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 Some help with twist- chain FWD, please?
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Ivo.M
Starting Member

Germany
28 Posts

Posted - 12/18/2016 :  18:13:38  Show Profile  Reply with Quote
Hello, all.

I am in the (slow) process of building a small series of three twist- chain FWD lowracer. I have just run into a small problem with the idler placement and I hope some of you might help me with it.

https://www.flickr.com/photos/147177895@N03/albums/72157676219184082

The first bike will have a Toxy front fork. Toxy (and Zox) have 1" steerers, so the axle for the power side idler is bolted through a little tube that goes through the frame just ahead of the steering head.

The other two will use Raptobike forks which ave 1 1/8" steerers. This is due to a tight budget, Toxy forks cost about 300 Euros, Zox forks cost about 200 (and are ugly), and Rapto forks are 90 Euros.

Here´s my problem: Even if I plan for the biggest Terrcycle idler, there is not a chance to get the power chain line far enough behind the fork crown with an idler bolt in front of the 1 1/8" steerer tube. How ever I try to draw it up, the chainline seems to foul the fork crown.

Sure, I could place a smaller idler behind the steere tube, but I´d really prefer the power side chain to be deflected by a large idler and run fairly parallel to the fork leg in order to avoid or minimize influence on the steering.

Maybe a Raptobike owner might like to chime in how his power side idler is attached, because from all the photos I have googled it looks as if it was attached somehow somewhere near the steerer without the bolt going through the frame.

My frame is going to be 50mm round tube with 1,2mm wall thickness. I am in dire need for an idea. Will a long nut brazed into the frame do the trick?

Thanks in advance,

Ivo

Edited by - Ivo.M on 12/18/2016 18:28:09

Speedbiker
human power expert

USA
3702 Posts

Posted - 12/18/2016 :  19:06:18  Show Profile  Reply with Quote
You can graze a nut to the headtube if you add a bracket that goes around the sprocket(pulley) to support the pulley's bolt. You can use thin (3mm)thick material if you make it wide(25mm). this way the bolt is supported on both ends and won't tear the nut off the frame. Use a button head style bolt so it is fairly flat with the bracket. Good luck with your fine design.
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alevand
human power expert

USA
2874 Posts

Posted - 12/19/2016 :  04:32:52  Show Profile  Visit alevand's Homepage  Reply with Quote
Ive never built a fwd twist chain, but Ive read that it works if the chain crosses the steering axis, chain ring or idler in front and driven cog behind. I don't how the the Terra cyle idlers are constructed, but Ive used 5/8 cr-mo tubing welded to the frame tube and slid the belt pulley bearing onto that.

C:
Tony Levand
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Speedbiker
human power expert

USA
3702 Posts

Posted - 12/19/2016 :  04:56:44  Show Profile  Reply with Quote
Tony, in order to prevent torque steer the tension side of the chain has to run parallel to the steering axis. Of course if you have a cassette on the hub you have to pick the most used cog from which to determine what is parallel. This means the chain will run behind the fork crown, which usually puts the pulley stud right beside the headtube, unless you run a Grelk-like very large pulley(great idea).
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Garrie L Hill
human power supergeek

USA
1689 Posts

Posted - 12/19/2016 :  06:20:47  Show Profile  Reply with Quote










This should help.





Garrie "carbon based lifeform" Hill
HPRA Co-Dictator of the East
for pics of some of my time and money sucking projects
http://s58.photobucket.com/albums/g277/cfbb/
and videos
http://vimeo.com/5513519


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warren
human power expert

USA
6050 Posts

Posted - 12/19/2016 :  07:19:49  Show Profile  Visit warren's Homepage  Reply with Quote
I love the curved tubing! Look up the Fenner pulleys (that's what's on the Barcroft, above), they are pretty cheap and work well.

Edited by - warren on 12/19/2016 07:21:54
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Ivo.M
Starting Member

Germany
28 Posts

Posted - 12/19/2016 :  07:22:02  Show Profile  Reply with Quote
Thank you, Garrie, but is that Barcroft fork a 1" one?
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Garrie L Hill
human power supergeek

USA
1689 Posts

Posted - 12/19/2016 :  07:45:00  Show Profile  Reply with Quote
Ivo, take a look at the fork blueprint. 28.6 mm, i.e. 1.12 inch. BTW, the Bancroft forks were all aluminum with canti studs, disc brake tab, and crown drilled for whatever brake system you might desire.

Garrie "carbon based lifeform" Hill
HPRA Co-Dictator of the East
for pics of some of my time and money sucking projects
http://s58.photobucket.com/albums/g277/cfbb/
and videos
http://vimeo.com/5513519


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Garrie L Hill
human power supergeek

USA
1689 Posts

Posted - 12/19/2016 :  07:49:07  Show Profile  Reply with Quote
Coincidently, I just got off the phone with Bill Cook. He will confirm later today. Beware of not building your frame rigid enough. His original build would exhibit some scary pilot induced oscillation of the front wheel when powering through a turn. BTDT and had the brown shorts to prove it.



Garrie "carbon based lifeform" Hill
HPRA Co-Dictator of the East
for pics of some of my time and money sucking projects
http://s58.photobucket.com/albums/g277/cfbb/
and videos
http://vimeo.com/5513519


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Speedbiker
human power expert

USA
3702 Posts

Posted - 12/19/2016 :  07:58:19  Show Profile  Reply with Quote
Garrie, how did the pulley mount? A threaded insert going all the way thru? Also notice the pulley is big enough that it puts the pulley axle ahead of the headtube.

I hope Balor read that bit about frame flex.
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Garrie L Hill
human power supergeek

USA
1689 Posts

Posted - 12/19/2016 :  08:30:13  Show Profile  Reply with Quote
Thom;
Yes, through the tube. And, yes, I hope he reads through carefully. Of course, we all learn one way or another...... some ways are less painful than others!

Garrie "carbon based lifeform" Hill
HPRA Co-Dictator of the East
for pics of some of my time and money sucking projects
http://s58.photobucket.com/albums/g277/cfbb/
and videos
http://vimeo.com/5513519


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Balor
recumbent guru

Russia
512 Posts

Posted - 12/19/2016 :  10:04:19  Show Profile  Reply with Quote
Well, I do read that, but my options are limited and besides, I've already had two spills on my current MBB from overcorrections due to steering inertia and I'm tired of that. Hopefully, it will not be any worse... anyway, pedalling loads are isolated at front triangle that should be superbly rigid. I do worry about frame fishtailing due to length and unorthodox rear suspension. We'll see. And I do actually intend to wear full body armor from my brief DH riding days when I'll be testing the hell out of the frame.
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alevand
human power expert

USA
2874 Posts

Posted - 12/19/2016 :  10:09:33  Show Profile  Visit alevand's Homepage  Reply with Quote
Has anyone experienced speed wobble on a MBB?

C:
Tony Levand
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Balor
recumbent guru

Russia
512 Posts

Posted - 12/19/2016 :  12:26:31  Show Profile  Reply with Quote
quote:
Originally posted by alevand

Has anyone experienced speed wobble on a MBB?

C:
Tony Levand



No, I did not. I doubt that is physically possible, you'll simply go down much faster than that I'm pretty sure.
But before you can go anywhere fast on MBB (at least my MBB, which is Zockra-inspired but has angle of about 65 with proper sag: https://pp.vk.me/c638425/v638425343/10a88/HtbEW1hUM9Y.jpg) you need to 'relearn' your steering, and pedal feedback is just one small part of that.

You have to learn to use slow, deliberate and controlled steering movements, because the front triangle has a metric ****ton of steering inertia due to entire drivetrain (including your legs) rotating with steering, and I = m*R^2 is a formula that even I know, R being distance from axis of rotation to BB.

I've spilled a couple of times before I learned that steering conventionally, using fast movements is a recipe for disaster because it is not humanly possible to damp this inertia the way you do with any other bike. This is like greatsword fencing vs rapier fencing - you simply cannot use same tricks because inertia involved is literally orders of magnitude larger.

Unfortunately, in some cases it seems you NEED to make fast and precise steering corrections, and it seems that the faster the speed, the greater the need for proper 'steering damping'. (Just like with motorbikes)

It steering MBB, it results in subconscious SCARY feeling the faster you go, even when not pedalling. It gets easier, but this feeling is completely absent on non-mbb bents I've tried, whether gunner-bar equipped M5 or tiller hamster bars (that, admittedly, had too much tiller for my taste) equpped highracer 26" Azub replica.
When I was going mere 40 mph downhill and was buffeted a bit by a passing truck I experienced one of the scariest moments of my life - I've barely regained control and felt that I've was one hairsbreadth of going down, perhaps under wheels of an other passing truck.
And this 'scary feeling' is common to all MBBs, as much as I know. (Though, since my Zockraish MBB is pretty low, Vendetta etc might be better off because higher CG bikes are easier to balance)

Anyway, while Jason did 100 kmh downhills on his Vendetta, he admitted that he 'holds for his dear life' and has serious problems with high-speed MBB behaviour, but being a tough guy he is, he just deals with that.
I'm not a tough guy, so I'm trying to fix that using resources that I have available - spare cash, cheap labour and lateral thinking.

P.S.
I have rather bad kinaesthetics, so it may be possible for gifted people to use legs to aid in steering. I'm still not entirely comfortable steering with one hand, not to mention 'hands off', so problem might not be as severe as I make it sound.
But since I'm much faster on MBB that on other bents I've tried, I'm convinced that some design modifications to MBB may result in a bent that is both fast and feels comfortable and safe.

P.P.S.
Now I think that 'speed wobbles' are impossible due to inherent 'unstable equilibrium' (wheel flop) of MBB that actually magnifies steering inertia and makes MBB behave kinda like a bent with negative trail - once you upset the equilibrium enough, you go down instantly.

Edited by - Balor on 12/19/2016 12:52:35
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Ivo.M
Starting Member

Germany
28 Posts

Posted - 12/20/2016 :  17:31:35  Show Profile  Reply with Quote
Dear friends,

why is it that when someone starts a thread on twist chain FWD that all of a sudden everyone is talking about MBBs?

Just asking. ;)
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Garrie L Hill
human power supergeek

USA
1689 Posts

Posted - 12/20/2016 :  17:58:28  Show Profile  Reply with Quote
Not I! ;-)

Garrie "carbon based lifeform" Hill
HPRA Co-Dictator of the East
for pics of some of my time and money sucking projects
http://s58.photobucket.com/albums/g277/cfbb/
and videos
http://vimeo.com/5513519


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Runxner
recumbent enthusiast

USA
441 Posts

Posted - 12/20/2016 :  18:56:55  Show Profile  Reply with Quote
Have you been on internet forums long?


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

USA
3702 Posts

Posted - 12/20/2016 :  23:04:59  Show Profile  Reply with Quote
Because your question was answered, you didn't get involved, and the thread moved on?
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alevand
human power expert

USA
2874 Posts

Posted - 12/21/2016 :  04:57:17  Show Profile  Visit alevand's Homepage  Reply with Quote
Yes, I derailed the post at the speed wobble, but this is another off topic comment in itself. Is the frame aluminum or steel? Use the largest idler (http://t-cycle.com/idlers-chain-management-c-41/idlers-c-41_9/elite-power-23-tooth-idler-p-273.html) an 5/8 tube through the frame ahead of the steering head, as Thom has suggested or use a idler pulley from mc master-carr, the 1/2 inch bushing can be knocked out and the webs drilled for lightness and less $. There, now we can close the post, problem solved, on to the next plunder.

C:
Tony Levand
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CincyRay
Starting Member

USA
31 Posts

Posted - 12/22/2016 :  18:22:58  Show Profile  Reply with Quote
.
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Patrick Bateman
New Member

USA
69 Posts

Posted - 01/10/2017 :  14:03:39  Show Profile  Visit Patrick Bateman's Homepage  Reply with Quote
quote:
Originally posted by Garrie L Hill











This should help.





Garrie "carbon based lifeform" Hill
HPRA Co-Dictator of the East
for pics of some of my time and money sucking projects
http://s58.photobucket.com/albums/g277/cfbb/
and videos
http://vimeo.com/5513519






You wouldn't happen to have a copy of the plans for the 700c version of this, would you?

I believe Bill called it the "Barcroft Montana"

He had an album of pics of it, but it's gone now:

http://www.bentrideronline.com/messageboard/showthread.php?t=104050

The Montana has a few interesting advantages:

1) Due to the use of a 700C fork, it should be compatible with the readily available Raptobike fork... which I happen to own :)
2) I personally prefer the better visibility and smoother ride of 700C wheels. YMMV

Edited by - Patrick Bateman on 01/10/2017 14:05:19
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purplepeopledesign
recumbent guru

Canada
673 Posts

Posted - 01/11/2017 :  12:33:27  Show Profile  Reply with Quote
Although you've already built the frame (I like those little tubular head tube gussets), if you can keep the drive idler as close to the head tube as possible.

Just as Thom says to keep the drive chain parallel to the head tube, keeping the chain as close as possible to the head tube also lessens any torque steer. Note that a wider tire will force the chain outward to prevent interference, minimally increasing torque steer. A thinner front tire can help, but that's if you don't need the larger contact patch. The amount of tire clearance is surprisingly small, maybe 5-10mm, even for tight turns.

:)ensen.

Those who claim to be making history are often the same ones repeating it.

Video of my trike
http://www.youtube.com/watch?v=MdSLRD_2vzc
Photos of my trike
http://www.flickr.com/photos/purplepeople/
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Ivo.M
Starting Member

Germany
28 Posts

Posted - 01/31/2017 :  14:43:31  Show Profile  Reply with Quote
I´m a bit late, but people: thank you very much for all your help.

Garrie, My calculation tells me that 28,5mm diameter for the Barcroft fork shaft equals 1 1/8", and per the drawing the Barcroft´s power idler is bolted through the frame right in front of the steerer as on most other FWD designs.

And: I have googled a fair bit and found pictures showing a Ti Bracroft Oregon sporting a Terracycle 23 tooth idler which is what I was shooting for anyway.
https://www.bentrideronline.com/messageboard/showthread.php?t=57815
So fingers crossed, this should work.

Warren, I am afraid the Fenner pulleys are not as readily available in Germany. What was their original purpose? Maybe I can source something similar this side of the sea if all else fails.

As to the speed wobble thing: The prototype frame is the same as my ICE- based lowracer and has a fairly massive 1,5mm wall thickness. Back then I had to find something that had the right inner diameter to safely clamp the ICE trike bits, was easily available, and for which the tube bending company had the right tools (I had had them mandrel- bent). The Zanzara will use the second length of tube I had bent. The ICE has a bit of frame flex in it, but the Zanzara will get some additional nice gussets around the headtube.

I am just finishing the 1:1 drawings, and with a bit of luck it´s off to the welders by the end of the week.



Great!
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warren
human power expert

USA
6050 Posts

Posted - 01/31/2017 :  18:26:16  Show Profile  Visit warren's Homepage  Reply with Quote
They are v-belt idlers. Like this: https://www.amazon.com/V-Belt-Idler-Pulley-Belt-Type/dp/B007IB084W This 3" one is tool small though. I use the 5" ones. I'm not sure what the German equivalent is.

Edited by - warren on 01/31/2017 18:28:10
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