Setting Motorcycle Rear Suspension Sag


Measuring seat height
Understanding how to set suspension sag returns dividends: a better handling motorcycle.

Although most of us would rather spend our time riding rather than wrenching, motorcycles — especially the older bikes we talk about and ride at Motorcycle Classics — require dedicated rider input to get the best out of them. It’s important to understand how your bike works best and why, and when it comes to basic prep, a properly adjusted suspension should be right up there at the top of any Best Practices list. No doubt, any sort of tuning takes time, but once you’ve gone through basic suspension setup a few times, you’ll discover that it doesn’t take that long, nor is it especially difficult.

Setting sag is important because it’s a baseline for proper suspension performance. Too little sag — meaning the springs have too much preload and are already bound up tight — returns a rough ride with poor road contact. Too much sag — meaning the springs don’t have enough preload — results in a bouncy ride and poor road contact because the suspension moves too much for conditions.

For this discussion, we’ll focus on vintage street bike rear suspension. The basic process for determining sag is the same front and rear, the difference is in how you adjust sag on front forks versus rear shocks. Many modern forks let you dial in preload and, if equipped, compression and rebound. Ditto with many modern rear shocks. That makes suspension tuning on modern bikes a lot easier. However, those are features you’ll rarely find on vintage bikes of the type we focus on in Motorcycle Classics.

With vintage front forks, the most you can usually do is adjust preload and make changes in fork oil viscosity and quantity. For the front fork, adjusting preload requires installing shorter or longer spacers and can be an involved process, depending on the bike.

With vintage rear shocks, you’re typically limited to changes in preload only. However, as shocks are something riders regularly upgrade, lots of old bikes still on the road are sporting modern rear shocks equipped with either or both rebound and compression damping, enabling further tuning. The bike we’re setting up here is a 1995 BMW K75. Our K75 was originally equipped with a 3-position, preload-adjustable Boge mono rear shock, but now wears an Ikon 3610-1009 with threaded spring collar preload adjustment and 4-position adjustable rebound.

4/30/2020 8:26:36 AM supplied the rear mono shock for this article. Embedded link is incorrect. Thanks.

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