XS650 Monoshock

[mathil]

You're getting close. Just make sure you figure in your damper angle into the equation as well. On that note, you know that the rocker plate mount below the damper and compress the damper upward, right? The way you have it now, you're super loading the swing arm and not passing any of the loads to the frame.

--Chris

How does one factor in shock angle? I saw a calculation for car suspension which multiplied by (sin*angle). Is that it?


From my research, I've set the linkage up in the fashion Ducati does, only I reversed the positions of the solid link and the shock. Does it matter which way they go, as far as forces are concerned? How could I better take the load off of the swingarm without changing to the underslung linkage yamaha uses?

 

[teazer]

I'm no suspension expert but it seems to me you are designing a whole new suspension a la Ducati using some R1 parts. I'd use one or the other if possible. The purpose of the linkage is partly to change the leverage ratio but it changes it progressively ie the ratio is different at fully extended to fully compressed. Ducati has a rocking linkage tat teh top on the frame, so supennsion forces are transmitted into the frame at that link like an early Suzuki Full Floater design in a way.

I'd start by bolting the shock to the swingarm and the top to that frame cross tube and just see how that works with a 1:1 ratio (no linkages). That will probably be too soft and will probably need to have either a much stronger spring and shock or a linkage. Linkage design can be to increase leverage over the range or decrease it. Depends on whether or not you want rising rate. I'd go for a straight ratio and keep it simple or maybe a slight rising rate if yiu have to fit a linkage.

Shock angle is a second order effect and it's teh angle versus the arc of travel that's important. Your shock will be close to right on that arc, so I'd leave that calculation alone for the moment.

Let's guess that your chosen shock is too soft. To make it "stiffer" we need to make it travel further than a straight motion which would reduce travel at the rear wheel. In other words, there are two leverage ratios to consider - first it's shock position versus length of swingarm. Move the shock further forwards and the action is softer compared to the shock mounted at the rear axle. Moving teh shock back towards the wheel reduces that mechanical advantage.

Thinking about it, a pair of shocks mounted more or less at the rear wheel on a 400 pound bike with 2x100 pound springs would have a static sag of about 1" and that's about right. Move them all the way forwards to say 4" from the swingarm pivot (5:1 leverage) and they would have to be say 5 times stronger. That would need 2x 500 pound springs. That works out to the same as one spring of 1,000 pounds/inch which is around 18kg/mm. A typical sportbike spring is 8-12Kg/mm, so we are off slightly, but close.

 

[mathil]

I'm no suspension expert but it seems to me you are designing a whole new suspension a la Ducati using some R1 parts. I'd use one or the other if possible. The purpose of the linkage is partly to change the leverage ratio but it changes it progressively ie the ratio is different at fully extended to fully compressed. Ducati has a rocking linkage tat teh top on the frame, so supennsion forces are transmitted into the frame at that link like an early Suzuki Full Floater design in a way.

I'd start by bolting the shock to the swingarm and the top to that frame cross tube and just see how that works with a 1:1 ratio (no linkages). That will probably be too soft and will probably need to have either a much stronger spring and shock or a linkage. Linkage design can be to increase leverage over the range or decrease it. Depends on whether or not you want rising rate. I'd go for a straight ratio and keep it simple or maybe a slight rising rate if yiu have to fit a linkage.

Shock angle is a second order effect and it's teh angle versus the arc of travel that's important. Your shock will be close to right on that arc, so I'd leave that calculation alone for the moment.

Let's guess that your chosen shock is too soft. To make it "stiffer" we need to make it travel further than a straight motion which would reduce travel at the rear wheel. In other words, there are two leverage ratios to consider - first it's shock position versus length of swingarm. Move the shock further forwards and the action is softer compared to the shock mounted at the rear axle. Moving teh shock back towards the wheel reduces that mechanical advantage.

Thinking about it, a pair of shocks mounted more or less at the rear wheel on a 400 pound bike with 2x100 pound springs would have a static sag of about 1" and that's about right. Move them all the way forwards to say 4" from the swingarm pivot (5:1 leverage) and they would have to be say 5 times stronger. That would need 2x 500 pound springs. That works out to the same as one spring of 1,000 pounds/inch which is around 18kg/mm. A typical sportbike spring is 8-12Kg/mm, so we are off slightly, but close.

As shown previously in this thread I did bolt the shock directly between the swingarm, right ahead of the tire, and the top of the frame. Way too soft. Would need a retardedly stiff spring as the motion ratio was something like 4:1. A linkage is needed in order to increase the motion ratio. Whether or not the ratio is linear, progressive or regressive isn't entirely measurable until I cut up some steel since the hardboard I modeled the system out of is flimsy enough to introduce significant(enough) error. All this is is a rough proof of concept. As for further engineering of such ratios, it's probably not important at all. The bike's going to have a 5' wheelbase...it's not exactly a performer. All I'm looking for is something that sucks up some of the bumps and doesn't bottom out. If it looks badass and has a shitton of moving parts...all the better.

I don't think there is any harm in using the Yamaha solid link instead of a Ducati one. Reusing the Yamaha part just saves me 100$ in tubing, threaded inserts and heim joints. The latter would be adjustable, but if I need adjustability in the link I'll build one...later. I'm planning to make the belcrank, lever, triangles, whatever you want to call them have a few holes in them for adjustability, and if I hit on a setting I love and see the plates as particularly weak I'll make new ones with just one hole location.

BTW, your math is wrong on your hypothetical situation listed above. Moving forward from a 1:1 ratio to a 5:1 ratio isn't just a simple "(spring rate)(5)" calculation. The difference between the wheel rate (what a shock mounted at the axis of the wheel sees) and a spring rate is a square of the leverage ratio. So in your situation with 2 100lb/in springs (lets combine them into one 200lb/in spring for the sake of math) the calculation becomes "200lb/in(5x5)" which is a whopping 5000lb/in spring. You can see how fitting a vertically mounted shock in front of a 21" wheel practically REQUIRES a linkage.

Any help from the big boys about where the extra spring rate goes? Is there losses from changing the direction of force/friction around bearings and such that effectively makes the spring rate act higher?

 

[mathil]

You're getting close. Just make sure you figure in your damper angle into the equation as well. On that note, you know that the rocker plate mount below the damper and compress the damper upward, right? The way you have it now, you're super loading the swing arm and not passing any of the loads to the frame.

--Chris

So I thought about it some more....


The loads should be about the same as if the shock were simply mounted without linkage between the swingarm and the frame. Before it was a load on the lower mount and a load on the upper mount...pretty simple. With the linkage its a load on the pivot of the rocker, which is attached to the frame, and a load on the swingarm, split between two points (lower shock mount and lower link mount. I may be wrong, as the last physics I took was in grade 12, but I think I divided the load from being at one point into two points. That would be a good thing....no?

 

[teazer]

As for teh physics, I forgot to include that factor. In the new position, the shock would only move by 1/5th of the amount and would need to be another 5x higher. That's an insane rate though and modern bikes don't need to be that stiff, so what gives?

I'd look at the R1 donor geometry and see how that could be tweaked to fit your situation.

Or what about a BMW shock, don't they have some mounted directly or are they all linkage?

 

[mathil]

As for teh physics, I forgot to include that factor. In the new position, the shock would only move by 1/5th of the amount and would need to be another 5x higher. That's an insane rate though and modern bikes don't need to be that stiff, so what gives?

I'd look at the R1 donor geometry and see how that could be tweaked to fit your situation.

Or what about a BMW shock, don't they have some mounted directly or are they all linkage?

Once a linkage is added the position of the shock no longer matters. What matters is the motion ratio (the distance wheel moves divided by the distance the shock moves). Linkages were invented to counter this exact problem, I believe by motocross people to allow a much greater range of travel. Just so happens they allow an adept engineer to design in progressions as well as package things more towards the middle of the bike, both things desirable for a sportbike. I suggest you read one of the online articles about differing motorcycle rear suspensions, they explain a lot. I'll try to post a link to one soon.

 

[Kanticoy]

I went through the same thing when I designed my monoshock suspension using direct linkage-less mounting. It was always too soft the way I had it mounted. I found a spring with the correct lbs. rating and rebound properties that I needed for the weight of the bike plus rider and ended up mounting it into a position similar to this in order to better use the available stiffness of the spring I was using. forgive my MS paint scribble, but there would basically be an over-slung s/a brace on top of the swinger containing a mount, and the top mount would reside somewhere your seat rail and center tube union. This serves two purposes. It provides the spring stiffness that you desire, and transfers the load of the spring into the frame by pushing it in a linear line into the frame tube.

It was of course adjustable further for finite adjustments using the preload adjusters and the rebound valving built into the shock. It ended up working out perfectly. You are basically moving the mounting point of the shock to the hypothetical "rear" of the swingarm, and causing it to travel in a linear motion rather than an arcing motion. This creates a stiffer ride in general, and due to the mounting point in relation to the contact point of the rear wheel to the ground, it isn't too stiff. It will of course have to be worked out in the correct angles, and I can't remember the exact formula that Tony Foale used, but for every cm of drop from a completely linear compression arc that you drop, you lose x amount of the springs available pound rating from the spring. Mine travels about 15* below the true linear line and it ended up being perfect. As silly as it sounds, I ended up using barbell plates to simulate the motor weight with me on the bike until I got what I was looking for. It took me a few tries. This is the basic concept in an arc-following visual:

This is of course only a suggestion, but it worked out perfectly for me.

One other thing to consider, is that there will be 2x the force on the swingarm bolt by using any monoshock setup since it isn't triangulated between 6 points on the frame anymore. It is only transferred at 4 with a monoshock. Usually calls for a little extra bracing.

Forgive me if I am providing any insight to you that you already know. Just making a few suggestions.

 

[mathil]

So I finally tracked down a copy of the Tony foale book and read it. Super good stuff!

There's a calculation for finding progression in the book, so I'll look at my system and see what's up.

As for extra stress on the swingarm pivot, it's true, it does get higher, mainly because the shock acts as a fulcrum for the swingarm, directing wheel load into the pivot. However, foale explains in his book that driving force from the chain is MUCH MUCH higher than any suspension forces.... Sometimes over 1 metric ton of force! I've upgraded to needle bearings in the pivot, so the load shouldn't be a problem. The only other obstacle I see is tying the force into the frame at the top. Wherever my pivot ends up I'll tie it into the backbone by way of some tubing in tension. Should do the trick!

One more obstacle.... Finding consumables for my ancient plasma cutter... An astroplasma 450. I found one site online that has them, but who knows if they have them in stock. There is no way I am cutting out this much 1/4" plate with an angle grinder!!

 

[ADPete]

use a jigsaw and lots of tap magic. they cut plate steel better than you think! use progressive cut blades. seem to last the longest...

 

[mathil]

use a jigsaw and lots of tap magic. they cut plate steel better than you think! use progressive cut blades. seem to last the longest...

I'll try that I suppose. The blade guide on my Jigsaw just shattered a few days ago...(jigsaw #7... I'm a cabinetmaker) so it might tend to wander a bit, but I guess I'll try cutting a bit outside and grind to finish. I'll be tacking the plates together for final edge finishing and drilling so everything is lined up. I've been way too busy to spend any time on my bike. Between finding a new shop, starting a new company and getting engaged my time seems to dissapear... ???

 

[ADPete]

I'll try that I suppose. The blade guide on my Jigsaw just shattered a few days ago...(jigsaw #7... I'm a cabinetmaker) so it might tend to wander a bit, but I guess I'll try cutting a bit outside and grind to finish. I'll be tacking the plates together for final edge finishing and drilling so everything is lined up. I've been way too busy to spend any time on my bike. Between finding a new shop, starting a new company and getting engaged my time seems to dissapear... ???

i know how it is. im on my second for the year... just play with blades some like to jog there own direction others just wear fast. it works good. just finish with a disc sander. lookin good so far!

 

[The Red Wonder]

Nice looking build so far. I am doing a similar...by similar I mean mono-shock set up on my bike as well. I used a program called Mathcad to quickly do all of the calculations for me. I made a picture of them, its probably best to go to photobucket and zoom in on it:

I also uploaded a pdf here: http://dl.dropbox.com/u/550017/Suspension%20Calculation.pdf


If you give me the information in teal for yours, I can plug it in to the sheet and tell you what linkage ratio you need for the given ride frequency that you want.


I will need this information if you do (US units preferred, but doesn't matter):

Your weight:
Expected weight of bike:
% weight on front:
Unsprung weight front: (wheel, brakes, ~.5 of fork tubes)
Unsprung weight rear: (wheel, brakes, ~.5 of swingarm, ~.5 of rear shock)
Spring rate of fork:
Spring rate of shock:
Rake angle:


The back of Foale's book has some information taken from bikes like the weight distribution as well as normal bike weight.


With this information I can calculate the ride frequency of the front, and then we can determine what you want your rear ride frequency to be, I would go with less than the front, and then plug that in, and it will spit out the linkage ratio that you need to achieve that.


Let me know if this helps or if it is only more confusing...haha! Good lucking build by the way!

 

[mathil]

Wow! That's a whole lot of freaking awesome information.

I got the Foale book, and have been perusing it. If you're still up for it I'll gather the required information and send it to you to calculate. A few things will be estimates (weight, front rake within a few degrees, and unsprung weight) but I'll try to get it all within reasonable limits. Once I know a reasonable linkage ratio the next question is whether I want it to progress, regress, or be the same. We'll see!

Again, thank you so much for this, it's rad. Exactly the type of thing you hope to find on forums.

 

[The Red Wonder]

Not a problem man, just send it my way and I'll send you the results. I am out of town/ away from my home comp until Monday, jst fyi.

 

[mathil]

So, here I am, a good 4 or so years later, and I've gotten back at my bike. In the interim I got married, took over the family business, and bought/fixed/sold a sweet ass Volvo P1800. I needed something to tinker with again, and the bike was a perfect fit. I'm doing my best to make time for fun stuff instead of working way too much and then crashing on the couch to watch netflix, so hopefully I get this thing done soon...

I decided to direct mount the shock with a top hoop setup...way easier. The Yamaha R1 shock is probably a little too soft, but it's still stiffer than the front forks by far. I'm also a 6'5" beast, so a new spring might be in order. 200 lbs or so higher might do it, but I'll ride it first to see how it works. There's a company that makes springs up to 1200 lb/in in the right size.

I've also painted and buttoned up the motor with fresh gaskets and seals and checked out all the bearings/gears to make sure everything is in order. When I got the motor some previous owner had already gone 1st oversize, and the pistons were still really tight. I slapped new rings on and called it a day... hopefully it runs well. Broke a clutch screw reinstalling (I'll buy the allen head type with the collars), and I'm gonna get a PMA from Hugh's Handbuilt instead of the grimey old alternator that was on it.

I'd been worried about the distance of the front and rear wheels from the visual "weight" of the motor for a long time, so I installed the motor to see how it was all working out. Slapped the stock tank on as well just to give it more of a "motorcycle" look instead of bare tubes and springs. I think the wheel positions have come out spot-on, and I won't have to rake out the front to move the front wheel farther forward. The front end will also come down around an inch, since I plan on using a flat billet upper triple clamp.

As far as a tank and seat go, I don't think I'd be happy with anything off the shelf, and since I'll be building something why not make it carbon fibre... I made a seat plug before, but I wasn't really happy with it so I scrapped it. Thought about using the Honda tank, but want something sexier... I think I'll be making a carbon "continental style" tank, and a seat to match. People sell the continental style tank in fiberglass, but they want 500$ for it...way, way too much when I can build something myself and have the skills/materials laying around. I don't particularly like the look of full on carbon tanks and seat pans, but I like the idea of a little bit of carbon poking out from under paint, just to give things some street cred.

Hopefully I'll check in with more updates soon...


P.S. if anyone has a spare, or knows the size of the right side spacer (spool type) for the XS650 drum wheel I need one. Pretty sure I have all the other parts, just the wrong spacer from the mag to spoke swap.

 

[mathil]

Also, don't worry, another brace will be added from the spring loop back to the axle plates so the spring doesn't push the hoop back into the tire

 

[TijmenK]

Rad, I'm in!

 

[mathil]

Started work on the tank, the hardest part of the tank/seat combo... Made a thread in the forum, check it out.... http://www.dotheton.com/forum/index.php?topic=60061.0

 

[mathil]

Made the skeleton for the seat today, 25" long, 16" seat area, 7" wide, just like the rear of the tank.

 

[50gary]

I appears to me that you may have some conflict with the hoop tube (rear shock mount) hitting the frame tube upon compression. You may have an interference with the chain path and the left side of the hoop tube? Third, you may have very little shock compression with that extreme lay down shock angle. When the rear axle goes up the rear shock mount goes up more than it will compress. Not trying to be a dick here just looking at the pictures.
Cheers, 50gary