February 22, 2018

Confessions of a Born Again Biker – Part 18 – SAG = L1-((L2+L3)/2)

Confessions of a Born Again Biker - Suspension SAG

I never thought I’d end up receiving an education on motorcycle suspension SAG courtesy of a weeping oil seal. Strange the things that take you in a certain direction.

This Confession begins when a smear of oil on the front right fork leg indicated that the seal was leaking. Having moved past the initial torrent of unprintable words, I was pointed in the direction of the omnipresent internet and told to learn about seal cleaning.

It seems that in the majority of cases, weeping oil seals are the result of tiny bits of debris getting past the dust seals and wedging themselves between the actual oil seal and the fork leg.

To fix the problem, you fashion a curved piece of thin plastic with a hooked end, and push it up between the seal and the fork leg and clean out the dirt.

There are stories of intrepid motorcyclists making these shims roadside, from takeaway coffee lids or old milk containers. One inventive rider even made one using 35mm film. You know, that stuff we once put in cameras before the digital age.

Should you find, as I did, that the milk cartons you have are not rigid enough to make a useable seal cleaning tool, then there are at least two manufactured versions available. These are the very thing you will wish to had bought ages ago when you find yourself in need of one.

Having tried the homemade variety, but disappointingly only able to reduce the amount of leaking oil to a light coating, rather than the dribble it was, my Tiger is going to Webbs of Lincoln to have the forks rebuilt. All of which brings me neatly, onto my education. Motorcycle suspension is a dark art – feel free to correct me if I have it wrong.

SAG = L1-((L2+L3)/2)

Confessions of a Born Again Biker - Suspension SpringsSuspension settings, up to this point, were just something that I changed if I didn’t like the feel of the motorcycle. I’d add in some preload, or take some out, and if the feel of the bike changed for the better, job done. I was being as scientific as a garden gnome.

SAG is important because it sets several other important factors, which control how your motorcycle handles. If the front or the rear suspension sits in the wrong position, then things such as the angle of the headstock may not be in the optimum position.

The best example I can think of are the lowering plates that some “vertically challenged” riders use. These plates allow the rear of the bike to sit much lower, consequently, the front of the motorcycle points up more than the manufacturer designed the bike for.

To compensate for the lowering the plates, the forks need to be pushed up through the clamps. Don’t ask me by how much. There is a complicated calculation involving rake angles and ratios.

Adjusting the basic SAG of your motorcycle ensures the geometry of all the other angles are in the range the motorcycle designer was aiming for. As it is always good to make changes from a known starting point, now is the right time to read the manual and ensure all of the suspension settings are factory standard. This way, when you make a tweak, you know where you started from and can find your way back there.

The other important factor when setting SAG is you. If you normally ride with a passenger, then they are needed too. It all contributes to setting the correct SAG value for when the bike is loaded up.

In what follows, I’m talking about measuring the exposed part of the inner fork leg. The same calculation can be done for the rear, just measure from the centre of the rear axle to a fixed point on the rear of the bike. Just remember to use the same fixed point each time.

Confessions of a Born Again Biker - HyperPro Rear Motorcycle shockThe first measurement, L1, is easy. L1 is the fully extended, no load figure. If you have a centre stand, then ask someone to push down on the rear so that the front wheel comes off the floor. Don’t pull the wheel; you just want to see how far the forks extend naturally. With the front wheel off the ground, measure the length of the exposed inner fork leg.

The second measurement you will need, L2, is the fully compressed and then allowed to rise back up figure. The bike needs to be on both wheels for this, so working next to a wall is the easiest way I found to stop myself falling over. With you on the bike pump the suspension a few time and let it come to a natural resting point, with you sitting on the bike, feet on the pegs.

Ask your assistant to take the same inner fork leg measurement and call this L2.

L3 is the opposite of L2. Ask your assistant to push the front of the bike up, so the forks extend, and then let it settle. As with L2, you need to be on the bike with your feet on the pegs, while your trusted assistant measures the amount of exposed inner fork leg.

Now add L2 and L3 together and divide by two, then subtract this figure from L1 and you have the amount of SAG – Simples!

If the amount of SAG you have is more than the recommended figure, then you need to add preload. If it is less than the recommended figure, then remove preload.

It is worth noting that adding preload does nothing to stiffen a spring. Rather it increases or decreases the amount of force required to make the spring start to compress.

And where do you find the SAG figures you need to work to?

Well, I read the manual and couldn’t find them in there. I turned to the internet and found nothing but arguments and contrary information. Even calling Triumph yielded nothing as they only talk about the default preload and rebound setting, nothing else.

So where is there a reliable source of SAG and other motorcycle suspension information? Teut Wiehn at TW Suspension in Preston is the best answer I came up with. Having learnt a little, I know I’m only skimming the surface when it comes to the dark art of suspension setting.

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