Practical gearing limits of derailleurs

[John in CR]

I pedal very little, mostly just on takeoff, hills, and crossing in front of oncoming traffic. I'd like to add some human power at the higher end of my speed range, but my bike is too fast. I have a 72 tooth sprocket, but before I cannibalize it for use in a crankset, I need to know the practical limits of a derailleur. ie What variation in length of chain can a derailleur accommodate? Would it be reasonable to try to modify one.

John

 

[nicobie]

Hi John,

On my bike I've got a 53t front and 11t rear gear and I can keep up with the motor until 29 mph or so and I'm not much of a 'spinner'.

Nick

 

[Zoot Katz]

A front derailleur cage can be modified with a grinding wheel to better match the profile of your large ring. I don't know that there'd be anything left though. 72T is larger than normal.

The rear derailleur's capacity should equal the sum of the difference between your front and rear gears.
ie: Front 52, 42, 30 equals 22T difference. Rear 11-30 equals 19T difference.

So, 19T+22T = 41T capacity. Most mountain and touring derailleurs can handle that.

 

[John in CR]

A front derailleur cage can be modified with a grinding wheel to better match the profile of your large ring. I don't know that there'd be anything left though. 72T is larger than normal.

The rear derailleur's capacity should equal the sum of the difference between your front and rear gears.
ie: Front 52, 42, 30 equals 22T difference. Rear 11-30 equals 19T difference.

So, 19T+22T = 41T capacity. Most mountain and touring derailleurs can handle that.

Thanks, that's exactly what I was looking for, and also for teaching me how to figure it myself going fwd.


Nicobe,
I want to be able to leisurely pedal assist at 35-40mph if I so desire. No, it's not so I can go flying by lycras at a slow relaxed cadence, but that's definitely a side benefit. My bikes will be EV's with human assist instead of pedal bikes with electric assist. Typical bike parts just won't get me there, so I'm starting with a 72t gear for much heavier chain, but the same link length as bike chain.

John

 

[dogman dan]

I put on a 58 tooth chainring on my commuter and the mtb rear derailur seems to still function fine, but I haven't really tried in on the smallest front chainring. On the street, I don't need anything lower than the middle front chainring and then only if the bike doesn't run. I just took the front derailur off. I'd love to see a pic of a 72 tooth, my 58 looks like a dinnerplate . The gear I run in most of the time is the second smallest back one, the smallest, 14t makes me work too hard , but it's perfect for the long shallow hill at the start of my ride to work. On the steep down hill, I still peadle pretty comfy at 32 mph in 14-58 gear.

 

[recumbent]

Wow, 72 is huge.

If you want more high speed then you could probably sacrafice (Or replace) the smallest chain ring, then just add a link or two of chain to make it work properly.
Actually, I'd buy a new chain if replacing a sproket, it might start skipping if you don't, if the other sprokets are older.

 

[nicobie]

I still peadle pretty comfy at 32 mph in 14-58 gear.

I'd love to find a 55 or 58t big ring.

 

[John in CR]

Wow, 72 is huge.

Sure it will look unconventionally large, but I think typical bike gearing range is really a waste with an ebike. I'm simply unable to assist at top speed, but I'd like to add at least enough to overcome rolling resistance and take some of the chunk the wind steals. I think I'll go with Dogman's approach and forget the front derailleur. Then I'll try one of Shimano's MegaRange cassettes with the extra large biggest ring for hill climbing and low speed assist. If that doesn't give me enough range, then get one of those stackable cassettes and use a front chainring for the largest rear ring.

John

 

[Zoot Katz]

Wow, 72 is huge.

Sure it will look unconventionally large, but I think typical bike gearing range is really a waste with an ebike. I'm simply unable to assist at top speed, but I'd like to add at least enough to overcome rolling resistance and take some of the chunk the wind steals. I think I'll go with Dogman's approach and forget the front derailleur. Then I'll try one of Shimano's MegaRange cassettes with the extra large biggest ring for hill climbing and low speed assist. If that doesn't give me enough range, then get one of those stackable cassettes and use a front chainring for the largest rear ring.

John

The derailleur may be able to handle the chain but you can get into issues when the cage and or body aren't long enough to keep from striking the gear. You're also running the chain at angle that increases wear and friction.

Make sure the big front ring clears the chains stays. You also want to mount it in what would be the middle ring of a triple crank.
Line it up straight with the middle cog on the rear. You'll probably have to play around with spindle lengths too.
Without a front derailleur, consider a chain keeper like the downhill and psycho-cross riders use. You don't wan't to throw a chain at 40 mph.

 

[OneWayTraffic]

Have you considered a rear internally geared hub (assuming that your motor is on the front)?

Or a dual drive?

 

[dogman dan]

The vendor of the 58 tooth on ebay was bear-state. I just looked and he hasn't posted another, but you could ask him.

 

[John in CR]

Zoot,

I understand the clearance issue creating a limit for the rear sprocket size and alignment, but you lost me regarding the chain angle and wear. Doesn't the derailleur line everything up to result in the same chain angles of bend regardless of sprocket size? I will be replacing sprocket(s), not adding more width to the stacks of gears front or rear.

Oneway,

I'd love a geared hub, but because I essentially need a significant overdrive, I still need a larger than generally available sprocket on the crank. The issue is everything easy is geared for pedal speeds, not a very relaxed cadence at 30mph plus. I've seen some large rings available for recumbent parts, but at premium prices. I already have this 72t sprocket for my e-moto that ended up too big. I just need to lighten it up with a cutting torch, machine narrower at the teeth to fit bike chain, and drill mounting holes, which seems much easier than a jackshaft with an additional chain and derailleur.

It surprises me that this kind of stuff isn't already available as ebike parts. With China putting 20 million new electric 2-wheelers on the road each of the last 3 years these types of issues should already be solved and readily available, or are their streets so crowded that ebikes are restricted to pedal speeds there?

John

 

[docnjoj]

Just in case, dont forget wheel diameter! I had to put my 16" rear whell on temporarily while fixing my 20" Bafang and the resultant pedaling speed is clown like! I cant pedal fast enuff to keep up with the motor over 12 mph! Almost like a powered wheelchair! It helped a bit to put a 52 tooth chainring, but it still is almost make believe pedaling on all but the steepest hills! If I could fit a 26" rear I could probably pedal well over 30mph!
otherDoc

 

[Zoot Katz]

Zoot,

I understand the clearance issue creating a limit for the rear sprocket size and alignment, but you lost me regarding the chain angle and wear. Doesn't the derailleur line everything up to result in the same chain angles of bend regardless of sprocket size? I will be replacing sprocket(s), not adding more width to the stacks of gears front or rear.

I was talking about "chain line". It's never been recommended to use the lowest, largest, innermost gear on the rear in conjunction with the highest, largest, outermost gear on the front and vice versa. That's why old ten speeds really only had eight usable gear ratios.

 

[John in CR]

Zoot,

I understand the clearance issue creating a limit for the rear sprocket size and alignment, but you lost me regarding the chain angle and wear. Doesn't the derailleur line everything up to result in the same chain angles of bend regardless of sprocket size? I will be replacing sprocket(s), not adding more width to the stacks of gears front or rear.

I was talking about "chain line". It's never been recommended to use the lowest, largest, innermost gear on the rear in conjunction with the highest, largest, outermost gear on the front and vice versa. That's why old ten speeds really only had eight usable gear ratios.

Got it, I'll go with your advice and align the big front ring with the center of the rear cassette.

Thanks,

John

 

[dogman dan]

If I use my big ring on front and the big cog on the back it clanks pretty bad. For some reason it seems worse than before, but I had to change cranks when I put the big ring on. But with the motor running I never need it, and when it breaks, like I didn't charge long enough and have to peadle home, I want the little front rings anyway. But with 72, you may want to actually use those inner cogs on the rear. Moving the big ring in to the center postiion might be good, but I bet it rubs on the frame if you try that. That was why I couldn't just put a 52 tooth road crank on my mtb. Send us pix when you try it out.

 

[John in CR]

"Clanks"??? Is that noise from too much angle in the chain due to front and rear sprockets being on planes too far apart? Yes, rubbing might be an issue on normal bike, but but this frankenbike is far from normal.

John

 

[dogman dan]

Yes, as you describe. The clanking is the chain links rubbing on the side of the sprockets when the angle is at the max. It's fine on the six smallest rear gears. Since I changed cranks to put one on that fit the big chainring, removing the tinfoil steel wallbike one, It's quite possible the ring sits further out than it did before. If it does, it's a good thing, since the huge ring doesn't rub the swingarm.

 

[swade]

Hi John,

On my bike I've got a 53t front and 11t rear gear and I can keep up with the motor until 29 mph or so and I'm not much of a 'spinner'.

Nick

Same goes for me. 408 @ 66 volts. This gearing allows me a bit of exercise, especially in the wind or up hills. I ride a specialized hard rock mtb. Had to switch the cranks, bottom bracket out for a road bike set up to accomodate the 53t front. I had to go wider with the bottom bracket too so that the big ring wouldn't rub against the back rear axle support. It was a PITA but I like the setup now.

 

[John in CR]

Hi John,

On my bike I've got a 53t front and 11t rear gear and I can keep up with the motor until 29 mph or so and I'm not much of a 'spinner'.

Nick

Same goes for me. 408 @ 66 volts. This gearing allows me a bit of exercise, especially in the wind or up hills. I ride a specialized hard rock mtb. Had to switch the cranks, bottom bracket out for a road bike set up to accomodate the 53t front. I had to go wider with the bottom bracket too so that the big ring wouldn't rub against the back rear axle support. It was a PITA but I like the setup now.

With 53t/11t and barely keeping up in the high 20's, I'm more concerned that 72t won't be big enough. I'm after leisurely assist in mid 30's, I already have pedal my ass off at mid 20's, and want something completely different. My longer distance between BB and wheel than normal upright bike, should solve any chain angle issues.

John

 

[spinningmagnets]

The Schwinn Paramount Chainring

 

[John in CR]

Now that's the kind of chainring I need with leisurely pedal assist at 45-50mph.

John

 

[grandmasterE]

They're not giving it away, but have you ever seen the Schlumpf High-Speed drive?

It's a bottom bracket implementation of a planetary overdrive system (1:2.5 overdrive) that is user selectable. You can run 34t front, and in overdrive mode it's an 85t equivalent.

Company:

http://www.schlumpf.ch/antriebe_engl.htm

and for sale in USA here (and probably others):

http://www.pbwbikes.com/product_info.php/manufacturers_id/16/products_id/57?osCsid=ee2d34f1abc24a1c82f880d06f9582cb

Seems like a really cool product, but probably more than most on the ES forums like the spend on the 'Bike' part of the Ebike. I'm coming from the bike side of things, so this price seems quite reasonable for a well built high end product to me.

E.

 

[John in CR]

That Schlumpf looks great, but I simply can't justify the $575 price, since that's about the cost of my 50mph hub motor plus controller plus DC/DC converter plus about 1kwh of Konion batt pack.

John