I had an amateur road racer write in earlier this year with some fairly detailed questions about suspension and chassis set-up. I was holding off on a response, to see if anything more ‘mainstream’ came in from the readership, but alas, there have been very few questions arriving in to the tech line via email. I find it tough to believe that in all the 20 or 30 people that read this column, no one has any questions other than ‘Super Gillus’ who has been driving me crazy about road race stuff! Anyway, his question came in as follows… ” What about basic road racing suspension set-up Canadian style, static and dynamic sags, (with ty-raps). Maybe something on Hi and low speed compression damping found on those very desirable shocks. Do you play with oil weight in modern, fully adjustable forks? Is it true that hydraulic adjusters tend to be more effective towards the maximum? Would you add more rebound damping when you add more spring preload? Do you end up using most of the suspension travel?”
Gone are the eloquent letters of the past, replaced by the machine gun emails. Anyway, Gilles has brought up some good questions. I have discussed the importance of measuring Sag in a previous article entitled “Suspension measuring and changing” which came out in February 05’. Static Sag is a term referring to how far the motorcycle sits into its suspension travel under its own weight. Dynamic Sag brings you, the rider into the equation. The starting points for these measurements are taken from the fully ‘topped up’ position, meaning the suspension is fully extended. I outlined in detail how to perform the dynamic sag measurement for the front suspension in February, and the rear is very similar. Have the rider sit on the bike, fully geared (leathers, helmet, butterflies etc.), feet on the pegs, while a spectator with bad timing holds the front of the bike by the fairing. The measurer then measures from the axle straight up (perpendicular to the ground, not the swingarm) to a point on the tail section, recording this number. The rider then dismounts with out kicking the measurer in the head. The static sag end number can now be recorded as the motorcycle now sits just under it’s own weight. The rider and spectator (if they are still around) can now gently lift on the foot pegs to top up the suspension. You can now record the top out measurement. There you have it, the static and dynamic sag can be calculated by subtracting your top out number from your measured numbers. For an example, have the rider settle into position and measure from the axle to bottom of the number plate. Say that number is 500mm, now the rider hops off and the measurement grows to 525mm. The rider and helper now gently raise the bike until the back tire is just about to come off the ground and the number climbs to 535mm. Now you have what you need, the static sag is the topped out number- static number (535-525mm) or 10mm of static sag. The dynamic sag is the topped out number – the dynamic number (535-500mm) or 35 mm of dynamic sag. You can now tease the rider about their weight as these numbers are a little on the high side for the racetrack. Typically on a road race bike, rear dynamic sag should be around 25-33 mm, with static numbers in the 3-5mm range. This is a ballpark, I have deliberately ventured on both sides of these numbers with top road racers when we are trying to achieve a certain suspension function goal. Amateurs should start in the low 30’s to high 20’s (dynamic) until they feel comfortable with the setting. Sending someone with limited experience out on the track with rear suspension too stiff can be a recipe for a high side. I am generalizing, but a stiffer sag number will limit the weight transfer rearward which in turn limits the available rear traction. Front dynamic sag numbers will always be a little larger, in the 33-45mm range dynamic, 8-15mm static. Please remember that these are ballpark numbers, to give you an idea of what to expect. If you are way off of these numbers, seek a second opinion. Proper suspension action is the grounds to a safe motorcycle. If you have a component binding, seizing or installed improperly, it may show up when doing these checks and alert you to a potential hazard.
Gilles next question deals with high and low speed compression damping on a rear shock. He has bought one of the new 05’ Kawasaki ZX-6 RR Sport bikes, and the rear shock has two compression adjusters. The OEM equipment is just getting better and better each year. The low speed compression adjuster mainly controls the fore and aft pitching of the motorcycle. As you apply throttle exiting a turn, the weight shifts to the rear of the bike, and as you apply the brakes the bike will pitch around, compressing the front suspension and unloading the rear suspension. These are all movements that are resisted by the low speed “clickers”. The high-speed compression adjuster tunes the resistance to the faster hits, or the suspensions reaction over the bumps. Changes to the low speed compression and rebound adjusters are what can typically be felt when pushing on the suspension by hand. However, a Fox Shox technician once informed me that their compression adjuster (offering only 7 or 8 options) was more of a med to high-speed adjuster. He was responding to my comment that I couldn’t feel any difference between #1 and #8 in the pits.
Oil weight is still varied today, but cautiously. It is not the big band-aid that it was 10 years ago. Remember that a Fork oil viscosity change will change the rebound, compression, low and high speed damping all at once. Suspensions are so much better now that whole scale, across the board change requirements like that are rare.
Whew, ok Gilles, we are running out of space. So, to answer your last three questions… not sure what you mean, not usually, and I hope not. There! So unless some of you write in with some other questions, I may just have to answer Gilles’ last three questions in detail!