Middrive..... and other chain mysteries

[cncman333]

Is there an advantage to having a mid drive and using two chains rather than one long chain? I don't think I need 30+gears so I was thinking about having a direct drive 1-1 from the cranks to a midrive behind the seat (maybe even running the chain through a guard so that you dont see the chain in front at all). Then have the front derailer on the mid-drive and the rear on the rear wheel. That chain would be roughly the same length as it is on the donor bike. The front chain can be routed with much less slack since it is direct with no derailers at all. Does anyone see a problem with that plan?

[PeterT]

one advantage of having different gearing between the front sprockets and the rear cluster, or wheel, is that you can manipulate the gearing ratios so that it gets easier to pedal, eg, connect a thirteen sprocket to a driven 12 sprocket, and you get a 1.08 increase in mecahnaical advantage, connect a 24 sprocket to the same driven 12 sprocket, and you get a mechanical advantage increase of 2, 44 gives you a 3.67 increase etc
have a look at the Down Under fast-Recumbent Quad F-RQ that i am building

[cncman333]

Correct me if I am wrong, but if you have a single gear on the crank to a differnt gear to the mid, then arnt you just making your high gears higher and your low gears less usefull? Would I not be better off having the gear range I am looking for on the mid-drive and the rear wheel and going 1 to 1 from the crank to the mid? I don't think you gain mechanical advantage except by increasing the effective gear.

Also beyond gearing is there an advantage to a middrive in using two shorter chains rather than 1 long chain?

[PeterT]

If you consider the speed at which you get your first gear cluster/sprocket turning, and then attach another gear/sprocket to that axle, then what ever gear you connect to that axle must turn at the same speed you get the axle turning at.
e.g. if your primary gearing is a 46/14 combination, on 20" wheels, you get the following speeds
11.5mph/18.5kph @cadence 60rpm
13.5/21.7 @ 70rpm
15.4/24.8 @ 80rpm
17.3/27.9 @ 90rpm
19.2/30.9 @ 100rpm
21.2/34.1 @ 110rpm

if you made your first rear gear cluster a "live" axle, eg remove all internal bearings, and run the axle on external bearings, then your secondary gearing will have to turn at the same speed as you are getting the first axle to turn at, thus driving your second set of gears at a faster speed, producing a faster end speed.

The down side to having such a set up, is that you start increasing the torque required to actually start turning the pedals, and can have too much torque so that you can't actually turn the pedals!

If you want a vehicle to climb telegraph poles, then you would build into the gearing a reducer effect, ie where the axle turns at a slower speed than the input gears, e.g. a 6/80 gearing produces a 13.33 reduction in speed to the final drive, whereas a 80/6 gearing produces a 13.33 increase in overall wheel speed.

The main issue with multiple chains is the way in which you can/can't tension them, you may need to have extra derailleurs to take out the slack in the chain, because it isnt as easy to adjust a secondary bottom bracket to adjust the chain tension when the chain has worn, as it is to move a wheel slightly backwards.

PeterT

[cncman333]

Well I have not found the right donor bike yet for the rear, so I have not decided on a gear range to use. If the bike I find has too high (or less likely too low) a range I can adjust it with the mid.

As for the chain adjustments on the front I am thinking that my adjustable boom will allow me to set the tension on the front chain and the wheel adjustment will do the back.

[TheKid]

Some of us have done mid-drives, including myself, Sir Joey, and Locutus. You can gear it any way you want. For instance, on my Streetfox, I ran a 34t chainring to a 34t chainring on the left side, then on the right side, I used the rear as it was, with a triple crankset to the rear cluster. Later, I switched to an internally geared hub, so the chain was on the right side, 34t front to 34t on the triple, then from the 24t on the triple to the rear hub. This achieved the gearing I wanted, plus eliminated all deraillers. All you need to decide is the range of the gearing, and how many gears you want. For example, let's say you want a very low gear for hauling and climbing hills, yet want a top speed of 20 mph, using a 26" wheel. You also want a maximum of 21 gears. You average 80 rpm. For a high gear, you'd need 80 gear inches. (GI) So if you use a 34t to a 42t on the mid drive, then use the 24-34-42 crankset that usually comes with Wallyworld suspension bikes, and replace the freewheel with an 11t-32t 7 speed freewheel, you'd have a low gear of 15 GI and a high gear of 80 GI. Plenty low for steep hills, and with bursts of 90RPM, get up to 21MPH, while maintaining 19MPH @ 80RPM. If you run the mid drive at 34t, the range with the same rear setup would be 19-99 GI, still low for average hills, but a high end speed of 23MPH @ 80RPM.

[cncman333]

Great, it sounds like the middrive will work the way I am thinking it should. What about having the chain enclosed from the cranks to the mid? (or at least the section that is in front of the seat) I see it every now and then, but not very often. Is there a reason for this? I am not using my bike for comuting in a bussiness suit, but I know some people do and I would assume the chains and front gear sets would all be covered. The main reason I wanted to do was to clean up the look of the front end.

[SirJoey]

CNC, my most recent was a mid-drive setup, & I'm real happy with it: http://forum.atomiczombie.com/showthread.php?t=2983

ONE of my reasons for this was what YOU said, a nice, clean look to the front end.
I even opted for no front brake, to avoid having any forward-running cables at all.

Also, the dual chains help eliminate the chain slap & bouncing, typical in long, single-chain setups.
Slack in the forward chain is easily eliminated with the tension of an old der:


http://img31.imageshack.us/img31/1287/forum4x.jpg

Additionally, nothing on the rear end had to be altered in any way. Cables, ders, even chain length all stay
exactly the same as they originally were from the factory, assuming you don't change the chainrings. :)

...and then there's the "WOW" factor. :laugh3:


http://img384.imageshack.us/img384/7131/sirjoeysigmedij1.gif

[cncman333]

Great build Sir Joey. Yes the clean look is certainly what I am going for, and being able to keep the drive componets and cables stock will make the build go smoother (hopefully). I still have some kinks to work out, but I think I know where I am going for the most part.

What about the idea of running the chain through the main boom and having the crank drive gear enclosed? The biggest problem I see with it is making the boom adjustable because you will have to be able to move the entire boom, and the issue there will be making it sturdy enough to avoid pedel wobble at high cadences.

[cncman333]

just a pic I found of an enclosed chain on a comuter bike...

[SirJoey]

Great build Sir Joey.What about the idea of running the chain through the main boom and having the crank drive gear enclosed?Thanx!

As for your idea, I think it would be a lot more hassle than it's worth,
not only to build, but to maintain, as well. JMO, FWIW.

If you're really concerned with exposed chain, I'd just make sure to use
a chainring that has the guard on it, & a tube for the chain. Simple & effective. :)


http://img384.imageshack.us/img384/7131/sirjoeysigmedij1.gif

[cncman333]

Simple and effective? Isn't the most complicated and convoluted solution always the best? <evil grin>

I am not sold on the idea of having the chain completely enclosed, but I figured that since I would no derailer up front I would be able to clean up the look with just a round casing between the cranks. But I am sold on the idea on a mid drive thanks to you... unless someone else has a reason to no do a mid-drive?

[SirJoey]

Simple and effective? Isn't the most complicated and convoluted solution always the best?Of COURSE it is! Mwah-ha-ha... :evilgrin:

One possible drawback... since your front crank is actually installed backward,
some guys have had problems with the pedals loosening.

I haven't had that problem (yet), but I haven't logged but a few miles on this
one so far, either. I can't really speak to this issue from experience, as this is
my first mid-drive setup, but I would imagine a little Loc-Tite would take care of it.

Seems to me, TheKid mentioned something about this in an earlier thread, not sure.


http://img384.imageshack.us/img384/7131/sirjoeysigmedij1.gif

[savarin]

The pedals have never loosened on my mid drive.
If they are done up tight enough there does not seem to be a major problem.
Take a spanner with you if in doubt.
I thought a mid drive would be a good idea with the huge range of gears but in practice found it was a waste of time.
I used a mid drive on my full suspension chopper as well, this time just to clean up the chain line. It works well in this context.
My next low rider will only have a 21 speed and no mid drive.
Here is one with a totally enclosed chain.
http://www.flevobike.nl/content/view/16/79/lang,en/
its a superb ride, a dutch lady rides one here in townsville, I can vouch for its class.

[cncman333]

Yes, the Flevobike looks very clean and streamlined, I like it. thanks for the link

[PeterT]

http://www.flevobike.nl/content/view/16/79/lang,en/
its a superb ride, a dutch lady rides one here in townsville, I can vouch for its class.

Vouch for its class or vouch for her class?