MotoMojo: Setting Suspension Height

Mar 04, 2015 View Comments by

Remote adjuster (2)Many folks ride their motorcycles and never give a thought to setting up their suspension to match the weight carried on it. A few minutes spent measuring and adjusting your suspension’s spring preload can yield great results, with improved riding and handling.

Street motorcycle suspensions are designed to sit with about 25-30 percent of their overall travel compressed when the rider and luggage are on the bike. A good sag setting (more preload = less sag) will let your motorcycle have about 70-75 percent of its suspension travel available to handle upward movement of the wheel. The 25-30 percent compression allows the wheels to drop into dips to keep the tire in contact with the road. That extra front sag or less rear sag makes a motorcycle turn more slowly and will also decrease bottoming resistance.

Measuring rear height without the rider. Measure to a fixed point that is directly above the axel.Measuring and Setting the Rear
Nearly every motorcycle has some kind of rear-spring preload adjustment, so let’s start at the back. Ideally you should have two (or more) strong helpers. Your motorcycle should be loaded with a full tank of gas, all the luggage you plan to carry, and the rider fully dressed in riding gear. First, extend the rear suspension by getting the tire just off the pavement (about the thickness of a piece of paper). If you don’t have a centerstand, you’ll need those helpers to lift it or use a motorcycle jack under the engine. Measure vertically from the axle to some fixed point above it on the motorcycle. Metric measurements are easier to work with for this so we will use them here. Next, mark this spot with masking tape. This measurement is R1 (rear 1).

Second, place the bike on the ground, and put the rider onboard in riding position with feet up. Your helper can balance the motorcycle from the front. If you frequently take a passenger, take measurements both solo and with a pillion, and note the settings. To account for linkage friction, take two additional measurements by pressing down on the back of the bike about an inch (25mm), and let it rise slowly. Now, measure the distance between the axle and the previous mark again. (If there was no friction in the suspension, the motorcycle would rise a little higher.) This measurement is R2. Third, have your helpers lift up the rear of the bike about an inch, and let it down slowly. Then measure it where it stopped. This measurement is R3.

Setting rear preload of a threaded collar shock with a drift and hammer. Sometimes there is little room for access.Now you’re ready to calculate the spring sag, which is the average of these two measurements. (If there was no friction, R2 and R3 would be the same.) The actual static spring sag = R1 – [(R2 + R3) / 2]. Calculate how much sag you should have using the guidelines of 25-30 percent of total travel. So, you can multiply suspension travel by say 0.275 (27.5%) to get recommended sag. (You can also look up total suspension travel in the manufacturer’s specifications.) Race Tech (www.racetech.com) sells its handy Sag Master measuring tool, which allows you to read sag directly without subtracting.

Spring Preload
If there’s too much sag, increase preload, if there’s too little, reduce it. If your bike has a remote hydraulic preload adjuster, it will be mounted near the rear shock. It increases spring preload when turned clockwise and reduces preload when turned counterclockwise.

If your bike doesn’t have a remote adjuster, the shock’s body may have a threaded collar, or there may be a stepped adjustment. This may require a special hook tool or spanner, which may be included in the bike’s tool kit. Preload is increased by tightening the main nut and decreased by loosening it. The lock nut must be loosened before changing the setting and tightened afterward to lock down the adjustment. With stepped adjusters, you simply twist the adjuster to compress the spring or the other direction to reduce compression. If there are two shocks, make the identical setting on the other side.

Measuring fork compression with a rider in place. Again, use a fixed point that is above the axle.Measuring and Setting the Front
Rider, passenger, and luggage weight doesn’t have as much effect on the front suspension, and many bikes don’t have front preload adjusters. Still, preload can often be changed by installing spacers above the fork springs or varying the length of existing ones.

Measuring front sag is similar to the rear but seal and bushing sticking friction (stiction) are more pronounced. Push down vigorously several times on the handlebars to settle the fork. Mark a leg with a crayon where the seal is located.

Using helpers, lift the front wheel just clear of the pavement. First, measure the amount with the fork extended. This is static sag (or unladen sag). Fully extend the fork and measure from the dust seal on the slider to the bottom of the triple clamp (or lower fork casting on male-slider forks). This measurement is F1. Second, put the rider onboard with feet up, while a helper balances the motorcycle from the back. Have another person compress the fork, and then let it rise slowly. Measure the distance between the seal and bottom of the triple clamp. This measurement is F2. Third, lift up on the fork, let it down slowly, and measure again without bouncing. This measurement is F3. Note that F2 and F3 are different because of stiction. True static spring sag = F1 – [F2 + F3) / 2].

When checking sag and stiction, the greater the difference between the measurements (pushing down and pulling up), the worse the stiction. Fork stiction may range from about 15mm to more than 50mm in a bad set.

To adjust sag use the preload adjusters, if available, or vary the length of the preload spacers inside the fork. You may like the feel of the bike with somewhat more or less sag than the guidelines; that’s OK. Your preferred sag settings may vary with chassis geometry, spring rates, road conditions, tires, and rider’s weight.

Text and photography by Ken Freund

Want to receive free Touring Tips, reviews, deals and contests, and additional content? Sign up for your free newsletter now!

Tags: , , , , , , , , Categories: Technical Tips