2 speed with gears or retro-direct chains...efficiency?

[John in CR]

I've got a motorcycle tranny to easily make a solid 2 speed. That means 2 gear pairs always spinning, one engaged and one not, plus a chain to the wheel and a chain or belt from the motor to the tranny. My question is how less efficient would this be than a 2 chain retro-direct rig using freewheels and a couple of idlers for engaging the outside of the 2nd chain?

 

[Harold in CR]

Don't know if it's possible to buy electric motor bearings for the shafts down here. That's what I used in Fl on the sawmill build. They are smoother and last a LOT longer.

What you are thinking is the same thing as Thud's tranny. Seems to work pretty well for the RC motor drives ??

You should have plenty of battery, unless it's for the cross country run.

 

[recumpence]

I would say the drag will be relatively high. But, if you are running tons of power, that is not a problem.

Matt

 

[John in CR]

I would say the drag will be relatively high. But, if you are running tons of power, that is not a problem.
Matt

Which one would have significant drag? Compared to reduction units the 2 speed dog clutch tranny is just driving 2 additional spur gear pairs instead of a number of pairs in the designed multi-speed use. The R&D would just have 2 extra idlers with bearings and an extra freewheel, but only one chain driven at a time (if I'm looking at it properly).

 

[AussieJester]

Got some pics please John? interested in seeing this...

KiM

 

[Miles]

This was the retro-direct schematic that I did, back in 2008. If someone feels like repairing it in Photoshop, feel free to

The top segment has the correct colors, the other segments need correcting and shifting.

Ref: http://www.endless-sphere.com/forums/viewtopic.php?t=7679&p=115729#p115729

 

[John in CR]

Thanks Miles,
I thought I paid full attention to all of the R/D options back when we were hot on that topic, but I didn't recall that specific method of obtaining the lower gear. As always, your layout is nice and clean. I was thinking dual sprockets at motor and wheel, with double idlers at the motor end for the low gear to drive the outside of the chain. It's not as elegant, but the primary cruising gear, high gear, enjoys the efficiency of just a straight chain with the only extra losses being 2 freewheels on the low speed side's drive and driven sprockets.

AJ,
No pics, unless you want me to take one of the raw gears, shafts, and dog. I grabbed the brand new guts of a moto tranny for next to nothing, and this one has everything done for me except the housing and a shifting mechanism for the dog...ie like Thud's, but with gears instead of chains. I've got a mountain to cross with me, a strong (somewhat heavy) bike, and 150lbs of batteries, so there's just no way around either a 2 speed or a helper motor for climbing.

I have a smaller diameter high power frock on the way for testing that boasts a 93% peak efficiency, and now that I have a build where I actually need to pay attention to efficiency, I'd like to understand the impact on overall efficiency at cruise in order to make an intelligent decision about whether or not to use it as a mid-drive (frock+helper motor or frock as mid-drive with 2 speeds). The motor has Delta/WYE switching, but I don't think that's quite enough to tackle a 10% grade for 15 miles nearly continuous, not with this load.

 

[Miles]

I was thinking dual sprockets at motor and wheel, with double idlers at the motor end for the low gear to drive the outside of the chain.

That would work fine. One of the pedal retro-direct designs at the beginning of the 20thC. used a similar configuration. Sometimes with one chain on the left side of the bike and one on the right, as an alternative to having both chains on the right.

 

[John in CR]

Exactly, it's just not elegant looking like yours. A bonus though is that once explained people can look at it and understand how it works. Too bad the geared hubbies still have freewheel clutch shortcomings, because epicyclic reduction and reversal is still my favorite R/D, and a geared hubbie has most of what is needed to DIY one with one chain from the motor itself and the low speed chain from the housing.

 

[Alan B]

Should not be hard to defeat the freewheel in a geared hubmotor. Sometimes they do it by themselves.

 

[John in CR]

Should not be hard to defeat the freewheel in a geared hubmotor. Sometimes they do it by themselves.

That's the problem. I think we want the freewheel to work, and they have issues in both forms, slipping so engagement doesn't work and not freewheeling. The geared hubbie solves the reversing direction issue internally, so 2 normal looking chains are viewable. The motor spins the opposite direction of the shell and a 4-5:1 gear reduction is already done. Spin the motor normally and the outer shell drives a low speed chain with say a 2:1 reduction in the sprockets. The tricky part is porting getting a hollow shaft attached directly to the motor's rotor to get it's output to the exterior. Then reversing the motor drives a chain with a 3-4:1 reduction on those sprockets for a nice wide 2 speed gearing.

The one issue with R/D is the bike won't roll backward, not a show stopper, but different, especially on something like my long heavy cargo bike though I do often manhandle it around, typically to do 180's in a parking spot where I can't easily roll it backward.

John

 

[TylerDurden]

This was the retro-direct schematic that I did, back in 2008. If someone feels like repairing it in Photoshop, feel free to

Screenshot from PDF:

 

[Miles]

Thanks Tyler! For some reason, I didn't keep the PDF...

 

[recumpence]

John,

The reason I said the drag would be high is from experience rebuilding motorcycle engines and transmissions. The gears are relatively wide and create a lot of drag when spun by hand. I am not criticizing your project, I am just answering the question you asked.

A transmission designed for 100+ hp will have high drag (relatively speaking) when driven by a much smaller motor.

Matt

 

[John in CR]

Thanks Matt,

So it's those bearingless gears spinning on the shaft and the 2 pairs of spur gears creating the drag you stated...Plus pulling the gears through the oil bath necessary too. I was trying to compare the 2 different 2 speed systems, R/D and a common tranny reduced to 2 speeds. In terms of efficiency, it looks like R/D would be a close 2nd to a separate helper motor and it's single freewheel. The losses in even a simple geared tranny must add up quickly to at least a decent handful of %.

If we could only figure out a way to roll and R/D bike backward without some kind of disengagement clutch for the wheel. 8)

 

[recumpence]

Of course, the drag is proportional to the RPM. So, if it was running at low RPM, the drag would be reduced. So, a deep reduction before the tranny would help.

Matt

 

[bobc]

I run a retro direct e-bike. It works pretty good, but I spend so little time in the lower gear it's probably not been worth the effort. I use cheap sprags on the jackshaft, 12mm on the belt and 15mm on the (lower) gears & these have held up OK for 250 miles - a 12mm sprag has broken in both positions now so if the 12 that's still in goes again I'll swap it for another 15mm. I get some noise and drag from the idling drivetrain but still do 25wh/mile at 20mph (if it could do 20 that is.. cough cough). this is the 2 gears part

 

[bigsi]

Fantastic. Where did you get all the wheels and belts from?

 

[spinningmagnets]

Previous post is from 2011, but...if Bob chimes in, I also want any info I can find...

 

[bobc]

Blast from the past.... I would have bought the gears from HPCgears in UK, & lightened them + further machining in my own lathe.
FWIW I got rid of the 2speed, round here I only need top & the extra drive was noisy & the sprags were unreliable and power sapping.
The belt is not an HTD, it's "synchroflex" or something like that & has run fine on the HTD pulleys for thousands of miles. That was a question of what lengths were available.
That bike has now died, I actually got me a lipo fireball due to laziness when charging (I got so I 'forgot' to do the regular balance charges & just bulked it about 100 times). Whoof! (it gave me plenty of warning & died in the garden....) So let that be a lesson