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Setting Motorcycle Rear Suspension Sag

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.

Setting Rear Sag

When it comes to setting suspension sag — front or rear — we follow Race Tech’s “Race Sag” method, as it provides the most accurate results. The basic calculation for setting Race Sag is:

Race Sag Calculation:
L1 = Measurement of fully extended suspension
L2 = Measurement of suspension with rider and gear, suspension settled after mild compression and release
L3 = Measurement of suspension with rider and gear, suspension settled after mild lift and release
Race Sag = average of L2 and L3 subtracted from L1

1. First, put the bike on its centerstand and extend the rear swingarm completely, making sure there’s no load on it. If you don’t have a centerstand, you’ll have to figure out some way to support the chassis so there’s no load on the suspension. On some bikes, you can lean the bike over on the sidestand and unload the chassis.

For the rear shocks, take a measurement from the center of the rear axle straight up to a fixed point on the bike’s frame or bodywork. If you can’t measure from the axle for any reason, pick a point immediately above as possible and make sure to measure as vertically as possible. Take your measurement and write it down under the legend L1.

2. Next, take the bike off its centerstand and have the rider — dressed in their regular riding gear — get on, both feet off the ground and in a riding position. The rider will need to hold onto something to stay upright, or have a helper hold the bike. If you’re planning on touring or continuous two-up riding, load your bike with your intended gear and/or passenger before checking sag.

This next bit is a little harder in that it’s somewhat difficult to assess whether you’ve done it right. It’s one of those things that gets easier/more intuitive with practice. What you want to do now is gently compress the suspension approximately 1 inch (25mm) or so, and then slowly let it settle back out. Don’t just let go; maintain downward pressure while letting up until the bike settles. It’s very much a nuanced, balanced process, so be patient and be ready to do it several times. Don’t bounce the bike. The point of this measurement is to take into account any friction in the suspension system that limits travel. With the rider steady on the bike, take another measurement and write it down under the legend L2.

3. Next, gently lift the bike 1 inch (25mm) to extend the suspension from its resting position. Keep in mind that lifting off compression is usually a bit harder than compressing. The exact distance isn’t as important as ensuring the suspension actually lifts off of its resting position. Let it drop back to resting position slowly and take a measurement. Write this measurement down under the legend L3. Like your L2 measurement, the L3 measurement takes into account friction on the system.

4. The measurement between L2 and L3 is, theoretically, where your bike would come to rest if there wasn’t any friction or other element such as shock linkage acting on the suspension. Race Tech’s Race Sag setting protocol considers this, by averaging the two measurements. This is done simply by adding L2 to L3 and then dividing by 2; this measurement is then subtracted from L1, leaving your static or Race Sag measurement.

As an example, if L1 is 140mm, L2 125mm and L3 130mm, your Race Sag would be 12.5mm, calculated by subtracting the average of L2 and L3 (127.5mm) from L1 (140mm).

5. Finally, set your preload as necessary to achieve the desired sag (see discussion below for sag specs). Tightening the spring reduces sag, loosening increases sag. Whether you have a threaded collar for adjustment or a stepped preload adjuster, put your bike back on the center stand to reduce sprung weight before adjusting. You’ll likely end up going through the measuring and adjusting process several times before finding the right final adjustment.

How much sag is enough?

It’s important to point out that the preferred amount of sag is a subject of some disagreement among both manufacturers and riders. Race Tech, for example, says rear shocks on street bikes of the type we’re looking at — meaning old — should have their sag set at 28-35 percent of total travel with 28-37mm of Race Sag. The stock shocks on our subject 1995 BMW K75 are rated at 100mm or roughly 4 inches of travel. Following the Race Tech model, that equates to a preferred 28-35mm of sag. However, for our replacement Ikon shock, Ikon suggests setting sag at 30-40 percent of travel. Assuming the Ikon has the same 4 inches of travel, that means a sag of 30-40mm. However, the Ikon appears to have as much as an extra 1-inch of travel over the stock Boge, which translates to only 24-32mm of sag. Further, some manufacturers — including Ikon — suggest maximum compressed spring length on bikes for which they have data. In the case of our BMW shock, Ikon’s maximum suggested compressed length is 240-260mm; Ikon says that any longer is too soft. At the front, Race Tech suggests an almost identical sag calculation of 28-33 percent of total travel or 30-35mm of sag for street bikes.

That said, Race Tech’s suggested 28-35 percent of total travel is a good starting point. Final adjustment is very much an issue of rider preference, but without knowing where to start, it’s hard to find the point where you feel your bike is working at its best, which in return makes you a more confident and safer rider.

If you’re running vintage-style or original rear shocks with 3-position preload adjustment, your preload adjustment is limited. That makes it tough to really dial in your preload, but knowing something is better than knowing nothing, and once you’ve gone through the process you’ll have a better idea of where you want your shocks set, and why. If you’re running modern shocks with a threaded spring collar, you’ll be able to fine tune your shocks much better.

We know that plenty of people think that if they don’t notice anything distinctly “wrong” with how their suspension is performing, why bother? For the simple reason that until you do bother, you don’t actually know how your suspension is working, and most of the time you’ll be astounded by the improved performance of a properly set up suspension.

And yes, depending on how you ride, this is an exercise you’ll want to do regularly. If, for instance, most or your riding is solo but you’re planning a trip, you’ll want different settings for each. Once you’ve established your baseline in different scenarios – solo, two-up, touring, two-up touring – you’ll have the necessary info to make it easier to adjust to changing need. Finally, note that you can get static sag measurements without following Race Tech’s Race Sag averaging, it’s just not as accurate, which, especially if you’re racing, means you won’t be working with the best possible data to tune your suspension. Coming up next: Setting front suspension sag.

Published on Dec 12, 2018
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