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Full suspension FWD MBB - some advice needed.

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    Posted: 13 March 2018 at 9:33am
After some ruminations, I think I'll be doing it 'as pictured' anyway.
It WILL bob, I'll just go with much less (or, indeed, zero) sag for smoother roads (of course it would result less useable travel, but this is better than nothing) and for really rough roads some bob is 'lesser evil'. Air shock allow for easy tuning.
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Originally posted by GeoffBird GeoffBird wrote:

I'll have to see the bike in the link when it is finished. Please Private Message me your email and I'll send you some stuff about my bike.

Nothing all that interesting, really, just a variation on a theme of a Zimin's latter bikes:

https://www.vershina.info/

His first MBBs were extremely unorthodox and likely work very well for what they were intended to accomplish (MBB with VERY little flop, pedal feedback and steering inertia due to BB located ON the steering axis), but it involves sacrifices I'm not willing to make :).

https://www.vershina.info/moi-velosipedy

As for rear swingarm, it is akin to Toxy ZR:
http://www.toxy.de/index.php/en/products/toxy-zr

Unfortunately, it might be scrapped in favor of conventional swingarm (or even no suspension at all) because the builder has little access to good tooling and improvisation only gets you so far.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 07 March 2018 at 9:01pm
I'll have to see the bike in the link when it is finished. Please Private Message me your email and I'll send you some stuff about my bike.
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Originally posted by GeoffBird GeoffBird wrote:

Sorry Oleg - I've been on holiday for a few days. You obviously have a good understanding of the engineering principals involved (and the ability to communicate them).

As you say, a secondary drive to the wheel gives constant anti-squat geometry. It also reduces unsprung weight and allows the derailleur to be put in a sealed box, rather than close to the ground. The downside is a slight loss in transmission efficiency - maybe 1.5%? This might still be better than a derailleur system covered in mud and corrosion though!
 
Do you have an article describing it like one for your T7? I'd very much like to read it.
 
Originally posted by GeoffBird GeoffBird wrote:

I like the cantilever rear suspension on your bike pictured above - very neat.
 
That is not a cantilever suspension, it simply a rigid fork used as a swingarm. Works pretty well, though travel is rather short.
 
I have a separate project (being done by an other person, I'm not a builder myself), that would (likely) have a cantilever rear suspension not unlike your RWD bent, with a much simpler front end.
http://imgsrc.ru/mashinki/49524008.html
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 06 March 2018 at 9:49pm
Sorry Oleg - I've been on holiday for a few days. You obviously have a good understanding of the engineering principals involved (and the ability to communicate them).

As you say, a secondary drive to the wheel gives constant anti-squat geometry. It also reduces unsprung weight and allows the derailleur to be put in a sealed box, rather than close to the ground. The downside is a slight loss in transmission efficiency - maybe 1.5%? This might still be better than a derailleur system covered in mud and corrosion though!

I like the cantilever rear suspension on your bike pictured above - very neat.
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Originally posted by GeoffBird GeoffBird wrote:

Yes, remote steering is tricky. I had it on my last bike. Complicated, heavy and it gets in the way of your legs:
 
In case of MBB 'pedal steering' forces amplified by leverage at remote are actually so great that they literally buckle your frame and play havoc with your steering. :( It would have to be massive and fully triangulated to work. I have some ideas, but I'm not sure if and when I'll be able to make it work.
BTW, 'twitchiness' from a long tiller would likely be helped by a steering damper. Too bad I cannot afford something like viscoset for now, and cheap ones from aliespress will interfere with my legs.
 
Originally posted by GeoffBird GeoffBird wrote:

I am coming to the conclusion that on an efficient bicycle, you can only have soft suspension on the drive wheel, because, if you get the geometry right, you can use the traction force at the ground to remove squat when pedalling. There is no such force on the other wheel. The bike above has beautifully compliant suspension, but the front bobs up and down on steep hills, stealing power.
 
Yea, but going for 100% anti-squat from chain tension is an impossible task, because it vary as you go though your gears, and overdoing it results in 'reverse bob' with suspension extending with each pedal stroke.
I think you've moved entire drivetrain on your bike under the rider for that very purpose, right? Very unorthodox, where can I read more about it?
 
It IS easier on a bent because your CG is pretty much fixed and you don't have to worry about raider standing up on his pedals and throwing off your calculation.
 
Originally posted by GeoffBird GeoffBird wrote:

Oleg, the geometry for front anti-dive is the same as anti-squat for an FWD bike, because both the horizontal and vertical force at the ground are reversed, so, as you suggest, your bike would be inclined to bounce when pedalling. If it does not then please tell me and I will revise my theory Smile.
 
Yea, of course, but as you see I've went with URT suspension for the very reason of NOT having to take chain forces into account, which is hard to get right. Going for concentric BB pivot would have added some welcome pro-dive (as in - pro-squat chain force that makes concentric pivot DF bikes hateful to pedal, but would likely work well in my case), but is too complex for me.
 
Anyway, as you surely know, bents are much less susceptible to weight transfer (and, hence, pedal bob) anyway - due to longer wheelbase and lower CG.
My diy MBB (that started it all) has 130cm of wheelbase, ~30cm seat height and rear suspension does not bob at all. It also has absolutely no problems with climbing steep stuff and accelerating with no wheel slip whatsoever. Too bad it weights 50 pounds :).
 
 
 
Originally posted by GeoffBird GeoffBird wrote:

It may help with pedal bounce if you have no droop travel on the front suspension - so, lots of preload on the spring, so that it does not move when you sit on the bike?

Hmm, interesting idea. Kinda like Giant NRS? A solid concept, though it would require a steeply rising shock rate to work properly (or a coil shock), and wider tires - small bump compliance is going to be abysmal.
 
Originally posted by GeoffBird GeoffBird wrote:

BTW, thank you for the kind comment about my bikes, even if it is not true Wink
 
Humility is good, but I think you can overdo even that :)
 
Originally posted by GeoffBird GeoffBird wrote:

I think you are right about the efficient transmission of an MBB.; even my FWD bike seems to climb more efficiently than my RWD bikes and that has a chain idler.
 
Yea, with FWD you can route the power chain line along the tubes and minimise frame flex this way. It is also much shorter... but MBB with its triangulated drivetrain simply cannot be beat. It does come with pretty heavy downsides that I'm currently trying to tackle.
 
Originally posted by GeoffBird GeoffBird wrote:

PS: I see you use SolidWorks. Me too!
"Use" is a very strong word in my case, "dabble" more like. I have no engeneering education and trying to turn my sketches into 3D literally gives me a headache :(.


Edited by Balor - 02 March 2018 at 10:06am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2018 at 10:26pm
It may help with pedal bounce if you have no droop travel on the front suspension - so, lots of preload on the spring, so that it does not move when you sit on the bike?

BTW, thank you for the kind comment about my bikes, even if it is not true Wink

I think you are right about the efficient transmission of an MBB.; even my FWD bike seems to climb more efficiently than my RWD bikes and that has a chain idler.

Here is a better view of the remote steering:



PS: I see you use SolidWorks. Me too!


Edited by GeoffBird - 01 March 2018 at 10:31pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2018 at 10:16pm
Hi Oleg,

Ha, yes, the roads here need suspension too! I was thinking of this Ukrainian bike, which I assume is one of his earlier bikes:



Yes, remote steering is tricky. I had it on my last bike. Complicated, heavy and it gets in the way of your legs:



I am coming to the conclusion that on an efficient bicycle, you can only have soft suspension on the drive wheel, because, if you get the geometry right, you can use the traction force at the ground to remove squat when pedalling. There is no such force on the other wheel. The bike above has beautifully compliant suspension, but the front bobs up and down on steep hills, stealing power.

Oleg, the geometry for front anti-dive is the same as anti-squat for an FWD bike, because both the horizontal and vertical force at the ground are reversed, so, as you suggest, your bike would be inclined to bounce when pedalling. If it does not then please tell me and I will revise my theory Smile.
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As for 'optimise' - well, you must know MUCH more about suspensions than I do, maybe you'll just see some fairly obvious way to change the construction? Maybe by adding an extra link?
Your FS FWD bents are nothing short of genius (that also makes replicating them a nearly impossible task).

I might be naive here, though, but I really like MBB efficiency, and our roads are brutal without suspension of some sort.
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My name is Oleg :).

A 'similar' one this one, not Ukrainian:


Ukrainian one is this, actually:



That's me riding it :) The builder moved to Russia and I've travelled to Vladimir test ride it. 
I can fully attest that front suspension works *wonderfully* - no brake squat and action is sublime - it swallows large bumps like there were never there.
Unfortunately, something like that is way out of my budget (plus, I don't like this chain routing, it kinda defeats most MBB benefits for me).

He also helped me finish my prototype with 'reverse' angle:

Unfortunately, while I new that remote steering on MBB would be hard, I've underestimated HOW hard would it be - I've tried a couple of arrangements, neither really worked. A tiller this long is definetely not optimal - while rideable.


Edited by Balor - 01 March 2018 at 5:18pm
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