2 chain gearing

beanfoto2

I have some knowledge of bike mechanics, but little practice.

However, I'm in China, I want a 'bent and they've never heard of them.

I have to DIY or find a man who does, ( THE WELDING DEFINITELY)

I 've done a lot of website research. Found lots of useful things. maybe.

I came across this one project where the guy had turned a mountain bike into a SWB 'bent.

Not so strange, but he'd made it into a two chain bilke by running the chain from the boom to the ( now crankless) existing double chainset.

Hmmn, tryable, maybe.

Then I looked at all the gearing software and sites.

If you mount a 3 ring on the boom, HOW do you factor in the 4th on the original chainset?

beanfoto2

C'mon somebody must know.

only answer I came up with myself is print out the answers of boom chainset (crankset to you Yanks) to cassette, input boom to middle chainset as a 3speed and then sit down in a comfy chair with herbal tea, calculator and piece of paper and redo all the computer generated answers.

Don't forget the aspirin and watch out for CTS

beanfoto2

I must not post when I'm tired, I must not post when I'm tired.

My reply, ( to myself), is Total Rubbish. dave, you're an idiot!

It came to me lying in bed,trying to get to sleep, ( that's why I'm tired).

You just calculate the fraction of the reduction/ increase of cog teeth boom to mid chainset and apply that to the gear inches table printout!


But you have to do it for both big cog and smallest cog for the boom. And big boom cog to big mid cog, ( still with me?), plus big boom cog to small mid cog to compare the results.

But the pic of the guy who built this shows no sign of premature greying of the hair. Must be a suck it and see, use what you have and THEN complain type of guy.

I'm not chances are I'll get a donor bike with one piece chainset and crank , saw off the crank and THEN find out it doesn't work gearwise. And be left with a amputee chainset that's no use to man nor bicycle.

Why not just try it? I'm not suck it and see. I still can't visualise, ( hey, I'm a photographer, that's how I work OK?) the connection between cog teeth and non sweaty, non knee grinding pedalling. I want the best compromise. I need a granny gear,( possible loaded touring in areas of China less flat from where I am, Tibet???), but 20 ish will do me. The bike I make also has to cope with urban traffic and have some turn of speed for safety's sake. So 60ish would be no good on the top end.

Build 2 bikes? OK.

Hey, I've seen pix of his man who's built a 'bent out of a Mountain bike with 2 chains and chainsets.....

This is where I came in.

I would like others to post, you don't have to have answers, but your input could prevent me from going mad due to this.

Coming!

beanfoto2

So 35 of you have looked but not one posted?

Doesn't anybody want to play?

beanfoto2

53 people.

No postings.

The silent majority?

tyler_derden

OK, I'll bite.



4th what?

I have a recumbent bike that has an intermediate gear. I made it from a gear cluster from a multispeed rear wheel (pawls removed). You can calculate all the gear ratios you want to, but since you're building from parts, just go ahead and build the thing. If you don't like the gear ratios after riding it you can always swap out a chain ring or two.

TD

__________________
"When I was a child, I spake as a child, I understood as a child, I thought as a child: but when I became a man, I put away childish things."

blazingpedals

I'm not sure I understand the question. But I'll try to give some general direction here. Starting point: you have two chains, one going from the chainring to the mid drive, and one from the mid drive to the cassette. I'm assuming the mid drive cogs are fixed, no shifting there.

OK. Now, if both mid drive gears are the same size, then there's no gearing change. If the gear coming from the front chain is going onto, say, a 20T cog, which turns a 27T cog as the mid drive cog going to the rear, then the chain going to the rear will move 27 teeth for every 20 teeth the front chain moves. That would be step-up gearing, as in letting you normalize the chainring size for a 20 inch wheel. conversely, the mid drive cog to the front being bigger than the mid drive cog going to the rear would result in step-down gearing.

If I remember correctly, the formula for gear inches is (C1/F1)*(C2/F2)*wheel diameter, where C1 is the chainring, F1 is the mid drive to the front chain, C2 is the mid drive to the rear chain, and F2 is the cassette cog. IOW, figure the gear ratio for each chain, then multiply together and apply the wheel size.

I hope that answered the question?

beanfoto2

Thanks to you 2 guys for posting, info should be useful. but both of you come from different directions to me.

To answer Tylers point first. I won't be building from parts, I'll be building from Bikes, and when I buy the second hand bikes I'll literally be looking at their teeth. I don't want to be buying a bike that will give me gear inches of under 20 to middling 60s, which would happen if I bought a BMX with it's 20 inch wheels and a racing bike with the rear cogs common over here. I may have low prices over here in China, but I' certainly not spoilt for choice, which is the reason behind this project.

Where did you put your mid drive? Wouldn't think it would be a straiight swop job for the chainset.

On to Blazing pedals.

Do you eat a lot of beans? Are your bikes wind assisted?

I'm talking about boom chainset to mutilated existing chainset, (double I hope), so we have to talk about step up and step down. I'm such a new boy that I still have to chant "High gear work hard low gear SPIN" and visualise block and tackle, ( I'm former yachtie weekend crew) to envisage how much effort you're expending. So run step up /down past me again. Which would give me decent high gears, so getting past the 20 inch wheel problem? I don't want to be a speed freak, China is too dangerous for that, just past pootlin' along.

So, both of you have nearly answered my questions but rased more questions. Thanks for your input.

In anticipation

blazingpedals

If you use the double (crankless) crankset with the stock gears for a mid-drive, say 42/52, typically the front chain would go to the 52T and the rear chain would go to the 42T. That would give you roughly 20% gear reduction, so you would have an 80 or 85 inch high gear. Vice versa if you swap chain positions, you would have a 20% gear increase, to 120 inches. Or you could replace one of the mid rings so they are both the same: normal gearing. One thing to watch out for is that you may need an extra spacer between the mid-drive chainrings or else the two chains will hit each other.

Another way to do this is to take the pawls out of a 5-speed freewheel so it doesn't click anymore. Then rearrange the gears so they're something like (inside to outside) 20-24-28-spacer-24. Mount it to the empty bb shell by screwing it on a bb adjustable cup. Shift the rear chain between 20-24-28 using a front derailleur. Treat the shiftable 3 cogs exactly as you would treat a triple crankest, except that you can't use the 5-speed cogs with a 9 speed chain. With a triple on the front too, and an 8-speed cassette in the rear, you'd have 72 speeds (probably quite a few duplicates, but a huge range.)

blazingpedals

I still don't think I answered your question, so I'll try some more.
Sorry, I tried to add some ASCII graphics, but they don't come out right. I'll just describe it:
*20 inch rear drive wheel with a 11-32 8-speed cassette
* Mid drive, a 42/52 crankset with the pedal arms cut off and the rings reversed - 52 on the inside and 42 on the outside.
*Front crankset 24-36-48
*Chain runs from the front crankset to the 42T outside ring in the mid-drive.
*Chain runs from the 52T inside ring at the mid-drive to the cassette on the rear wheel.

In this setup the mid drive is not shiftable, so the front chain is always on the 42T and the rear chain is always on the 52T. Let's say for simplicity that a 20 inch wheel has an actual diameter of 20 inches. First, step back for a minute and mentally verify that each time the mid-drive completes one revolution, the front chain will move 42 teeth and the rear chain will move 52 teeth. So the rear chain is moving faster than you are pedaling by a factor of 52/42. Your highest gear (starting at the front) would be 48-42 in front and 52-11 in back. The gear would be (48/42)*(52/11)*20, or 108 inches. your low gear would be 24-42 in front and 52-32 in back, which would calculate out to: (24/42)*(52/32)*20 or 18 inches.

Having a shiftable mid drive increases your gearing possibilities, but makes it a whole lot more complicated to keep track of which gear you're in. Let's leave it at that for now.

beanfoto2

Thanks for all your mental effort. The mid drive is looking like a solution for those 20" high gear problem. Wonder how much the Chainset weighs over here? I Don't think the shiftable option is a goer tho' as it's twice the mechanics to maintain, and go wrong, and I'd be forever looking down and back to see which the cogs are and then trying to do calculations with only 20 fingers and toes!

ASCII graphics? I can find <, >, and ^ but nothing for a downward pointing cog tooth.

Muchly thanks once again and I'll try to post on progress, ( other things are NOT going well in China at the moment, so everything is on hold).

blazingpedals

One last thing I forgot to mention before. The front chain in my example needs to have some mechanism for taking up the slack. There is 28T difference between the large and small chainring, so if the chain is tight when on the 48T ring, you will end up with 14 inches (half of 28) of slack when you're on the small chainring. A derailleur cage with two pulleys can be mounted in the return chainline.

NazcaRider

I have two possible solutions, but both would involve purchasing some new parts:

1). Use the middle crankset from a tandem bicycle. That way you'd have three rings for gearing, plus a fourth to run the chain forward to the pedal crankset. I think that's how most dual chain bents are made.

2). Forget about having multiple chains. Just order some chain tubes from HP-Velotechnik or some bent dealer in Europe. It shouldn't be difficult to design the bent so it has a straight (upper) chain line. This method would give you 27 usable gears, since crossing from inner to outer sprockets isn't an issue. I have this on my bike and I highly recommend it.

I only skimmed the previous posts, so I don't know if this was already discussed. If so, then sorry for being redundant. Good luck!

NR

beanfoto2

You're not redundant, just on short time!

If I read correctly your point 2) you're suggesting keeeping the middle chainset, but having one chain? Muist be one hell of a one chain!

Not to difficult to design? I'll be working from a built bike then hacking and sawing , plus getting someone else to weld other bike bits. This is the Suck it and See design school!