Progress report

I have been struggling with writing blog posts of late for a variety of reasons.  Firstly, I am working from home far more often.  This deprives me of most of my “train time”.  The half-hour commute (each way) is a golden time for me to catch up on things (such as writing blog entries and professional development AKA “coding for fun”)  It seems strange to gain an hour a day at home, but lose this time, but that is the natural evolution of my day. – When I am at home and finish work, I do things that I can do at home…  When I am on the train, I do things that I can do there.  With less options available to me on the train, my time tended to be more focused.

The second reason the blog posts are irregular, is that the restoration project on the RGV is taking up a lot of time and effort.  I have some admissions to make: I am not much of a mechanic, so mechanical tasks take me longer than they really should.  Also, the garage is a place where I “potter”.  It is a form of relaxation and as such, inordinate periods of time are required to achieve any real progress.  I have also been trying to stick to a monthly budget.  – This restricts the rate of progress I make on the bike.
Most of my work is guided by the experience of others.  The earliest model RGVs are now over twenty years old and there are plenty of enthusiasts (a large proportion being in the UK) who frequent an RGV forum.  I have yet to really come a problem with my bike, that someone hasn’t experienced before and written about on this web site.  It really is a fabulous wealth of information.

So, what have I been up to?

The swing-arm bearings for the bike felt fine, so I did not disturb them.  The dog-bone (linkage) bearings for the swing arm were not.  After losing several hours and inventing several swear words, I took the part to a local mechanic to press the blind bearings out.  I lack the specialised tool to remove them, and the “drift and hammer” technique to bash them out proved fruitless. I felt somewhat justified when I went back to collect the part, as the mechanic had found them far harder to remove than he was expecting!

Knick-knack paddy whack, give a dog a bone!

Based on discussions found on the site, I purchased a GSXR 600 rear shock absorber for the bike.  There is some question as to whether the spring will be too “heavy” to allow the correct range of motion, but track bikes do tend to be sprung more heavily than road bikes.  One forum member posted the measurements he took after fitting the same part. Weighing approximately the same as I do, he was able to set the suspension correctly, so I figure I have a sporting chance too.

Thing on a spring!  One of my favourite games on the C64

At the moment, I am in the middle of performing a top-end rebuild for the bike.  The bike still felt like it had good compression, but I wanted to check the condition of the cylinder barrels and it had been a long time since it had a “freshen up”.  Nothing comes un-done easily on a bike engine last touched ten years ago.  I have learnt that the hammer drill setting can be used as a substitute for an impact driver… sometimes…  At other times, it has led me to my other discovery, that a Dremmel is a good tool, for cutting new screw heads in a screw with a butchered head…
There is no end of “bling” that you can purchase for an RGV.  This also means, there is no end to the amount of money you can spend.  I am still managing to avoid the trap of spending money on good looking parts that do not help finish the project – but I do have my eyes on a couple of items. 😉
I am however reaching a point where I have a tough decision to make.   My budget is currently “in the red”.  I allocated myself a monthly spend, and I am about six weeks ahead of that, at this point in time.  Even using mental arithmetic and not trying to be precise about the budget, I can see that finishing the bike on time and on budget will be a story that belongs in the “fiction” section of the local library.  Going back to my New Year’s Resolution, that means I should be planning on taking the VFR out on the track.  But, to be honest, I’d rather spend the money that would cost, on “finishing” the RGV this year.
I use the term “finishing” loosely, as I said, there is no end of “bling”…

Motorcycle Suspension Part 3 – Compression and Rebound

Previously, we looked at the role that springs play in motorcycle suspension.  Suspension is comprised of a spring and a shock absorber.   The shock absorber’s role is to limit the rate at which the spring compresses and expands.
Ideally, a motorcycle wheel should not lose traction with the road surface.  On a perfectly smooth surface, this is not hard to achieve!  For the wheel to grip properly, it must be pressed to the surface with some degree of force.  Luckily for us, the weight of the motorcycle provides us with this downward pressure.
Suppose for a moment we have a motorcycle without any form of suspension:  Not only is there a lack of springs and shock absorbers, but the tyres themselves provide no flex.   Once we are moving forward, the motorcycle has inertia in this forward direction.

Bike moving forward

When this motorcycle hits a bump, there is no way of “absorbing” the bump.  As a result, some of the horizontal forward inertia changes to vertical inertia.

Bike moving over bump

Once the bump “ends”, the ground “falls away”.  The motorcycle being subject to gravity loses its vertical inertia and returns to the ground.  The vertical velocity is not overcome instantaneously. Rather, it is subject to the laws of gravity.  Depending on the speed of the motorcycle and the size of the bump, the wheel(s) will remain in the air for some period of time.
With the benefit of suspension, not all of the motorcycle needs to be subjected to the vertical inertia caused by the bump.  If the bump is small, the springs may absorb the bump.  As we saw previously, the spring will “want” to return to its state of rest.  The spring will expand in whatever direction cannot suppress the force that the compressed spring now has.  If the bump has ended, the spring will likely expand downward, otherwise the spring will expand upward.  The spring extending upwards is effectively transferring the vertical inertia to the rest of the motorcycle.
As noted previously, the spring continues to oscillate shorter and longer of its “at-rest” state, until all its kinetic energy is expended.  The suspension unit helps regulate the extending and compressing of the spring, by forcing oil to pass from one area of the suspension unit to another.  The oil can only pass through small valve holes.  Because a liquid cannot be compressed, the oil passes through the valves at a set rate.
Adjustable suspension allows the size of these holes to be altered, allowing a faster or slower rate of oil transfer.  Different systems handle the transference of oil in different directions.  That is, when the shock absorber is being compressed, oil is transferred by one set of valving, and when the suspension expands (or rebounds) a different set of valving operates.  The first set of adjustable valving gives you the “compression” setting, and the return valves gives you the “rebound” setting.
In recent years, suspension has been further refined by having two lots of compression valves.  These are known as “high speed” and “low speed” compression valves.  This “speed” refers to the speed that the suspension compresses.  Fast compression is needed to handle surface irregularities (i.e. bumps) whereas “handling issues” arise from slow compression motion.  Unfortunately, I have not had the chance to study suspension with this feature, so how the shock absorption works and which valving is used is as much a mystery to me as the next person.  (Unless they happen to work for Ohlins).
As you may well be aware, oil viscosity affects its flow rates.  A more viscous oil will therefore be slower to transfer through the valves than a “light” oil.  Oil has a tendency to become less viscous as it heats up.  Forcing oil through small valves tends to lead to the oil heating up.  (The kinetic energy of the oil movement is exchanged as heat, due to the friction caused by the fact that the oil cannot move freely.   This of course affects the compression and rebound rates of the suspension.  The bumpier the surface, the more this becomes an issue.  This is an unavoidable fact, but that does not make it desirable!

To combat this, suspension units have been designed to carry as much oil as possible.  More oil, means more oil to heat, meaning more energy required to do so.  A bigger oil reservoir also means a larger surface area in contact with the oil, drawing excess heat out of the oil.

That is about it for how the parts of suspension fit together.  How to make the most of the suspension you have, is a story for another day.

Photo attribution:

http://www.flickr.com/photos/nathanwells/1084352488/in/set-72157601369471448/ courtesy of Nathan Wells.