Welcome back folks,
Ah yes, the geometry of a motorcycle, something that, even back in the 80’s when this bike was designed, scores of people had input and men in white jackets with clip boards made sure it was perfect. So it’s just as well some hamfisted bearded chap with a an angle grinder thinks he can hack away at it to make it better….oh the joys of custom motorcycles.
Better is a bit of a strong word, different is probably more suited. I want to lower the bike slightly. As it stands (or stood, it’s in 100 bits now) it has a 21″ front wheel and is quite a tall gangly bike. I am in the process of getting a 19″ x 2.5″ front wheel made up which will beef the front end up. To add to this beefiness (?) I want to lower the bike slightly too.
After stripping the forks apart and instantly forgetting how they go together (despite laying them out best I could), I left them sat on my work bench for a few weeks whilst I worked on some other parts of the build. Once i cleared my feet, it was time to start the head scratching…Cue me staring at a diagram of the forks trying to figure out how the hell they work so I can start chopping bits off!
As always, I figure the best way to learn how something works is to try and explain it through the medium of poorly drawn sketches!
The forks are fairly simple in make up, they consist of a stanchion (large shiny bit), a lower fork tube (the other half), a spring and a damper (oh god I feel my mechanical engineering degree coming back to haunt me) and some seals to keep the oil in.
The function is simple – the forks absorb and react to any change in the road ahead of you such as pot holes, speed bumps, kerbs, people, police stingers…you know, usual stuff. The suspension ensures that the bikes progress isn’t hindered and mechanical damage doesn’t happen every time you run over a bump and more importantly keeps your wheels on the ground.
Now I was going to go all tech nerd on you and explain the height of bump vs speed = a rate of change required in the forks to counteract this (the answer is lots), but:
- a) this post would be 18 million pages long and interesting to few and,
- b) time is precious at the moment. I HAVE A BIKE TO BUILD IF YOU DIDN’T KNOW, JEEZ!
So find a condensed guide below that’s only 18 pages instead.
As you hit a bump, something has to give way in the bike so you don’t feel the brunt of the bump and shit your pants. The springs purpose is to move under load and absorb bumps. Springs will deform a certain amount depending on their spring constant i.e. their ability to move from its equilibrium point. Let’s take a purely hypothetical example. If a spring has a spring constant of say 83kg/metre and I decide to sit my fat ass on it (I weigh 83kg), then that spring will deform by 1 metre! See below crude sketch:
This movement can be summed up by Hookes Law, which connects the forces, deflection and spring constant into a handy to remember formula (cold shivers harking back to my mechanical eng degree).
Of course it gets more complicated than this because we are humans after and we always want the best. The above example was too easy to understand. Springs can be wound differently so that if you compress it more, you get more resistance. There are different types known as Dual rate and progressive springs.
If we use me as an example again and I stand on a dual spring with the first half that has the same constant (83kg/metre) and then the coils tighten up and the second half is 166kg/metre then I wold only compress the second half by 0.5metre.
Progressive strings take this theory further by adding more winds. this gives varying level of response for low and high speed bumps. Meaning as my fat ass compresses the springs, the amount of force required varies, because the spring constant is varying over the spring.
Still with me:
If we only had springs and nothing else uncontrolled then this spring would always be bouncing (like Tigger) and more than likely throw you off your bike. We need to control this movement so the bike goes back to its happy place i.e not trying to throw you off and kill you.
Dampers are used to control the spring and to settle it down to the equilibrium point (i.e. normal riding position). The concept is fairly simple:
- You have a piston with holes in it inside of the fork. The fork itself is filled with oil. The stanchion fits tightly around this piston
- As the spring and stanchion move under load, the piston subsequently moves through the oil with the holes in it. (remember my shady sketch above, they are all connected)
- The resistance felt by the oil pushing through the holes settles the spring.
- This resistance varies depending on the speed the oil wants to pass through. If you want the piston to move faster, you get more resistance (high speed/larger impact) and less resistance for slower.
- The resistance can be tinkered with by altering the piston hole size and the fork oil.
See another work of art below too help:
The Spring and Damper have to be set up correctly to give you the best riding experience. To get the perfect suspension, you want forks that react the correct way to your riding environment i.e. you aren’t going to fit dirt bike suspension to a Yamaha R1 because they are designed for completely different purposes (yet…)
Shortening your forks
So one way to undo all that engineering that Mr Yamaha spent so much effort perfecting is to tinker with the above variables and completely ruin the handling of the bike! Kidding on (kind of).
There are several ways to lower the forks, and depending on how pikey (can I say that these days?) you are, helps you decides on the best method:
Most Pikey: This method relies on cutting the springs down by the amount you want to lower the forks and making spacers to fit. The issue I have about cutting the springs is that if the suspension has a progressive or dual rate spring, you can drastically alter the spring constant (as the spring length and spring constant are related) – Which means in laymans terms, you end up with a much stiffer spring which can drastically alter the handling.
Least Pikey: You speak nicely to the likes of Hagon to build you the shorter spring you want in the same parameters (if you liked the handling) and this will give you a lowered bike but with the same handling characteristics.
Reversible/lets hope this works Method: This is the method I went for, because looking at my shock absorbers, there is a big spacer at the top of spring and the stanchion.
Whilst sketching this out figuring out how the damn thing works I concluded that the spring + spacer = how high the stanchion will protude the lower fork slider
So without cutting my spring, the other way to reduce this height is with a shorter spacer. This means my handling stays intact. The downside is that I have reduced the overall travel by the amount I have cut off, so the chance for my forks to bottom out or top out increases. The only way to tell whether this will impact me is to ride it and put it to the test!
I went straight to screwfix and bought the nearest size pipe (comically in a 2.4m length) and cut a couple of spacers 1.5″ shorter than the original
This was now ready to be put back in the fork. Simples.
One of the fiddly bits of assembling forks is fitting new fork seals. I watched a guide online that said buy a fork seal driver. At £35 for one and being a tight git, I set about making a home made one. Initially I thought about using some larger pipe to slide over the stanchion, but I read a guide where someone used electrical tape to drive them in place. Very cheap and very effective. See below (also in video lower down):
I had my new gaiters delivered from KEDO in Germany to go on, as well as the freshly vapour blasted fork lowers.
I then rebuilt the forks as per the instructions (Twice…I may have forgotten a key component *Facepalm*).
LOOK HOW GOOD THEY LOOK! I am very chuffed the way they have turned out and they are shorter! They are now currently sat waiting to go back on the bike.
I have had a bit of a week of it with waiting on bearings to arrive, slowing progress, but this weekend should see a rolling chassis with the engine in the frame.
More updates to follow,