6 minute read
Suspension Setup
INSIDE TECH By John Sharrard Front sag AND a few questions
Last month we discussed, in detail, how to set the rear shock’s preload properly to achieve target rider sag. Everyone should do this on every motorcycle! Even the cheapest bikes have facility to adjust the rear preload as rider and passenger or gear weight changes and has a dramatic, and sometimes even dangerous effect on a motorcycle’s handling. We discussed the procedure and I promised that this month we would discuss the front sag and how to adjust the preload properly. As discussed last issue, the first step is to get a ‘front top-up’ number. The easiest reference point to measure front wheel travel is along the shiny chrome fork tube. On an inverted fork where the upper fork tube is larger than the lower tube, we will measure from the fork seal wiper down to the cast aluminum portion of the foot. This number is often the same as the full available wheel travel, but is occasionally more. In our example, let’s say it is the very common 127 mm (strangely close to five inches... lol). On older conventional forks where the smaller male fork tube is up in the triple clamps, we will measure up from the fork wiper to the lower portion of the bottom triple clamp.
Now, to extend the suspension fully, the front wheel has to be off the ground. A centre stand is ideal, but you will need to get the rider to lean on the back of the bike as centre stands are usually biased to get the rear wheel off the ground. A gentle push down on the back of the bike will pop the front tire off the ground. If the bike doesn’t have a centre stand, you can carefully pry it left onto the side stand, or use other proper stands to allow the front wheel to dangle freely. Obviously, you cannot use a front stand that goes under the two fork legs; these are common for brake service and checking tire pressures and they are common at racetracks for the installation of tire warmers, but they do not unweight the springs. Now that the front tire is elevated and you are ready to begin, a mistake is often made when doing sags. Yes, we need to measure our top-up reference at this time, but, we need to ensure the forks are fully extended! There are small top-out springs inside your forks that are
designed to soften the impact when a motorcycle’s front wheel leaves the ground. This is common on an off-road racing or trail bike, but will also occur on a street bike during a wheelie. Without these top out springs you would hear a possibly damaging BANG when the front suspension extended and the wheel left the ground. So, at this moment of measuring, you can hold and position your tape measure with your right hand and gently push down on the front fender or wheel with your left hand to see (or confirm) that the top out springs are fully compressed and the front suspension is actually fully extended. You will usually see the exposed portion of the fork tube grow a few millimetres as you push down and gently come to a firm stop. Then you can take and record your accurate top-up number on a piece of paper or in your phone.
The rider can now release their weight off the rear of the bike and it is ready to come off the centre stand. Now, drop the bike off the centre stand and have the rider hold it vertically from the side. I will push gently down on the triple clamps to make sure the bike isn’t sticking up and then I will bend down and take my front measurement again. This new measurement of bike weight only is called the ‘static number’ and once subtracted from your top-up number will give you the ‘static sag.’ The static sag will be commonly 20 to 30 mm less than our top-up on a five-inch travel street bike. If it is much more than 30 mm, the springs are too soft. Now, ask the rider to hop on the bike and a third person can balance the rider as they get into riding position with hands on the grips and feet up on the pegs. If I suspect the fork is sticky, I will reach in and give the triple clamp a gentle push down to make sure it is where it wants to be. Then, I bend down and take my last number, which we call our rider-on number. In our example of a 127 mm travel front fork, we are looking for a rider sag of 35 to 40 mm (top-up minus rider-on). If we are at 50 mm, I will stand up and add 10 turns of preload to the fork caps. The vast majority of fork caps containing preload adjusters will have a 1.0 mm pitch thread. So, we can expect a 1 mm change in sag with every turn of preload... easy peasy. If sag was 45 mm, add five turns to hit our 40 mm target. So, if you can’t hit the target number with the number of turns you have available, chances are, you need stronger springs. If your fork caps are low quality cheaper units that have no adjustability, you can take the fork caps off and fabricate longer spacers; however, if you need much more than 15 mm of preload on the springs, they are undersized for you. Don’t be surprised or annoyed that you need stronger fork springs, this is very common in North America as most riders are heavier than the 150 to 175 pounds that the Japanese and European manufacturers seem to target their designs for.
Now, something we all love to do... Let’s switch gears a bit. After we do suspension service in our shop, many customers will trailer their bike home and often ask me about tying a bike down. They may start off by asking, “Is it true that pulling a bike down too hard is hard on the fork?” (Or fork springs, or seals.) Well, the answer is strange. The quick answer is no. You pulling the bike down with tie downs or ratchet straps is not much different than what it experiences bouncing down the road or trail. But... a fork seal operates in a wiping or sliding motion. This motion tends to allow dirt or contamination a chance to wipe off the fork surface as the oil pressure builds inside the fork during normal operation. What the fork thinks is strange is to be cranked down to one spot (which builds air and oil pressure inside the fork) and then jiggled in your trailer. If (and it’s a big if) there is just a bit of dirt stuck in the seal, or there is a small groove or imperfection in your fork tube at this point of being tied down, the fork can leak during the trip. The fork may not have leaked at all during normal operation, but can leak during this odd combination in the trailer described above. You can’t predict or prevent this (other than knocking your wipers down occasionally to keep your seals as clean as possible), all you can do is try to crank the bike down as little as possible to avoid travel with high pressure in the forks. Transporting without securing the front of the bike necessitates a Pit Bull-style rear axle restraint system that doesn’t compress the forks at all, or alternately, using four straps pulled down gently rather than two straps pulled down aggressively.
Anyway, for what it’s worth, that’s my two cents for this month! So, ride respectfully in traffic, carefully in the woods and we may see you out on the trails! IM
