How to rework rear sprockets for mid drive systems...

[ykick]

UPDATE: We now have a video showing this. Highly recommended if using gearclamps as shown in video to use both of them to hold cassette in position

Here’s a photo show and tell about the wonderful rear cassette modification I’ve learned from other mid drive users.

Standard 9 speed rear derailleur and the resulting poor chain line connected to a single front ring:




The straightest and best chain line is somewhere in the middle of the cassette gear cluster. By the time you reach granny or greyhound positions the chain is suffering a fairly extreme angle.

Do we need all those gears/cogs in between the High-Med-Low ranges? Nope, those are needed with human legs looking for every ounce of efficiency they can scrape together. When you tug 1500W of battery power on the chain the need for “narrow” bicycle ranges becomes unnecessary.

And who wants to spend all their time shifting? These modern and robust BBSHD drive kits can jump 10 teeth at time without any problems or stress. What we really need for them is 3-5 wide gear ranges.

So in this example I’m going to rework 3 gears to 14T, 24T, 32T or High, Medium, Low gear ratios.

We start with a basic 8 speed cassette gear cluster. I like cheap steel Sunrace 8 speed gear sets as they most often don’t use rivets to hold the gears in shape:





Even if you do have rivets, they can easily be ground away so that disassembled looks something like this:






Don’t worry about keeping them in order because it seems they’ll only assemble correct orientation on the cassette spider:

[ykick]

Basically make up whatever combo you wish. I’m doing 14T, 24T, 32T on this spider

Here’s the resulting 3 speed range
:

The inner-outer gears serve as spacers. I actually changed my mind and moved the 3 speeds out towards the axle one more cog space.

These tools are very helpful for doing this sort of work:




Next thing to do is to adjust the “limits” so that we’re only shifting the 3 desired gears:
The Low limit needed a longer screw to accommodate the new narrower range.

[ykick]

Resulting 3 speed derailleur chain line looks good now:

[islander20]

I did this with my cassette. Huge difference, well worth the fairly minimal effort. I kept 5 gears though. I have a tiny amount of play in my freehub, hoping it was like that before I did the mod.

[commuter ebikes]

I have a hub motor with the very common Imperial threaded male freewheel mount. I use a common freewheel, which happens to be a 5-speed Sunrace 14-28T freewheel. I don't have room for a 7-speed freewheel, which I what I wanted because the smallest cog has only 12T.

I see that you did this with a cassette. Would I have any luck trying to do this with a freewheel?

[ykick]

Hmm? I watched the video but never got anything about how to re-arrange cogs on the freewheel gear cluster? Great info for cleaning and reworking the ratchet pawls and stuff but I'm not seeing a way to move the gears around?

[commuter ebikes]

ykick, you are right. When I posted that video, I assumed that one had to execute the work in the video in order to allow the freewheel cogs to be unscrewed. I assumed that the inner shaft had a lip which prevented the cogs from becoming unscrewed.

Upon looking at my freewheel, however, I see that there is a lock nut securing the cogs in place. In order to rearrange the cogs on a freewheel, which are threaded, one uses a pin spanner to remove the aforementioned lock nut at which point one is free to arrange the cogs in any order desired.

I still plan to take an 11-13T cog off of a 7-speed freewheel and put it on to my 5-speed freewheel, but I have yet to see if the threads match on the two different cogs that I plan to switch out.

Thirty lashes with a wet noodle for posting an irrelevant video!

[MIKEinNC]

I have a seven speed Falcon freewheel, on which the cogs slide onto the hub and are secured with a threaded lock ring. I disassembled it with the intent of rearranging the cogs, but found that the diameter of the hub is stepped in size. The inner (larger) cogs have a larger center aperture. Has anyone found a freewheel on which the cogs can be rearranged ?

[ykick]

Thanks all for the input and we'll hopefully get some more savvy computer folks to better clean this mess up? But yeah, it's proven a good way to solve some of the annoyances created by the marriage of so much power thorough single chain ring applied to rear gear clusters.

I've also done it with some 9 speed derailleurs but I prefer older 8 speed for this: cheap, solid and easy to source.

[paxtana]

Thanks for this! Did what I could to clean it up, added a top pic for a not-good chainline since that pic was broken on the post and I couldn't recover it.

[wagonrd]

Thanks for the info ykick.Using your knowledge, I modified my 8 speed 11-40 sunrace so that te 40 tooth cog is in the 5th position ( counting from the outside in). Three cogs with lesser teeth are mounted to the outside, with the 11 tooth cog as the final cog. I set the derailler limits so that I only shift cogs 2, 3, 4, 5. This setup allows me to install the luna cycle mighty mini 30 tooth sprocket on my BBSHD motor and thereby keep the chain line functioning within its limits. Now , when I run out of battery juice on some hilly dirt road I can pedal the bike home using the 30/40 ratio instead of pushing it

[HutsonDave59@Gmail.com]

I'm getting ready to build a mid motored mountain bike with a 9 speed cassette. I am curious as to what affect the increase of gear diameter from one gear to the next has on the shifting performance/efficiency. Thanks

[ykick]

I find little difference shifting 2T increments or 10T increments.

Performance/efficiency? How efficient is it to change up/down 2T 4X as opposed to 10T 1X?

The distinction here is that bicycle gear cluster increments designed "narrow" for leg powered systems. When a motor tugs 750-1500W on a chain, narrow range gear increment's no longer a benefit. And IMO, it adds a lot of wear tear to any derailleur system while also wasting a bunch of time fiddling with gear selection more than is really needed.

[calfee20]

That is a good question. I have come to the conclusion that it depends more on the quality of your rear derailleur than on the spacing of the sprocket sizes. All of my bikes have been around $500 and the shifting was not the best with a wide variety of cassettes and freewheels. I just bought a Paratrooper Pro, $1000 and the shifting is flawless.

[Giant Rider]

I have a good quality Sram derailleur that shifts rock solid when tuned properly, even under moderate leg power so with a little luck it should be fine. I am not going racing here all it needs to do is get a fat old man over the hill and through the woods. LOL

[ApexBM]

Appreciate the guide... My Sunrace CSM98 9-speed gets here today to begin modifications. Cogs to select from are: 11, 13, 15, 18, 21, 24, 28, 32, 36.

Drive cogs will be 15, 21, 28, 36 with spacers on the outsides.

My initial thought was to try and center the chain line on the middle 21 and 28. Does this make sense? also are there advantages to specific spacer cog order/placement other than centering the chain line on the driven cogs?

[ykick]

I honestly don’t know if you’ll ever need a 32T? I’ve yet to need anything bigger/lower than 28T on 26” wheels running a 42T chain ring. But you are in SF so 32T might be a good idea?

I do find myself sometimes wishing I had something a little smaller/higher where I run 24T. 21T might be a good idea there? A couple narrow increments in the middle ranges could be useful?

As far as what order to replace the cogs used as spacers? Obviously, the large ones won’t work too well outboard. But I just play around with everything until it looks good. I get funny looks from bicycle mechanics who notice this odd arrangement….

[ApexBM]

I do a lot of throttle only and have 29er/700c wheels so I find the granny gear useful for takeoffs after I turn the first revolution with the crank.

[Kocho]

30t on the rear and 42 on the front is what I have on my 7 speed freewheel with BBSHD on 26" mtb wheels. I do use my granny gear on several slopes during my commute. Yes, for very short sections. But, on one of those sections, I need to help with the pedals as a bit over 1,100W (the max. on my programming on a 52V battery and 20A current limit) is not enought to get me up that hill on throttle alone. 1,500W might do it, but I'd rather pedal. On the other hand, 42t front and 11t rear is only good for pedaling along up to about 25mph max (even a bit slower for a more comfortable cadence). Yes, I can pedal at 30mph, probably a bit more, for short sprints, but for miles on end I seem to settle on no more than 22-25mph with 42-11t combo and 26" wheels. Ideally, I think I'd want 11-40+ on the rear and 44t on the front, I think. But they don't make 7 speed free wheels with such range. Might need to rework an 8 speed cassette with bigger spacers to get a 7 speed spacing. Or get an 8 speed setup (change my shifter and derailleur) and rearrange and use maybe 4 or 5 gears only, like shown here ...

[ykick]

Odd numbers to me my friend. For example, my 26” BBSHD Fatty tugs 42T front and ‘never use anything smaller than 14T rear. About 1000W from battery capable of 28-30MPH on flat, smooth road calm day (no wind).

Do my legs input anything at that motor RPM/cadence? Nope and these Bafangs are “geared that way” internally. If you drop down to 11T rear you might contribute something from your legs (and wreck your chain) but the electric motor is running in such an inefficient RPM power band that whatever your legs contribute will be negated by heating of the motor windings, IMO.

This bike tackles crazy steep stuff 12-16MPH on 28T rear gear pulling roughly 1000W battery power. 32T rear granny accomplishes the same climbs on a little less than 1000W but only making about 8-10MPH.

[Kocho]

Hmm, you are right in principle that too high a gear might bog down the motor, but I think at the around 20mph and pretty much flat, and in low PAS, this is not the case. I'd gladly hear anargument that proves me wrong thpugh, as I don't want to torture the motor if you think I'm doing it. But it does not seem this way. I'm usually in PAS 2 or 3 on the 11t rear, goung on a slight downhill, and adding just a little from my own leg-power. That's under 200W electric plus a few W from me. That seems to work well, keeping the bike in the 22-24mph range drawing just 2-3A. I don't feel the motor is struggling. If I put it in PAS 5 (about 500W) or more, it will go faster easily, but I don't want to go that fast on that trail. On the same path going back uphill, I'm down to 20mph on the same PAS (200W) and usually go down to 6th gear instead of the 11t 7th if the speed goes below 19mph or so at these low PAS levels.

For me, if the bike won't accelerate nicely with more throttle, then it means I need to downshift. In the situations above that is not the case - I can accelerate, it's just that I'm using very little power so the air resistance is enough to keep me at asteady speed at low power levels.

I've experimented to see if a lower gear vs. higher gear would propel me faster for the same power, and in the ranges I ride it does not - the higher gear achieves higher top speed at a given power. This tells me the motor is more efficient at those speeds at the slightly lower RPMs. Again, this is at certain speed at certain inclines, not a generalization. I feel at higher RPM power drops vs mid-range or low RPMs on the BBSHD. Thus seems similar to my Zero SR electric motorbike, whuch would pull very hard from a stop and through mid range up to say 70mph, then would gradually reduce power output above that (air resistance notwithstanding), probably due to back EMF ...

[ykick]

200W probably isn't losing much Wattage to heating the windings because it's s small amount of power to begin with. I avoid anything above that power drain/demand at those low motor RPMs.

I don't mean to preach but 11T is killer on chain. I had a very knowledgeable bicycle mechanic tell me to never ever use less than 12T. Especially with possibly 1000W of power or more tugging on the chain on that small sprocket.

[Kocho]

Your 26" fatty has 4" bigger diameter tires, right? I ride on 2" tires, you are on 4". That might be enough of a difference?

[ykick]

I don’t believe ICE is good comparison here partly due to the fact that higher ICE RPM’s usually require more fuel/air mixture even with absolutely no load. Electric motors reaching their RPM/V limit require some electrical power too but if the windings aren’t shedding any waste heat there’s really not much loss of efficiency.

Unfortunately, Bafang chose internal gear reduction which doesn’t seem to cooperate with legs on 48-52V systems, IMO. Perhaps it’s hold-over from the 36V era BBSXX’s?

Interesting stuff to be sure. I’m probably in the minority about how I ride but I only attempt to add leg power if I’m riding at “human” levels in the 15 MPH range. Once over 20MPH I find it very difficult to maintain the motor RPM in what I feel is an “easy/efficient” place and still add any leg input.

This is also one of those BBSXX things very different from hub motor setups. Hub motors, 52T front -11T rear gearing, legs can usually add a little (100-150W) while trucking along @ 25-30MPH.

[Kocho]

My wheels and tires are a hair under 25-1/4", that's with 26" wheels and 1.95 profile tire. I suppose they might get to 26" diametef with 2" or slightly lfatter tires.

I agree the motor gearing could be better setup for lower cadence at higher motor RPM...

Btw, got the Mirrycle and installed. Will see how it works. I did not want it to stick out from my handlebars, so installed it rotated under the grips.

[ykick]

I've often wondered if Bafang simply left the gearing ratio (internal planetary) the same as it was back when these drives started life on 36V? Perhaps there was even 24V back in the day? That might explain why we see such a poor match to natural leg cadence running 48-52V motor RPM's?

Mirrcyle installation is "one of those things" I've seen done several different ways. I do it as seen in video but some folks arrange them to suit their needs/wants. Whatever accomplishes a decent view of whatever's approaching from the rear is the point.

[ykick]

Here's a quick video compilation from this am - 1st part simply trucking along paved flat level, calm day ,PAS 7 42T-14T ratio (doesn't max much above 1000W) around 30MPH

2nd part simply coasting downhill behind a roadie about 20 MPH neither of us under any pedal to compare "coasting" capability. He didn't begin to pull away until he tucked down. Screw that, I'm all about comfort and more comfort. In that order....

3rd part is a seriously steep long climb in PAS 7 using 42T-28T 15-16MPH

I usually let the Bafang wind up and max out RPM then work gearing around that. It seems to stay cooler = more battery energy reaching the driveline, IMO.

[islander20]

I love my mirror, I have a mirrcycle. I don't know how I ever rode without one.

[commuter ebikes]

So $12 could make the difference between getting killed or moving out of the way? Seems like an easy decision.

[Onemorejoltwarden]

A fine complement to mirrors is the Varia.
Pricey, but once you try it , there is no turning back.
Looks back to 160yards, back where you can't hear esp the 'lectric cars

[LI-ghtcycle]

Nice solution! I have to ask though, why not just use a good 3- speed hub like the Nexus Inter-3 instead? Even on a derailleur framed bike, you can get a chain tensioner or just use the derailleur as tensioner, then you have a rock solid 3 speed, and you can swap out the cogs on the IGH for making your gearing range more custom. : )

[ykick]

Cheap, no lacing required and it's not limited to 3 spd. This rework can do 3, 4, 5 gears using any ratios desired.