    • hipsabad

      belts are what i like best about harleys

      • spiff

        I say it’s their paint.

        • Sayyed Bashir

          Also the hydraulic valve adjusters. Never have to adjust the gap.

    • John Ferguson

      Is there a recommended change interval for the belt, or is it “if it’s damaged/worn, change it”?

      • Sayyed Bashir

        There is no recommended change interval. If it is well adjusted and lined up correctly, it should last for ever.

  • JMDGT

    Shaft drive. Know it. Live it. Love it. There is nothing wrong with chain drive.

    • Sayyed Bashir

      There are plenty of stories about shaft drives, both on and off road, especially on the pot-holed roads in Canada.

      • JMDGT

        My personal preference is shaft drive. Chain drive as good as it is still requires more maintenance than I want to do. I always thought it a messy business. Belt drive depending on the application is also good. The service intervals of shaft drive are longer and I don’t have to do it. That is what I like about it. I know guys who have never lubricated the chains or replaced the belts on their bikes. My philosophy is the less maintenance I have to do myself the better. If I personally have to do it I will but I prefer not to if I can help it. Ride safe.

        • Sayyed Bashir

          I have chains on both my KTM 1190 R and the Suzuki Bandit 1250S. Just lubricate them every 600 miles. Takes 30 seconds on the center stand which both bikes have. No adjustment necessary. People ruin their chains prematurely by unnecessarily adjusting them. Some slack is necessary because of the chain tightening at the furthest point of the suspension arc and because of uneven wear. Least cost and weight and most efficient way to transfer power to the rear wheel. Also allows for speed vs torque adjustment by changing rear and/or front sprockets. Much easier to repair/replace the chain in the middle of nowhere.

          • JMDGT

            Like I said before there is nothing wrong with chain drive.

          • Jon Jones

            ” People ruin their chains prematurely by unnecessarily adjusting them.”

            Great point. Seems either chains are completely neglected or killed with love. I see too many chains that are WAY too tight. The bike’s owner thinks he’s on the ball with maintenance and adjustment. Hint: Chains need more freeplay than you think.

          • Sayyed Bashir

            Chain freeplay spec on my KTM 1190 R is 40-45mm (1.57″ to 1.77″). It is not just owners but dealers too. Yesterday I was going on a ride with a friend who has a new Husqvarna 701 and I noticed his chain was straight across instead of sagging like mine. I checked and it was too tight. I told him he was going to ruin both his chain and engine and he said he had just taken his bike to the dealer for a one year check-up and they had gone over and adjusted everything.

            This what it says in the KTM 1190 R manual: “If the chain is too taut, the components of the secondary power transmission (chain, engine sprocket, rear sprocket, and bearings in the transmission and in the rear wheel) will be under additional load. In addition to premature wear, this can cause the chain or the countershaft of the transmission to break in extreme cases.”

  • Dootin

    somewhere between the chain breaking and falling off the sprocket.

  • Chocodog

    The older I get the more I have to adjust my belt!

  • Old MOron

    JB, your perspective is balanced, well-informed, and practical.
    How is it that you found it necessary to get divorced?
    I guess marriage is not about adjusting chains, eh?

    No, it’s about wearing them!

    “Come on, that’s a little man joke.”

    • Sayyed Bashir

      Divorce is about freedom.

    • elgar

      Know why divorce is so expensive? Because it’s soooo worth it!!

      • john burns

        Hey, do you know why the husband usually dies first?
        He wants to.
        Ahahaha…