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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