1983 XV500 Cafe Racer

[Popeye SXM]

Nicely described, I hope those non engineers here now have a better understanding of rear suspension, converting to a mono set up is often more difficult than most invision. I think the best way to learn to is to try to do it yourself

 

[Karlloss]

An easy way to see if you are in the ballpark is to measure the full travel of the shock with the spring removed. It need to be quite close to the travel you expect to see in your own set up.

For the record, I have taken the spring off and measured the total travel of the shock. The travel does suffice for my design.

I understand what you are saying, essentially it is the total clockwise movement equals total anti-clockwise movement, principles of moment. The anti-clockwise force in this case is the force of the spring, which due to the nature of the spring actually creates a variable force which increases as the lever (crank) rotates, i.e. the spring force in N/mm, which is a compound force from zero to full compression.

The amount of compression of the spring as the lever rotates through its arc varies due to the spring being a linear movement versus the rotational movement of the lever in relation to the top mounting point of the spring.

I agree with your observations, however what needs to be remembered are that as the lever moves through its arc, the spring will be compressed. The compression of the spring will be accumulative (compound) as it shortens, which means the force will increase through the arc of the lever. However as you have explained, as the lever moves through its arc towards the extreme of its range, the linear movement (that which compresses the spring) becomes less than when at the 90 degree point. But as it is still compressing the spring, a greater force will still be needed than when in the first range of movement before the 90 degree point. Essentially it means the suspension will feel soft moving to firm and then slightly softening but not enough to be an issue.

As for the actual damping of the shock unit, this I completely understand, I am familiar with how shock absorber/dampers work.

What needs to be remembered is that how the bike will be used and considering Kawasaki had these characteristics on their 2004 ZX10R which most road riders never noticed, I'm confident that my project will be fine, especially as it will struggle to get over 90mph.

 

[brad black]

sounds like you've convinced yourself. in reality it'll probably work no worse than a lot of even factory stuff out there that is mis-sprung or applied or overloaded. it's not like you're going to have another one of these done well to compare it to. people ride some amazingly crap stuff in blissful ignorance.

the lower shock mount needs to go down and back. i say do it once and do it right. at full compression the lines between 1/ swingarm pivot and lower mount and 2/ lower mount and upper mount need to be at an angle of 90 degrees or less.

 

[Karlloss]

sounds like you've convinced yourself. in reality it'll probably work no worse than a lot of even factory stuff out there that is mis-sprung or applied or overloaded. it's not like you're going to have another one of these done well to compare it to. people ride some amazingly crap stuff in blissful ignorance.

the lower shock mount needs to go down and back. i say do it once and do it right. at full compression the lines between 1/ swingarm pivot and lower mount and 2/ lower mount and upper mount need to be at an angle of 90 degrees or less.

It’s not about convincing myself, it’s about what is feasible within the limitations of what is available. Your comments appear to be condescending, which I do not welcome.

Likewise I’m not going to be riding in blissful ignorance and I’m not building crap stuff. My work is done well. No suspension set up is perfect, even the big manufacturers cannot achieve that.

As for doing it once and doing it well, my build has been remade several times due to trial and error and learning as I go. It is a garage built bike, it is about learning long the way, which is what the majority of builds on this forum are. Clearly you are an expert that gets everything perfect 1st time, maybe you should be advising the big manufacturers. If you don’t have anything helpful to say, then perhaps say nothing.


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[brad black]

this part is not done well. simple as that. look at the maths of it and do it better, that's my point. jmobius has been trying pretty hard too. clearly you're capable of the fabrication, so there's no issue there. suspension is a science. objective, not subjective, and if you do it right it'll make a much better result.

if it's because you don't want to spend the money to get the right shock (as most here won't) then just say that. a custom shock to suit the requirements will fix your issue pretty much. it needs a longer tail so you can lower the lower mount.

 

[brad black]

If you don’t have anything helpful to say, then perhaps say nothing.

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pointing out that you're doing it wrong and offering advice on how to make it better is as helpful as any of us can get.

blowing smoke up your arse will not make this better.

 

[jpmobius]

For what it's worth, I have a comment and a couple of observations. If helpful, great, if not, please keep sailing, you're doing some great work. Personally, I tend to work through the numbers a fair bit, but I get tired of it pretty quickly, and once I think I am in the ballpark, I'm ready to fire up the welder. The fact is that not only do things usually get adjusted due to the realities of fabricating, but the assumptions used to get the numbers in the first place are only approximations of the real world. My observations are these: Your set up will definitely work. You may (or may not) have to fuss with finding an acceptable spring, but that is just a nut and bolt problem. The other thing, is that, even if the kinematics are ideal, how well the bike handles likely will be limited by how well the swing arm pivot behaves with the very different loads applied to it. It had comparatively little to do as most of the weight (and loads) it has to deal with now previously were fed into the twin shocks and old sub frame. I'm not saying this will be a problem, just noting that everything in any design is relative compared to perfection - even if you start with a blank sheet of paper and have a fleet of experts doing the design. If everything didn't have some sort of compromise, we'd have just one motorcycle chassis by now that everyone would use exclusively. If this were my project and I felt like I was ok with it as-is, I'd get it up to a rolling chassis and strap enough weight on it to simulate the final product. I'd install my shock, and, well, jump up and down on it to see what the spring was like. If it is anywhere close, I'd expect to get some idea as to whether or not the regressive rate of the linkage made much of a problem. I agree with you, and would bet it's not a real problem as is. But if you have second thoughts after such a scientific lab test, you can make the changes now without having to repaint the frame and basically build another motorcycle.

 

[brad black]

what if you moved the top shock mount up into you subframe mount and ran the shock along the top of the frame tube with a rocker and pushrod from the swingarm?

 

[Karlloss]

But if you have second thoughts after such a scientific lab test, you can make the changes now without having to repaint the frame and basically build another motorcycle.

That is my thought. I’m going to build it up, yes the spring will be regressive but how much it will be noticeable remains to be seen. As I mentioned in a previous reply, I’m going to build the bike and ride it before painting. This thing is being built on a tight budget, so I have to work with what I can and that does mean compromise is some areas, as long as it’s reasonably good and not dangerous then that’s fine.


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[Karlloss]

Today I managed to fabricate the left hand top shock adjustable mount so I could actually mount the shock and test out the angles. When on full comprehension the shock is as near as damn it at 90 degrees to the swing arm pivot, it is tight to the frame but there is 5mm of clearance (the perspective of the picture doesn’t show this).

I loosened the spring off to enable the shock to go to full compression and then did it again without the spring in place. I might move the lower shock mount back by 5mm yet still. Anyway disaster adverted.




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[jpmobius]

It is important when examining these things to stay focused on the individual elements. The kinematics, or mechanism of how the shock is actuated is completely unrelated to the swing arm itself. The actual swing arm, that is the link that goes between the rear axle center and the axle center that the arm pivots on could point in any direction. What matters is the link that goes between the pivot the swing arm rotates on and the pivot the lower shock mount swings on. If you align your square on this link, you will appreciate how much larger that 90o this angle is. Obviously, since the lower shock pivot is welded onto the swing arm, the relation between the orientation of these two "links" is fixed, and so the one must depend on the other. Perhaps it would be helpful to imagine two individual swing arms. The one you have, and another tiny one that only goes to the shock with the two welded together. That is actually what you have, it just does not look like it because the primary visual feature is the tube that makes up the main arm. Your shock is not aligned with that arm, so you have to imagine what the arm that actuates the shock would look like to get a handle on the angles that are important. In fact, the tubing that makes up the arm is equally irrelevant when analyzing this (or any) system. It of course makes sense to align the tube on centers of the "link", but think on those "banana" shaped arms needed for exhaust clearance.
The link is still center to center, and the angle of the banana is irrelevant. It is often quite easy to misinterpret mechanisms when the pivot centers are not directly in line with their connecting structure - you have to see past the visual illusion. You can do this easily by making a diagram that has only the pivot centers drawn the correct distances from one another. Do that, and you will see that the link to the shock is a long ways away from that to the axle.

 

[Karlloss]

Yes I see what you mean. The original reason for changing the lower mount was to ensure clearance between the rear wheel and shock. No that is has bags of room I will move the lower mount back and down. Live and learn hey.


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[Karlloss]

Having listened to the views on here, I decided that I would extend the lower mount of the rear shock. So I made a soft metal clamp to grip the damper rod in the vice. I then attempted to unscrew the lower mount, but it wouldn’t budge, so I apply heat, but still it wouldn’t budge.

So giving up on that I decided the only way to remove the mounting eye would be to machine the mount casing until I could see the threads.

Mounting the shock on my mill, I measured the damper rod to ensure that I didn’t cut to deep before exposing the threads.

So I milled to the required depth and still couldn’t see any threads. I decided to go a bit further, but still no threads. Anyway after some more messing about I’ve found the reason I couldn’t find the threads. The reason being is that there isn’t any. It appears the lower mount/eye is fusion welded onto the rod. At first I thought it was an interference fit. But when I got another shock of the same type, it was clear to see it was only welded onto a 3mm washer. I find this quite surprising.


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[Karlloss]

I’ve decided to move onto getting the rest of the bike sorted as the rear suspension was taking too much time and affecting my motivation. Although I’ve called my project a Cafe Racer, I’ve decided to make it a Brat Tracker. I’ve mounted the handle bars.




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[Karlloss]

In order to make brackets and other stuff, I need to bend plate and stock. Getting fed up with using a vise and hammer to bend things badly, I’ve make a press brake.










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[Karlloss]

Cut out some 2.5mm plate for rear mounting brackets for the fuel tank. Bit more finishing to do and then weld onto the tank. Really pleased with the press brake as I can turn out quality parts.




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[Karlloss]

I’ve welded the mounting brackets to the tank and made some mounts and brackets for the front of the tank. I was going to use the original mounts on the tank, however these were in the wrong place for what I wanted and they were not even symmetrical, so the tank sat on the piss. Hence I’ve made a load of new mounts.

Now that the tank is mounted, I’m going to build the exhaust and make some rearsets.




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[canyoncarver]

Nice work.

 

[Karlloss]

Thanks. Its been a long journey so far. I've broken the back of the fabrication work now, so hopefully I'll get it finished this year.

 

[Karlloss]

Decided to do some fiddly jobs today. I’ve rewired the switch gear. It’s cheap Chinese, but I’m impressed with the quality, near as damn it to Jap switch gear. The reason for the rewire is that the kill switch was arranged to make the connection when in the off position, which some systems use as they earth out the ignition system, thus ‘killing’ it. However on my set up I want it to make the circuit when in the run/on position.

The internals allow for the repositioning of the contacts. I removed the original contacts with a view to re-using them, however this was not to be.

To make new contacts, I ordered some copper rivets, which fitted perfectly, then just soldered the wires onto the crimped rivers. All works a treat now.




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