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alevand
human power expert

USA
3478 Posts

Posted - 10/09/2016 :  05:11:33  Show Profile  Visit alevand's Homepage  Reply with Quote
I have started to build a MBB-VPS.

I have a problem with the crank arm interfering with the spokes, though. Due to my limited inseam, I placed the BB as close as possible to the tire, this makes a very short chain. Due to the low Q, the chainwheel is quite inboard. I angled the BB so the chain line intersects at mid-cassette, but by doing this, the left crank arm is inside the spokes. I may either find a 5 inch unicycle arm online, make one or cut on up or reweld the BB straighter.

The geometry has a lot of trail, like the Sofa Bike, but won't be subject to wheel flop due to the VPS. Static head tube angle is about 45 degrees with nearly zero rake. Wheel base is 50 inches with a 13 inch seat height. Space frame is over the shoulder. Both wheels are 700c.

C:
Tony Levand

Edited by - alevand on 10/09/2016 05:17:26

Balor
recumbent guru

Russia
704 Posts

Posted - 10/09/2016 :  12:18:26  Show Profile  Reply with Quote
"This thread is useless without pictures!" (c)

I've thought about VPS and MBB, but could not wrap my mind around it.
Btw, short crank arms are available in lots of lengths on ebay (square taper, racing BMX), and using square taper you can vary q-factor by quite a bit. Cheap, too... heavy, though.

Edited by - Balor on 10/09/2016 12:18:57
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alevand
human power expert

USA
3478 Posts

Posted - 10/09/2016 :  14:42:56  Show Profile  Visit alevand's Homepage  Reply with Quote
Since the frame was only tacked, I adjusted the BB so the chain lines with the second sprocket, left arm clearing the spokes by 1/4 inch. Looks like some shoe interference with the frame, diagonal left front member will have to be a little higher..


C:
Tony Levand

Edited by - alevand on 10/09/2016 14:51:39
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PUGZCAT
recumbent enthusiast

Canada
466 Posts

Posted - 10/10/2016 :  09:27:58  Show Profile  Reply with Quote
What does "VPS" stand for?
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Balor
recumbent guru

Russia
704 Posts

Posted - 10/10/2016 :  09:37:32  Show Profile  Reply with Quote
Virtual Pivot Steering, like on his other bents. Very interesting, but mechanically complex.
Given that MBB with conventional steering systems has severe drawbacks (steering inertia, wheel flop and pedal feedback), making one with VPS may solve some or all of those.
I'd be very interested to see what he had come up with.

Edited by - Balor on 10/10/2016 09:39:22
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PUGZCAT
recumbent enthusiast

Canada
466 Posts

Posted - 10/11/2016 :  06:19:12  Show Profile  Reply with Quote
Thanks Balor, VPS could actually work well in this application. Tony is the pitbull of experimentation.
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alevand
human power expert

USA
3478 Posts

Posted - 10/17/2016 :  16:45:20  Show Profile  Visit alevand's Homepage  Reply with Quote
I am deciding one how to install the links. Front triangle tacked and ball joint in place.

C:
Tony Levand
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PUGZCAT
recumbent enthusiast

Canada
466 Posts

Posted - 10/21/2016 :  17:28:29  Show Profile  Reply with Quote
I look forward to seeing pics and videos.
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Runxner
recumbent enthusiast

USA
452 Posts

Posted - 10/22/2016 :  10:21:20  Show Profile  Reply with Quote
Only choose gear ratio/spoke counts where the crank meshes with spoke gaps

Team Low-Life
Lowracer Test Pilot/Evangelist
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alevand
human power expert

USA
3478 Posts

Posted - 10/23/2016 :  05:20:39  Show Profile  Visit alevand's Homepage  Reply with Quote
Thanks for the brilliant insight, Aaron. I haven't had time lately to work on the mbb-vps, still considering link attachment points. I need some sort of triangulation around the cassette and attach underneath, the left side is more straight forward.

C:
Tony Levand
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Speedbiker
human power expert

USA
3835 Posts

Posted - 10/23/2016 :  07:54:20  Show Profile  Reply with Quote
Hey Tony, do you expect it to behave any different from a headtube mbb? If so, why?
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alevand
human power expert

USA
3478 Posts

Posted - 10/23/2016 :  12:26:33  Show Profile  Visit alevand's Homepage  Reply with Quote
Nothing more than what I described in the first post.

C:
Tony Levand
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Speedbiker
human power expert

USA
3835 Posts

Posted - 10/25/2016 :  12:58:41  Show Profile  Reply with Quote
I didn't really see any clear goals. Certainly not your usual innovation.
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Balor
recumbent guru

Russia
704 Posts

Posted - 10/25/2016 :  13:24:58  Show Profile  Reply with Quote
Yea, it would be very interesting to see whan exactly you are trying to accompish.
Based on my mbb experience, it would definetely pay to try and make wheel flop, trail-dependent wheel rise/fall and, above all, steering inertia as low as possible.
SOME pedal feedback is definetely benefitial, provided it is not overwhelming - at least for burst climbing and sprinting efforts.
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alevand
human power expert

USA
3478 Posts

Posted - 10/25/2016 :  18:08:51  Show Profile  Visit alevand's Homepage  Reply with Quote
Well, I wanted dual 700c, light weight with a relatively low seat hight and shorter wheel base than a lwb. The MBB seamed easier than a linear drive, which still would get in the way of steering. The VPS allows me to see over the front wheel, unobstructed,except for my knees. To counteract pedal steer I wanted a lot of trail, the VPS does this without wheel flop, as the steering becomes neutral at some steering angle and behaves like negative trail at further steering. So I would imagine that it would behave like a Python near full lock, which from the videos Ive seem, are fairly maneuverable.

C:
Tony Levand
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Johnsfwdbent
recumbent enthusiast

USA
125 Posts

Posted - 10/25/2016 :  18:42:41  Show Profile  Reply with Quote
Tony,
What I learned with my limited experience on my home built MBB (just over 5500 miles).
I started with a 55 degree seat, then 40, then 35, now 25 degrees or so.
What this did was to decrease the ratio from the BB to steering axis verses hand position to steering axis.
BB to steering axis ( headset) 17" hands to hs started at about 10" now about 15"
I used to get a pretty good upper body workout, now not so much. Also as the seat leans back i use stronger muscles to counter steer.
Another thing that I find really helps is having my arms as straight as possible.

Who cares if you have pedal induced steering as long as it is manageable.

Also another thing to consider is the tire scrub on inner thigh if you sit close to the front tire. (On turns)
Moving the headset closer should reduce this. of course I don't know how this affects VPS.

John


Edited by - Johnsfwdbent on 10/27/2016 20:26:18
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Speedbiker
human power expert

USA
3835 Posts

Posted - 10/26/2016 :  02:20:27  Show Profile  Reply with Quote
John, we came to pretty much the same conclusions when we built the Cruzbike prototype.
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Balor
recumbent guru

Russia
704 Posts

Posted - 10/26/2016 :  03:00:45  Show Profile  Reply with Quote
quote:
Originally posted by Speedbiker

John, we came to pretty much the same conclusions when we built the Cruzbike prototype.



In fact, some 'fine-tuned' amount of pedal feedback seem beneficial indeed.

Here is a rather old snippet from Burrows, but it greatly impressed me:

https://vimeo.com/80482788

Transcript:

"Using the muscles other than your legs sub-maximally allows you to recycle lactates your hard-working legs are developing, which actually gives you more power in the system".

Sounds extremely plausible:
https://www.ncbi.nlm.nih.gov/pubmed/10776896/

It neatly explains why MBB designs excel in anaerobic 'burst climbing' (even without the 'swinging boom' factor that allows you to actually add arm power to leg power - and I fail to notice that in my case), much better than my own 'tissue hysteresis' theory (though I still think that this is an important factor, but it is now delegated to secondary roles).

It is often noted that recumbents are 'leg-limited' while DFs are 'respiratory-limited'.. and so are MBB recumbents, and it is certainly my own experience, too.


It seems that dual-propulsion recumbents should be even better than MBB, but unless you are doped though the roof - it will only work in for short sprints, just like it was noted by Warren in his 'Things NOT to do'.
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Johnsfwdbent
recumbent enthusiast

USA
125 Posts

Posted - 10/26/2016 :  08:03:33  Show Profile  Reply with Quote
Baylor,
On my fixed bottom bracket bikes I find sitting up straight helps me climb faster, or so it seems!
But even if it is the same speed, when I crest the hill and sit back, my legs feel less tired and I can pedal harder.

John
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Balor
recumbent guru

Russia
704 Posts

Posted - 10/26/2016 :  08:17:21  Show Profile  Reply with Quote
quote:
Originally posted by Johnsfwdbent

Baylor,
On my fixed bottom bracket bikes I find sitting up straight helps me climb faster, or so it seems!
But even if it is the same speed, when I crest the hill and sit back, my legs feel less tired and I can pedal harder.

John



Didn't notice that, though I've tried after seeing that tip on Cruzbike forums. But my level of recline on MBB is not very drastic.
It might be possible that with higher bottom bracket and low seat you may run into an other recumbent pecularity - lower leg blood pressure. (up to and including numb toes that I've had on my first recumbent).
Since I had tilting hamster bars on my FWD, I could not really sit up during climbing. It might be quite possible that on steep inclines sitting up is beneficial. It also works different muscles.

Anyway, a good point - making bottom bracket as low as possible, to help gravity pump blood into legs. They have valves and whatnot to help blood flow UP, not down. There is a certain drop in blood pressure in your legs the higher they are.
Though I'm not sure that anyone tried measuring pressure during a workout.
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Speedbiker
human power expert

USA
3835 Posts

Posted - 10/26/2016 :  12:23:39  Show Profile  Reply with Quote
I think you are coming to false conclusions about low bottom brackets in general. On the lowracer that preceded my NoCom the BB was a very high 11" over the seat. This accounted for much of it's speed as I won many sprints, but I also rode races up to 12 hours with no issues. With a best of 240 miles I see little validity in your idea. A lower by on a mbb can help pedal steer, but kills the speed. Maria Parker's V had a surprisingly high BB setup when she won RAAM.
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Balor
recumbent guru

Russia
704 Posts

Posted - 10/26/2016 :  12:46:20  Show Profile  Reply with Quote
Well, now we are likely treading into territory where personal anatomy reigns supreme. My first recumbent had BB about 10" from seat bottom. I've got numb feet about 1/2 hour into the ride, and it got so bad eventually that I've stop now and then because of my feet getting painfully numb (no, not an oxymoron in this case, unfortunately).
BB 3 inches above seat on my MBB - no issues whatsoever.
I've been using SAME shoes, so it cannot be due to that. *shrugs*

Maria's high BB is not likely intentional, it is due to her short stature and the way you make correction for size on Vendetta... though I guess they would make her a custom front triangle with shorter chainstays if she would develop circulation problems.

Anyway, MBB is great for me exactly because you can have extremely low bb without making tradeoffs in crank overlap or total seat height. And it certainly does NOT make me lose any power - I've been pretty much instantly faster on my MBB (once I've got confident enough to ride it without fear of falling over), especially uphill.
This article I've already linked to also suggests that BB should be at above seat level:

http://www.ruedalenticular.com/2013/06/estudios-biomecanicos-en-bicicletas.html
Though I'm unsure about validity of their methods.

By the way, are you absolutely sure that seat - BB distance is positively correlated with power output? That would actually require a controlled experiment, 'different bikes' does not count because other things are not equal. (That goes both ways - maybe if my MBB had 10" of seat to BB distance, I'd be much faster - at least for half an hour, before foot numbness set in).
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Speedbiker
human power expert

USA
3835 Posts

Posted - 10/26/2016 :  13:00:11  Show Profile  Reply with Quote
I'm not absolutely sure of anything. But my body of research and experience is substantial. As such, I apply these ideas within my group and the results across a very wide spectrum of the sport speak for themselves. Perhaps you feel what works for you is enough to sound authoritative. My ideas resulted in numerous world and county records, race wins and national championships. I'm certainly not right all the time, but whether you want to believe it or not, my ideas get top results on a world wide scale(or at least the nine countries my riders are in).
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PUGZCAT
recumbent enthusiast

Canada
466 Posts

Posted - 10/27/2016 :  12:32:06  Show Profile  Reply with Quote
I hope Tony gets it built soon, it may be the better mouse trap, err recumbent. I too find high bottom brackets relaxing, no problems with numb feet. It would be great if someone could figure out a fwd, rear steered recumbent that didn't lose stability at 23 mph, in a perfect world.
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alevand
human power expert

USA
3478 Posts

Posted - 10/28/2016 :  06:44:16  Show Profile  Visit alevand's Homepage  Reply with Quote
quote:
Also another thing to consider is the tire scrub on inner thigh if you sit close to the front tire. (On turns)
Moving the headset closer should reduce this. of course I don't know how this affects VPS.



Yes, the pivot is fairly low to avoid raw thigh, may need some sort of wrap around fender.

C:
Tony Levand

Edited by - alevand on 10/28/2016 06:45:57
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alevand
human power expert

USA
3478 Posts

Posted - 12/10/2016 :  17:40:37  Show Profile  Visit alevand's Homepage  Reply with Quote
I attached the links today.



C:
Tony Levand
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