How can I change shock position without altering geometry?

macribs

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Been following the Descendence bikes open-source thread.

And what strike me is that most of todays frame has the rear shock mounted almost horizontal leaning forward into the triangle.
I am sure the designers have a good reason for that - but for e-bikes that kind of take away valuable space.

Then I came over this photo, the new Yamaha Electric. Now there is something different.
Shock is tilted backwards. Giving room for mid motor and batteries.

yamaha-pes1-ped1-electric-motorcycles-production-2014-6.jpg


Will this shock angle in any way make the bike handle worst then if the shock was tilted the other way - leaning forward?
Does anyone have an idea to maybe create a pivot point that can counter act for any problems such a shock angle might have?

If I was to modify a DH frame or an e-bike frame kit would changing shock position ruin the handling of the bike?

Any and all thoughts on the matter will be appreciated.
 
Hi all, it's my first post here :wink:

The shock is horizontal but pressed by a linkage, not directly by the swingarm. IMO that's a bit too complex and heavy to put on a ebike.

More explanations here: http://en.wikipedia.org/wiki/Suspension_%28motorcycle%29#Mono-shock
 
Generalizing too much, the more vertical the shock, like in the pic above, the more the ratio between wheel travel and shock travel is close to 1 to 1.

So like in the old school motorcycle shock, the shock has to be longer to get long travel suspension. Linkages, or angled shocks allow one inch of shock to allow more inches of wheel travel, which can mean a shorter shock, or more travel from a long one.
 
dogman said:
Generalizing too much, the more vertical the shock, like in the pic above, the more the ratio between wheel travel and shock travel is close to 1 to 1.
True, if the shock is located near the wheelshaft (like in old motorcycles). But think oh Archimedes... The swingarm is used as a lever, so if the shock is located near the jackshaft, 4" displacement at the wheel will only compress the shock for 1" (for example, depends where it's located exactly).

dogman said:
Linkages, or angled shocks allow one inch of shock to allow more inches of wheel travel, which can mean a shorter shock, or more travel from a long one.
I do agree with you for the angled shocks, but not for the linkage.
The actual need for a cantilever system (linkage) is to have a proportional stifness over the whole displacement of the shock: smooth for lower displacements, and stiffer for longer ones (excuse my English, maybe not the best you could read :wink: ). Here is another explanation with the Pro-Link system from Honda: http://world.honda.com/CBR250R/chassis/page03.html
 
speedmd said:
Assuming all the travel ratios are identical, you would need to strengthen the top sections of the frame to handle the shock loads.


Well triangulation will sort that out. So then a pivot/linkage system is what I need then?
If the linkage work properly the mounting point should not affect the shocks handling of jumps, pot holes and what not?
 
Diva said:
Hi all, it's my first post here :wink:

The shock is horizontal but pressed by a linkage, not directly by the swingarm. IMO that's a bit too complex and heavy to put on a ebike.

More explanations here: http://en.wikipedia.org/wiki/Suspension_%28motorcycle%29#Mono-shock


Well most DH bikes already comes with linkage/pivot points, right?
So it would be a question of getting the right geometry and make the linkage strong enough without adding too much weight.
 
Macribs,

Different shock angle by changing the frame anchor point, assuming all else equal, changes how the spring rate changes over the full travel. Linkage can help mitigate this. If the suspension pictured didn't have linkage to compensate, then that shock angle would result a very decreasing rate of change in shock compression per unit of travel, which I'm sure is undesirable. A progressive spring would help offset it, but I doubt you could find one sufficient without getting too heavy, so the designer used linkage to change the effective angle of the spring.

If you try to mimick the shock position in the pic, don't underestimate the forces involved and the structure required.

MadRhino told me once that it's desirable for the shock to point toward the headset. He's a downhiller with tons of experience on lot's of bikes, so I'd consult with him regarding why that is.

If your plan is to use a motor with a direct single reduction to the wheel, then instead of worrying about shock angle now, first focus on how to get the motor centered in the swingarm pivot as the optimum, or on the swingarm as close to the pivot as practical. Not only does this free up space and multi-task one of the strongest points on the bike, making a lighter bike possible, but it eliminates chain issues for a more quiet ride. Without the ICE noise, chain noise gets ugly in a hurry. It also changes how the suspension reacts under acceleration, because the motor is no longer pulling on the chain from a point anchored in the frame with a force causing compression of the rear suspension. The net result reported by Toolman2 is anti-squat during acceleration that feels quite different. How strong that is depends on the angle of the rear contact patch and the pivot. I don't notice it on my bikes, because I'm so heavy and those with dual suspension have long swingarms and relatively low pivot points.

John
 
A quick reply as I am on the way out, will comment more when I am back home again. Just a run to the store to secure some weekend fluid. Got to keep hydrated you know.

For coilovers progressive (is that the word I am looking for?) springs have been used in MX, all sorts of racing for several decades. Soft in the first part of compression and tighter further down. Hard to bottom out.

This spring I got to play with an ATV for 4 weeks with some mad air shocks. They looked kind of like dual reservoirs on the bottom and top. I guess they mimicked the progressive coilovers. They where all light alloy. Don't know the weight but that is not important either as they a way to price for an ebike anyway. Can't remember exact the pricing, but maybe like 3K $ a piece. So unless people can afford to coat their handlebar with Swarovski's those are not suitable for e-bikes.

Does something like that exist for DH market or e-bikes? A lighter and smaller air shock like that would have to deal with a lot less forces then a heavy ATV. If something like that is possible it might just be a winner. Soft enough for daily use, yet tough enough so it will not bottom out when air borne. And the nature of air shocks you could do instant tuning to fit the firmness or style of riding each day even. Something like that might work along with a linkage?

Hm mid motor on the swing arm. I've been picturing the mid motor inside the triangle, guess because that is what I've seen so far.
Moving the motor backwards would free up space - but wouldn't the weight of the motor still be unsprung?

I mean not as bad as a large hub in rear wheel but still the motor would be attached to the one part of the bike that is unsprung?
This idea is well worth considering John. If there are not to much of a compromise anchoring the motor back there the motor anchor alone would free up much needed space for batteries.
 
The effect on unsprung weight is directly proportional to its distance from the pivot compared to the distance the wheel axle is from the pivot. In the wheel, like a hubmotor, is 100%, and centered in the pivot is 0 effect on unsprung weight. If the frame is DIY, then the only problem comes into play if you want function pedals, though the guys with Motopeds don't complain too much about their extra wide cranks.
 
John, I've been doing some drinking thinking.
Your point of correct chain line, less tension and wear do to reduced flexing of chain/belt due to shock compression/swing arm movement are all valid points. But couldn't that be achieved also if motor is mounted inside triangle? By using reduction/jackshaft the final sprocket could be exactly where you pointed out the better place for the motor. All the way in the front of the swing arm - closest to the triangle.

Might be the beer talking but wouldn't that jackshaft accomplish the very same thing without getting additional unsprung weight, as the weight of the motor would be in the triangle and thereby fully suspended?
 
speedmd said:
You don't need a jack shaft if you go with a double pivot suspension setup and dial in a virtual pivot point to be at the motor shaft.

Not sure I understand what you are saying. Double pivot setup?

I was thinking that I would avoid swing arm mounted motor both for avoiding unsprung weight and to avoid the need for a stretched swing arm. Yes I know there are upsides to longer wheelbase from a stretched swing arm, both there are down sides as well. And keeping the wheelbase will help keeping the bike nimble, fast responding and maneuverable.

If the gearing works out so I don't need a reduction then I gladly drop a jackshaft, any bikes uses a double pivot setup that you know of so you can maybe post an image, that way it will be easier for me to understand exactly what you mean.
 
The pictures are typical production setups and designed mainly for shock and pedaling response. The concept can be used to give you a pivot position at the motor shaft where ever you can fit the motor. You will have to reverse engineer it with something like motion analysis in a good solid modeling program. Some of the good folks here on ES may be able to help more.

719888d1346093476-blur-trc-pictures-2013-santa-cruz-blur-trc-frame.jpg

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in auto applications----------------------------------------one below { here } only adapted for bike
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Love this group. It challenges me to find answers to life's persistent questions.

Modeled some various double center link suspensions like the ones I pictured above to see what drove the pivot point in and out and how easy it would be to dial it in somewhat to get to a motor shaft location. It looks to be much simpler than I had thought originally. I got better results with the opposing rotating links like used on the santa cruz, and could easily move it anywhere I wished, from just forward of the rear wheel to overlapping the front wheel. Nice smooth chain length over 8 plus inches of suspension travel. Certainly no need for jack shafts / idlers with this type of setup. Not sure how to take screen shots on solid works to best show. Will work on it some more.

cheers
 
speedmd said:
Love this group. It challenges me to find answers to life's persistent questions.

Modeled some various double center link suspensions like the ones I pictured above to see what drove the pivot point in and out and how easy it would be to dial it in somewhat to get to a motor shaft location. It looks to be much simpler than I had thought originally. I got better results with the opposing rotating links like used on the santa cruz, and could easily move it anywhere I wished, from just forward of the rear wheel to overlapping the front wheel. Nice smooth chain length over 8 plus inches of suspension travel. Certainly no need for jack shafts / idlers with this type of setup. Not sure how to take screen shots on solid works to best show. Will work on it some more.

cheers


As I am starting from scratch I've been looking for a donor, and it seems I will be starting from this bike. A scott genius Long Travel 30.
Can you see if you can make anything out of this? Shock mount point makes all of triangle usable space, but it might be in conflict with the perfect placement of the sprocket for minimal travel and best possible chain line.

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Well it seems it might be possible to move back the lowest mountpoint for the rear shock. We are not talking major work, just maybe an inch or so to the back to make room for a jackshaft where the mount point is today. Will that small movement alter the geometry or the functionality of the rear shock/linkage/swingarm?

I mean if that is doable without messing up the ability to handle bumps, jumps and what not I could easily hammer a jack shaft into shape where the low mount of the rear shock is today - and then have more or less an optimal placement of the front sprocket and a correct chain line for a left side drive.


To be honest there are so many things to read up on when building for the first time, it is a little overwhelming at times. Especially for people like me with no engineering background or even the faintest basic understanding of electricity and how motors work and why.
I don't think there is room to get a new piece of software to play with right now. Cos I wouldn't find the time. Between family life, work and some social chores now and then it is hard work to read up on all the topics needed to a build of an e-bike. Even when using guidelines and shortcuts like modding a donor frame rather then doing it all from scratch.
 
Unless you mount the motor to the swing, you will always have chain length issues with the suspension when mounting off pivot. If you mount the motor to the swing arm, they go away. It may be best to start off with something small/simple to get a better feel for all the other stuff you will get up to speed on in a first build. Anything you do to the suspension will most likely change how it works significantly and may make it inoperable/ unstable. Hard to say without looking at the exact changes.
 
Well ending up with useless geometry was one major reason why I decided to go for a donor bike rather then sketching it all up from scratch.

So what if I stretch the swingarm and place the motor as close to the pivot point as possible?


I have been thinking of ways to get a dual drive in and for a long time I was sure the way to make it fit was to mount it vertically in the seatpost tubing. So the motors hang from the top, outward on the seatpost of the triangle with a reduction and the reduction jack shaft in the pivot point. And for many bikes that would have worked, but I can't see any ways I can make that work for this specific bike/frame.

The thing that got me to lock on this frame/bike was the full usable triangle, that would make it easy to mount batteries and controller(s). Now I am not so sure anymore. Seems the linkage and rear shock mount point will make things even harder, and that was never my intentions.

I have been having the Yamaha PED 1 sort of as inspiration when I looked for different frames to start the work from. But even though this design give a fair space to batteries and controllers it seems even Yamaha and all their designers struggle with the same thing. How to place rear shock to maximize triangle space without ruining the chainline and chain travel along the movement of the swing arm. Notice the amount of slack in the chain.

img01.jpg

yamaha-pes1-ped1-electric-motorcycles-production-2014-5.jpg



It seems they have had better luck with the street version PES1. Notice they are using belt and pulleys. So the travel should be way less, right? The rear shock is here mounted horizontal under the triangle, not really useful for an motard/enduro/freerider style bike as the clearance to ground would take an impact with such a design.


yamaha-pes1-ped1-electric-motorcycles-production-2014-1.JPG

yamaha-pes1-ped1-electric-motorcycles-production-2014-8.jpg



I am almost at the point of just making a steel jiig and start an tiresome path of trial and error. As it seems even the big guys are making compromises, and in the end if it all comes down to trades off anyway it might take less effort going the trail and error path then keep looking for the best and also acconomically feasible donor frame/bike. At least such an approach will be a fantastic learning experience, where I will see first hand what is the result of subtle changes. And as long as I am learning I can use the cheapest possible steel which also makes the welding so much easier and faster. Hm where to score a tube bender?
 
Thx for that great reply, that drawing really says it.
As of right now frame project is on the back burner as I got a ready made one.
But there will always be next build :)
 
No problem. Glad I spent a bit of time on it as I have been wanting to see what drives the pivot distance/location. I also want to build a mid drive that can regen, not require a jack shaft and has some latitude where the motor can be located and still not have chain issues. I think the counter rotating dual link shows promise.
 
speedmd said:
No problem. Glad I spent a bit of time on it as I have been wanting to see what drives the pivot distance/location. I also want to build a mid drive that can regen, not require a jack shaft and has some latitude where the motor can be located and still not have chain issues. I think the counter rotating dual link shows promise.


Make sure you do a build thread when you start your project, very interesting approach. And some fine work done so far.
Also there are some working on a rele that let you choose between Delta/Wye switching on the go. Future looks bright for mid drives.
And there is also more frame kits coming for mid drives. And for motors there are just so much to choose from. From ultra small and ultra light motors to real beasts that can reach 30kw burst. And people experimenting with various gearboxes.

For the ultimate play bike mid drive will rule, light weight and all weight centralized. It will make for super responsive ride with power on tap no matter what the situation calls for.
 
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