1983 XV500 Cafe Racer

ChopperCharles said:
Yeah, I see that now. I was replying to the first page info, and I see now that there are four pages :)

Nice looking build! I wish I had the machines you do! So jealous!

Charles.
Thanks for the support. Been a long road so far, I'm hoping to get it finished in the next 6 months.


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JadusMotorcycleParts said:
Love that damper rod set up. One of the cleanest/best integrated set ups I have seen!

Thanks.

It took a bit of working out, I'm quite pleased with it. I didn't want something that stuck out in front too much, I wanted to keep it as tight as I could.


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Things have been slow lately, managed to get some bits done today and started to pull the dents out of the tank.

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Not the perfect job, but a lot better than it was.


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I needed to cope some tube to make support struts. Not having a 22mm hole saw, plus in my experience hole saws don’t cut very neat.

Luckily my milling machine has a tilting head, although it is not that rigid when tilted, but it did the job.

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Managed to find some time today in between working and doing other work related things.

I’ve machines up the lock stops. For such a simple and basic item, it’s surprising how long it takes to make them.




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After a long break due to work and life getting in the way, I’m now able to get some time in on the project.

I’ve had to remanufacture the rear shock mounts as it was fouling the rear wheel on full compression. Whilst it looks the same, it’s actually been modified a lot, the lower mount on the swingarm was cut off and a new piece welded in which is 20mm higher and 15 mm closer to the swingarm pivot for the lower shock pivot. The upper frame brackets have been trimmed to accommodate this.

Although not in the pictures, I’ve also bought a new spring, which is 130 n/mm, a vast improvement on the existing one which is 95n/mm.

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A lot of nice work so far. Regarding your new rear suspension setup, you may want to reconsider. Your original setup created a reducing rate of spring and damping, but I expect not exceptionally severe. The new setup will be very noticeable. Even if you only ride the bike casually, I expect you will still find the back end behaving quite poorly. If you increase the spring pressure to the point that bottoming out is not a problem, the normal ride will be very harsh. In general, (in this case) the angle between the link between the lower shock mount and swing arm pivot and the shock body should be acute at rest and come up to 90o when at max compression. That way the spring force increases to maximum as the suspension is compressed to the end of its travel. Your original scheme was nearly the opposite of this, but now is much worse. It also looks like the spring will be very close to contacting the frame when at full compression, but hard to tell just by looking at the pics. Not saying the bike will be unrideable by any means, but certainly disappointing considering all the work involved. The original lower pivot had much better geometry (again, just from looking at the pics), though without being able to lean the shock forward at the top would need to be lower to get good geometry. I don't have a simple solution. At least not one that does not involve pretty much starting again. For such a simple mechanism mechanically, there is rather a bit to figure out if you want it to work well, especially if you have to design around an existing shock. You simply cannot overcome poor geometry no matter what you have for a shock, and I would bet it also impossible to construct a progressive spring to neutralize the issue given the physical space available. As a guideline, start with calculating the length of the link you need to get the wheel travel you want with the shock travel you have. Then, you can see where that link can be in relation to the swing arm pivot and still fit on the bike incorporating the above noted angle to the shock arriving at 90o at full compression. This can easily be impossible depending on the space in front of the tire, and the available real estate above to fit the other end of the shock. In your case, it looks like the tire is too close to the swing arm pivot to mount the shock low enough to achieve the angle you need, and if you raise it like you have, the frame prevents you from leaning the shock forward to achieve the needed geometry. One solution is to move the lower shock pivot much higher and further back (like the stock location) so you can escape the frame conflict and lean the shock forward, but this changes substantially the shock travel and spring rate. Alternatively, you move the lower mount down in front of the tire and move the wheel back to get the clearance you need. Which of course changes the wheelbase etc. and gives you even more things to consider.
Anyway, some things to think about. It takes a lot of dedication and perseverance to do such a project, and especially so to scrap part of it to do over, like you have already done. Respect!
 
Jpmobius, I see the point you are making. I had considered moving both mounts higher, however at this stage I'll suck it and see. I've bought a stronger spring because I want the rear to be firm. My intention is to build the bike and get it running/riding before painting the frame and swingarm. As the angle of the shock isn't too severe at full compression, I don't anticipate the spring force varying a noticeable amount, I maybe wrong, but hey if I am, then I'll tear it apart and make some changes. Its all about the experience and learning ;-)
 
That's the spirit! Keep in mind it is not he angle itself (within reason) that is the issue, but the change in angle between links through their travel that changes the rate. Great idea to run it before paint. Something I always do myself. Designing and fabricating parts takes a lot of time. As does all the pretty work. Assembling a whole bike after everything is sorted out is trivial - shame more people don't make it their game plan.
 
I have learned so much reading this thread. This is an awesome build indeed. I am really enjoying the conversation around the rear shock/spring and its mounting with the geometry in question. I too am planning on converting my bike to monoshock, and I am learning everything I can before I even turn a wrench.

I cant wait for updates, though I realize this has been a long build. WELL DONE!!
 
Kunphushun said:
I cant wait for updates, though I realize this has been a long build. WELL DONE!!

Unfortunately it has/is a long build, life tends to get in the way, but I will finish it. Glad you’re enjoying the story.


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Having listened to the feedback, I decided to modify the rear suspension top mount. I will also make some alterations to the bottom mount.

The modification I’ve made is an adjustable top mount. It’s taken me a couple of days to make this, if I had a CNC mill it’d be no more than a few hours, however my mill is manual feed and therefore takes a lot longer to turn out parts. But hey CNCing it would be ‘too professional’ and this is a garage build.

There is still a bit of finishing to be done, but I’m happy with it so far.






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As usual, some very nice work! Unhappily,I believe you will not see much effect from your very nice adjuster. I scribbled some lines over a previous pic to illustrate. I know how much work all of this takes to do the first time and what it takes to alter, so if nothing else, maybe it will be useful the next time around if you decide to keep things as they are.

The red lines are what you have. I realize there is some parallax due to the angle of the camera, but I think it is slight enough to ignore. As the swing arm rises, it becomes easier and easier to compress the spring (thin lines move toward fat lines). The shock/spring offer maximum resistance when the angle between the link that pushes on it is at 90o, and as this angle grows, the effort needed to continue compression goes down. Conversely, when the angle is less than 90o, as the angle grows so also does the resistance of the shock to being further compressed until it maxes out at 90o. Keeping your existing lower mount, you would need to rotate the upper mount very far forward to create this condition (green lines). As this real estate is consumed by the engine, this is obviously not possible. Rotating the whole green system counter clockwise around the swing arm pivot enough to clear the existing engine and chassis results in the desired effect (orange lines). The desired effect perhaps, but certainly not the desired fabrication task as now both mounts would need to be altered! I know it is bad news, but I hope you find it useful at some point. You can bounce the bike up and down in the shop before you paint it to get an idea of how important this may or may not be.
 

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Originally I thought about placing the shock to the side of the frame as per the green line, but it became messy. I understand the point you are making, in reality the red line (lower mount) doesn’t move as much as it looks like it does in the image. I am going to reposition the lower mount, might even make an adjustable mount.

I’m not too concerned, the proof will be in the pudding as they say. If it turns out to be a bit crap, I’ll do it all again. The way I see it, some people still ride hard-tail bikes, so how bad can it be.


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Karlloss said:
I’m not too concerned, the proof will be in the pudding as they say. If it turns out to be a bit crap, I’ll do it all again. The way I see it, some people still ride hard-tail bikes, so how bad can it be.

Totally agree. But I'd bet with so much work in building it, you'd prefer it works reasonably well at the end! I really like side mounted shocks, especially on just one side. Of course that creates some serious torsion issues that have to be reckoned with in the swing arm. It's always something!
 
jpmobius said:
Totally agree. But I'd bet with so much work in building it, you'd prefer it works reasonably well at the end! I really like side mounted shocks, especially on just one side. Of course that creates some serious torsion issues that have to be reckoned with in the swing arm. It's always something!

Taking on board your observations I've been doing some research. Before I explain my understanding from the research, please note that I am not challenging your view in argument, I am posing some questions so that I can better understand suspension geometry.

The research I have does indeed suggest the effective spring rate of my design will be digressive. As the 2004/5 ZX10R has a weakly digressive rear suspension and Honda introduced a significantly digressive rear suspension on their 2001 RS250 race bike I'm not too concerned, albeit both bikes suffer from problems driving out of corners, but my bike is not a racer and will not be ridden in such a way. Whilst finding out that mainstream manufacturers have built digressive suspension gives me some comfort, I am still trying to understand the mechanics of how the rate is changed via a variance of angle relationship.

As I see it when the spring is at an incline, the acute angle from swing arm pivot to lower shock mount, the spring effectively needs to compress a given greater amount to provide a given amount of vertical movement in much the same way front forks work, i.e. as the forks are not vertical, so the fork springs have to compress about 1.1mm for every 1mm of vertical axle movement. That means the effective spring-rate at the front axle is about 20% greater (as there are two springs) than the sum of the fork spring rates.

Now bear with me whilst I try and articulate my understanding. With a shock the top mount is the point of reference for the transfer of force? Therefore with my design as the swing arm and therefore the bottom shock mount moves through its arc, the relative position of the of rear wheel to the bottom shock mount in the horizontal plain actually increases in length which creates a longer lever thus enabling the force of the spring to be more easily overcome, giving the feeling of a weaker spring?

If my thoughts are correct, then I would also think about the incline of the shock which using the understood action of front fork springs, when inclined will need more force to move the same vertical distance. Therefore would this mitigate or even neutralise the action of the arc of the lower shock mount.

I've made a diagram to help explain my logic, not sure if my logic is correct though.
 

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Ok, since you obviously have been putting some thought into all of this, I'll take a shot at explanation without a diagram.

First, there is nothing pertinent in comparing the action of a telescopic fork and the linkage of your rear suspension, even with the simple rear system system here. The reason is that the telescopic front suspension has straight linear action that has no change due to mechanical articulation like the linkage at the rear. The rear shock itself is in fact identical to the fork leg, but unlike the front, the rear has the linkage that adds additional complexity. For the sake of simplification, let's ignore the importance of the angle between the road surface to the angle of motion of the wheels, or even that of any of the suspension components. I'm not saying there is nothing to talk about here, just that it clouds the topic at hand and is another layer of designing the system overall. Here we are only looking at the rate of change in the rear suspension due to the effect of the linkage only. So lets look at just the bare bones of such a system.

Consider a simpler model that contains the same bits as the one on your bike. Paring it down to just the required components, we have a single link that pivots at one end on a fixed base. Think main bearing on a crankshaft. The other end is attached to our shock/spring assembly. The other end of which is again attached to a fixed base. Exactly like your bike, just imagine that you have cut off the swingarm just ahead of the shock mount and mounted the frame in a vise. Now imagine the shock is much longer and has lots more travel - enough so that you can rotate what's left of the swing arm 180o, from straight down to straight up, and that the upper shock mount is directly above the swingarm fixed pivot. Just like a crank, connecting rod and piston, except the piston is fixed and the con rod is replaced with the telescoping shock. Let us imagine rotating this model clockwise, imitating the rotation of your swingarm when viewed from the right side of your bike. This may seem like an extreme and unrealistic scenario, but looking at the extreme condition often makes it simpler to visualize the way things change through the motion.

Starting at the due south position, you can easily see that rotating the crank, say 10 degrees compresses the shock hardly at all. You get a lot of rotation with very little travel of the shock. If you were using your hand instead of the shock to resist the motion of the crank, you would find it very difficult. Conversely, rotating the crank against the shock would be easy at this point - in fact infinitely easy at due south, where there is no compression of the shock at all. Crank it 1o, and you could easily begin to compress even a very strong spring. but 2o will be harder, 3o even harder and as the crank rotates to 30 or 40 degrees will become very hard indeed. However, at 40o, you are compressing the shock significantly more distance than at 30o, and massively more than at 3o. This increase per degree of rotation will continue on until somewhat past 90o and then fall back off at a similar declining rate of compression vs. rotation until 180o, where tiny rotation of the crank essentially makes for zero compression of the shock. The reason that the max distance traveled by the shock is past 90o crank rotation is because the actual maximum shock travel per crank degree is where 90o between the crank arm and shock occurs, and actually has nothing directly to do with crank degrees. With this in mind, look again at 0 crank degrees; the crank/shock degree relationship is zero. At a few degree, the crank/shock angle is very acute. As the crank rotates further, this angle becomes less and less acute until it becomes 90o. This is the point where the shock/spring has the most mechanical advantage over the crank. If you were to re-attach your swingarm, this is the point where the system will place the highest spring rate and damping due to the linkage. So you can see, if you were somehow to use this exaggerated rotation/travel, at the beginning, at 0 crankshaft degrees, your effective spring rate would be zero, and there would be no damping at all regardless of shock and spring, but both would rapidly increase to max as the shock/crank angle increases to 90o. This is typically the concept one starts with in thinking over something like a suspension, as you likely want very low spring/damping resistance for tiny impacts like a cigarette butt, but need much more when hitting a pothole.

But consider what happens after the 90+ degrees of crank rotation when the crank/shock angle gets to 90o. The angle continues to get bigger, but the shock compression per degree of rotation begins to fall. Where it will be very hard indeed to keep compressing the shock at 90o, at 130o it will be much easier, and at 180o it will be so easy that the only resistance to rotation will be from the bearings, and the shock will have again zero damping and the spring will have zero rotational effort. In your current configuration, you are somewhere in this second half of this hypothetical model, where the farther you rotate the arm, the less per rotational degree you compress the shock.

This may seem to be a fairly dire situation, but there is a bit more to consider. The spring does not offer a constant resisting force to the compression of the system. It accumulates in proportion to the compression distance. So even though the mechanical advantage due to the linkage of the shock/spring over the swingarm is getting lower the farther it rotates, the spring pressure is still increasing because it is only dependent on the distance it is compressed; the farther it is compressed, the more pressure it exerts. The shock, on the other hand, is not. It's function is dependent on the rate at which it is compressed. It does not care how far it is compressed, only how fast it is compressed regardless of where it happens to be travel wise. This means that even if you have a digressive linkage, depending on how digressive and what the spring rate is, you can still end up with enough spring pressure. The damping however will be reduced the farther the travel regardless. The other aspect is that the actual degrees of rotation in the swing arm are fairly small compared to the 90o of our model, so as long as you are a lot closer to the 90o end than the 180o end you will see fairly small digressive rate change.

Returning to the first 90o+ of crank rotation, where we are in the progressive portion, with the simple scheme where the swingarm itself is the crank, in addition to designing an appropriate crank/shock angle, there are some other important considerations. As previously noted, the swing arm actually rotates only a small amount. With the shock directly attached to the arm close to the arm's pivot, the distance traveled there is consequently very small even at the intended 90o at max leverage. This requires a very strong spring and robust damping to resist the very long leverage, as the distance the spring/shock will compress is very small. This is where all of the many linkage systems come into play. If you add connecting rods (dog bones) and bell-cranks/rockers to the system, you can add a lot more travel, which will require a less strong/robust shock/spring. Additionally, you can create a lot more rotation in the rocker compared to the swingarm, and glean the benefits of a very steep progression akin to our 180o model (if desired). It all gets a bit tricky, as such systems commonly have digressive rates at some links and progressive rates at others, and it's the net effect that counts. What is likely to be important to you is your shock/spring selection. Many shocks are designed around these more complex setups and so need weaker springs and have longer travel than your system might require. So you should try to use a shock that had a non-linkage setup like yours as its original implementation. 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.
 
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