What about the offset?

Started to read a book called Motorcycle Dynamics, which dives into the physics of handling bike in quite a details. That brings me back to the Geometry -post where I mentioned the steering axle and fork main offset. Offset can be used to adjust the trail, but it has some other implications as well.

One thing offset does is that it reduces the up and down motion of the front when turning the handlebar. Since the steering is not in upright position the center of the front wheel lowers. The plot is an example of calculations with 32 degree caster angle (angle of steering axel from vertical line) and 32 cm wheel radius.

Calculations  assume still some silly stuff like dimensionless wheels and hence no roll at all. Plus I'm severely hangoverish from yesterdays party at the garage so I'm not promising anything of this is 100% correct

So what? Good question, I've no clue about what all that means. But at least pumping front up and down is not nice for handling since it makes it heavy. If there is 150kg of weight on front wheel you will be pulling that up and lowering down in every turn.

Random pics maybe related to something but probably not

Harry potterism of building bikes

There is something funny and fascinating about the whole culture of craftmanship around building bikes. First of all, there is no internet here. If I need to find someone to measure and bore the engine I will not find that person by googling it. They way these masters are found is by knowing someone, who knows someone who knows a person with a beard like Dumbledore and a skill of resurrecting engines that are know to be dead for decades.

These dumbledores can be found in addresses like seven and half of a street that probably is not on a map. After finding the building there will be about dozen iron doors with no signs of what so ever, so one needs to be good at guessing, have a guide or go knocking on all the doors until the right one is found. When the right one is found there will be someone speaking in dialect barely understandable to anyone who was not born on a bike - and he will be asking who sent you. If you answer that question right you may be granted access to dungeon of parts, tobacco smoke, machinery and bikes that are so hand made and customised I'm willing to believe they are constructed atom by atom.

So cool.

Pic not related but the whole blog looks so damn boring if there are no pics in the posts. This is from Norrtälje show in May 2015. Or was it June.

Visualizing the frame

That is about how it is going to be. Tape is a guide for an eye on how the frame is going to be. I'm not sure about the oil tank - it's funny but, well it's funny. And also it might turn out to be very impractical in shape and difficult to fit.

It's not going to be a chopper. Thing which sometimes seems to raise strong opinions.

There was a swap meet and afterparty some weeks ago where I tried to get to know and into discussions with different people who might be able to work with the engine and the transmission. Engine at least needs to be measured and most likely bored as I'm expecting little damaged. Anyway, as part of these discussions I got plenty of opinions what the bike should be. Especially there were couple of nice and very skilful chopper builders who really seemed to struggle with trying to comprehend why an earth someone would not build a chopper. They were all like "why build a bike at all if it's not going to be chopper". Good question, but building bikes makes no sense anyway - its extremely expensive way of getting possession of something highly impractical. Still not making a chopper, at least not from this one but the next time I just might.

And I did promise to pay these aforementioned chopper dudes a visit and maybe learn something more about what they do and how.

Cleaning up

The rims were found to be round enough to keep working with. So the next step was to get the beautiful assistant participating in contamination removal procedures.

Measuring roundness

I'm keeping the old rims for two reasons. The real reason is convenience and money - since they are there and they seem sort of decent why waste money for new ones. To everybody else I'm telling that I'm saving the precious change percents because it's much cooler reason than being cheap or just broke.

But how decent the rims actually are? I did very scientific measurement for roundness and the spatial displacement (see the pic). I put the wheel on solid stand where I can rotate it and then I have the cord with a bolt pendulum with a tape marker on it. This was actually pretty accurate, and it gets better with a hint of modern tech -- put the wheel spinning and take slow motion video with a phone. Gives easily accuracy less than millimeter.

They are round and flat with displacement less than the mentioned one millimeter.

Geometry

I know I said something about digging into the geometry of the frame and angles. I did some of it but never wrote about it. Mostly because I didn't bother to draw any pictures or do any real math - but here is some of the thinking I did.

Let's start with a pic that I took from the tool that knowledgeable people nearby have made.

There are three different measures that typically are discussed; First one is called trail and is the measure J in the pic. It is the distance between the point where imaginary continuation of steering axle hits the ground and the point where the tyre touches the ground.

Bearded wise men, much like Gandalf, can tell that about 10cm of trail is good. If the trail is less the steering becomes volatile and if it's too much the bike becomes heavy to handle. Heavy meaning that it's almost too stable at the middle but there may be a sudden kink at certain angle where steering collapses to a side. So, as many times is in life, too little is as bad and so is too much, but for different reason.

While we try to aim in getting the trail to close to the magical 10cm we have some freedom of choice. We have chosen the tyres already so those measures are fixed. But if we stretch the front we can adjust the height of the steering axel top from the ground. I'm not going to do that because I don't have to. If I was making a chopper then it would be inevitable. But we still have couple of things left we can adjust - the rake angle, which is the angle between the steering axle and fork and the offset of steering axel and fork neck.

The measures in the picture are somewhat close to what makes sense but not final for my bike. I will have to see the rake angle in its place when the rest of the frame is ready.

But the million dollar question I was pondering on was that why exactly in mathematical sense the trail needs to be something and what is the physics behind when it is wrong? There are two main reasons why bikes can be ridden at higher speeds to begin with. The explanation typically given in the driving schools, or at least given to me, is the precession - i.e. momentum related to rotating object and it's reluctancy to change it's position. Fine, I buy that and it is part of the reason but not the whole truth. You can try this with an old fashioned bike that has fork with a little angle there in between -- turn the handlebar 180 degrees and try driving that bike. I bet you a beer that you will be eating grass in a minute (try it on grass and not on paved road for the sake of your health).

What I'm trying to say is that when bike moves, there are other forces in to action than just the precession. One main thing is the dragging force of the front wheel and the line of action compared to the point around which the wheel actually turns. And the point around which the wheel actually tries to turn is not really where the wheel touches the ground but the line of steering axel. This is easy to understand and see if you lift the front into the air -- steering axel is the rigid part attached to the frame around which the whole moving part of front tries to revolve around.

So loosely speaking the momentum that will try to turn the wheel back to center line after displacement and around the steering axle is dependent on the distance of between the two points - the trail. At the extreme if trail would be negative it would mean that the whole set up is labile. Much like balancing pencil on your finger tip in upright position compared to keeping it in the same position by hanging it from the upper end. But this is not exact explanation and I'm not going to do the math. Not at least right now but maybe some day.

White walls

Something I felt I must have, are white wall tyres. These arrived to my office today and I just unpacked them second ago to see how they really look like. They seem to be about the right size I guess and pretty as a little piggy.