7 minute read
Bumps
INSIDE TECH By John Sharrard
Bumps: why we feel them, why we don’t Part 2
Last month we discussed the very first component of ride comfort on today’s motorcycles, the amount of suspension travel our bikes have. We touched on the different categories of bikes, from street to dual purpose adventure bikes right up to full off-road or motocross bikes and why they need more or less travel. We came to the conclusion that more travel is better for comfort, but what are the negatives of too much suspension (or wheel) travel? To quickly recap for those that missed it, the bumps are smaller on the street, if we don’t count potholes, so eight inches of travel or more just drives the seat height up and can make the bike vague and flexy. A bike that is too high limits the number of people that will purchase it and too much travel, combined with the grip a warm tire has on pavement, can result in unwanted and unnecessary pitching front to back and wobble left to right. We then closed the column saying we would touch on optimizing the suspension your bike has now. So, rather than go and buy a bike with more travel, let’s see how good we can make the bike you have now!
The very first thing we can all do that doesn’t cost a cent is to check the rider sag on your current bike. Basically, all you need are two friends, a paper, pencil and a tape measure. To begin, the bike should be loaded as you most often ride it. If you have a back pannier with tools, or your riding partner, they need to be included. Often when we do sags here at the shop, the main rider and passenger may both come and we will do sags for the rider by him or herself, and then the passenger hops on and we measure again, and then we will discuss a spring or preload solution to cover both scenarios. Or, if the passenger only hops on five percent of the time, we may just coach a small preload correction for this combination if the numbers dictate that we should. To be most accurate, the sags should be done with your helmet, boots, gloves and all your other gear on as well.
To prep the bike, we look up vertically from the centre of the rear axle and find somewhere convenient on the body above to measure to. On a race bike, it may be a corner of the number
plate or a seam in the body panel. If the bike has big saddlebags that make it difficult, we may remove the saddlebag to get into the frame and place the bag on top of the rear seat to maintain weight accuracy. If an opportune top measuring point isn’t blatantly obvious, a piece of masking tape will do just fine. Now, we need to top up the rear suspension. If the bike has a centre stand, perfect, pop it up on that. If the bike has a robust side stand, the rider can often ‘pry’ the bike towards them carefully to extend the rear suspension and lift the rear tire off the ground. A motocross bike can go on a track stand for this step as they often don’t have a side stand. The rider should not be taking the first measurement! The person doing the final measurement needs to do the first measurement! Not only for practice (you would be surprised how many people can’t read a tape measure), but also the technique and the actual tape measure needs to be the same.
Now with the rear wheel off the ground, the measurer can insert the blade of the tape measure into the hollow of the rear axle and measure up to your top reference point. At this point, I may move my top reference point a little so I have a nice even number on my tape measure, say 500 mm. Write this number down and call it your ‘rear top-up.’ Now drop the bike down onto its own weight and take the measurement again with the rider just balancing the bike from the side with the handlebars straight. For a typical street bike with 125 to 130 mm travel we would expect to see something like 490 to 495 mm in our example. This second measurement is called our ‘rear static number.’ The static sag is then top up minus ‘rear static number’ equals ‘rear static sag.’ Write these numbers down. Ask the rider to gently push on the seat and release a few times. The number should come back to the same spot every time. If it doesn’t, you generally have sticky or rusty linkage or swingarm pivot bearings.
At this point, the bored but requested third person comes into play. They need to stand at the front of the bike to balance it while the rider takes a seat and puts their hands on the bars and feet on the pegs. Give them a moment as this is usually a new exercise for both these people. They will argue for a bit, the rider will lose confidence and put a foot down once or twice while accusing the holder of not holding them up straight. The holder will then counter and accuse the rider of leaning to one side... It is really quite entertaining. Once settled, ask the rider to gently bounce a couple times. They will, without fail, strangely misunderstand ‘gently’ and stand up and aggressively jump on the pegs a couple times, or endlessly it seems until I ask them to stop. If you have neglected to put up the side stand, the rider will hit the floor with it while jumping on the pegs and pitch the bike right, causing a repeat of the foot down and argue cycle again. Then I explain what the word gently means. We just want to break friction to see if our third rider-on measurement is good and repeatable. We are not trying to bottom the rear shock or dyno test the damper during this process. So, once all theatrics and explanations are completed, you should be able to get a repeatable rider-on number and you can have the rider hop down and start apologizing to the holder person, while insisting that they were right the whole time... I’ve seen it a thousand times.
Now, we have in our possession our hard fought top-up, ‘static’, and rider-on numbers. To recap the obvious, the static sag equals top-up minus ‘rear static’ numbers, and the rider sag equals top-up minus rider-on numbers. Often erroneously supplied to me over the phone, even by experienced racers, is the ‘static sag’ minus ‘rider sag,’ but this difference means nothing; both sags have to be taken from fully topped up. To circle back to our example, a street bike with five inches of travel (approximately 125 to 130 mm) should have a rider-on sag of about 30 mm, while race bikes are a tick tighter at 25 to 26 mm. Again, make sure all tools, clothing and passengers, if applicable, are on board for this measurement. If you are at 40 mm, you need to add preload to get to 30 mm. You may have a multi-position steel cam, aluminum threads, or a handy knob to turn to help you out. If you can’t get down to 30 mm or you can but lose all static sag getting there, you need a stronger spring! Your goal is to hit the target rider sag while maintaining at least three to five mm of static sag. If you can, both your spring rate and preload are good for you and your application! Next month we can discuss the front fork and sag procedure, but the process is the same, with the targets closer to 40 mm rider and 30 mm static. Until then, ride safe, keep your stick on the ice and wear your mask, or better yet, your helmet! IM
