Single Speed Conversion : Optimal Chainring-Cog Ratio

[athead]

Hey guys
converting to a Single speed with my Ebikekit 500watt engine 36v battery and wanted to know what an optimal chainring to cog ratio would be to get the most out of my engine? I have to go 16T on my rear hub (since that's what they sell) , and was thinking 48T on the chainring for a 3:1 ratio. I've scoured Sheldon Brown and other webpages on regular single speed conversations, but obviously having an ebike gives me more leeway. I've been riding for 2 years daily (on my ebike), pretty strong and in good shape, just hate spinning out at 30miles per hour downhill-want to get the most out of my engine without going with a bigger battery. I live in a river valley so I literally ride up hill both ways when crossing through town). I've always kept my previous geared conversions on the lowest gear.
Thanks for your thoughts

 

[NeilP]

You could always go back to the Sheldon Brown site and look up the gear ratio calculator...work out what your current meters development / gear inches figure is... then using that as an input, work the other way...to find what you need on your current setup.

The calculator I am thinking of takes wheel size/ and pedal crank length in to consideration as well as front and rear teeth numbers, so you should end up with the same pedaling force give or take a few %

Here is the Sheldon Brown one

http://www.sheldonbrown.com/gears/index.html

but now looking at it...there was a much better one on another site...but cant find it now...I'll keep looking

 

[spinningmagnets]

I have read about high-speed E-bikes, with owners who wanted to contribute pedaling as much as is possible. They go out of their way to get the smallest rear sprocket, which I seem to remember being 11T. The front chainring I have seeen that seems to be popular is the 52T. Here's some high-tooth chainring links:

http://endless-sphere.com/forums/viewtopic.php?f=28&t=11690#p178009

and some fun pics of giant chainrings:

http://endless-sphere.com/forums/viewtopic.php?f=3&t=11323

You might consider a front sprocket-cluster that has two chainrings (or even three?) with a simple chain-tensioner. It seems people who have never had E-power often still 'think' they want a lot of gears in the rear, but when they finally ride it around a bit, they soon realize they only use a few of the gears.

Much frustration has been experienced trying to fit a wide E-hub with a 7-gear cluster into tight rear drop-outs, only to find out later that the "drop in" 5-gear would have been adequate. I've seen builds with only 3 gears in the rear (derailler style), freeing up the front bottom-bracket to be 'clean' with no derailler and a single chainring (also makes a front-half chain-cover a more viable option)...

 

[dogman dan]

I'm not so sure what you are wanting to do, pedal up more speed or help pedal up hills.

48 front 16 rear would be pretty good for riding the top speed of the motor on the flat, about 23-24 mph. For getting up the hills easier, mabye just a bit lower, like 44 front.

When I put my first commuter together, I didn't like 48-14 much at 48v too low, and found a 56 tooth front. I loved it, but only because I could still shift to lower rear gears.

 

[amberwolf]

converting to a Single speed
<snip>
I have to go 16T on my rear hub (since that's what they sell) , and was thinking 48T on the chainring for a 3:1 ratio.
<snip>
just hate spinning out at 30miles per hour downhill
<snip>
literally ride up hill both ways when crossing through town). I've always kept my previous geared conversions on the lowest gear.

If I understand correctly, you're looking to be able to contribute pedalling power on the faster *downhill* runs, *and* on the *uphill* runs, too, yet you want a single-speed gearing setup?

That confuses me, because you can't do both with only one gearing ratio, at least, not unless you really want to be cranking hard on the uphills and spinning pretty fast on the cranks on the downhills.

Hopefully I'm just misunderstanding.

 

[jbond]

After riding around on an e-bike for a bit with a BPM motor and a 21mph top speed, I think I'd like a 2 speed front and a 5 speed rear with something like 38-53 front and a 14-28 rear on 26" tyres. The only reason for the 38 front is to get home when I run out of power and have a nasty hill. So I could do without the front mech and just swap the chain by hand. Having more than 5 speeds on the back just seems like overkill. The Shimano Megarange I have at the moment means I'm only actually using 6 gears normally.

I wouldn't go for a single speed unless you have built a moped where you don't pedal much at all. Any kind of hill will be painful and/or you can't keep up on the downhills. Fixies are ok in town where all your speed is 5-15mph and there are no hills but out in the real world, they don't make a lot of sense even if they look neat.

 

[Kingfish]

I do not run with single speed-gearing, however I am most often in the same (highest) gear unless I hit a very steep hill or I have a system failure (rare these days). My goal has been to pedal as fast as I can to keep up with the bike. The original ebike (P0) ran with a gearing of 13:44 using a 7-speed freewheel (13-34) and a 22-32-44 chainring. Then I upgraded to 13:54 (22-38-54 chainring) just before my cross-country trip last summer and this made a huge difference on the top end. During that ride, 29 mph was comfortable and sustainable; in commuter mode (balls-to-the-walls urban aggression) 33 mph was about the limit (barring tailwind and downhills).

The new ebike (P1) has a gearing of 11:44 using a 9-speed freehub (11-34); it is a tiny bit slower gearing when shifted to the highest however the bike is lighter, has modern components, and in commuter-mode I can push it to 34 mph and hold it there consistently. I have a 48T chainring ready however the bike will require replacement of the front derailleur and a crank axle spacer to move the gear farther away to clear the frame. Last night in my search for other goodies I came across a 2-speed racing crankarm that had an attached 38-53T chainring; it was monstrously expensive carbon-fiber stuff – but at least I know that make them now.

The key to all of this is the final ratio; providing your bike can take you there the top ranges are (for a 26” wheel):

~ OLD eBike ~
13:44 = 1:3.385 (about 26 mph)
13:54 = 1:4.154 (29-33 mph)

~ NEW eBike ~
11:44 = 1:4 (28-34 mph)
11:53 = 1:4.82 (est. 37-42mph)

Your experiences will vary: The primary factors affecting speed – other than gearing are…
Controller configuration, motor winding ratio, battery volts and power delivery, harness wiring, weight of bike frame, aerodynamic coefficient, present physical condition, riding style, the clothes worn, weather, and terrain. Commuting I find makes for a beautiful model to evaluate these conditions.

Like the wind, KF

 

[spinningmagnets]

The dogman and Amberwolf are right about needing at least 2 gears to have an uphill/downhill system. You don't have to apologise for wanting a single cog in the back, some people just like it that way.

You could install a two-chainring set and derailler on the BB, and then juggle chainring tooth-counts until you found the two that "scratch your itch" the best (we all have our own personal preferences).

 

[athead]

Wow, been sick a couple of days so I'm just now checking back..first, thanks for all the helpful feedback guys. To clarify, I only mentioned the "uphill both ways" because I'm also downhill both ways and want to maximize my contribution downhill ...I do just fine going up the hills...but after a long day on my feet, I love to go, as has been said "balls to the wall." I wanted to go with the single speed setup for minimalist reasons, it's a my year round commuter and I'm tired of skipping gears/stretched chains and have come to see my ebike as a mobile exercise bike, and well, I'm a glutton for punishment. The responses have given me a lot to consider- I ended up getting a TransIt Weaver Single-Speed Bike from Performance Bike with a 38T Chainring and the 16T cog from ebikekit. I'll try it out and make upgrades per the advice given.

 

[dogman dan]

Ok, now I undersand. Ebay may have a 52 tooth that can fit your crank, or a 52 crank that fits your bike. At the very least you will for sure want a 48.

I've done a lot of riding, like about 4000 miles on my 56 tooth crank. I kept it on 56-14 on the open road, but then would drop down 1 or two gears for urban riding in the stop signs or 6% hills. So 52-16 oughta be good for simiar gearing to my stop sign gear.

That big crank is now on my racing bike, and I can pedal with signifigant power at 35 mph. 56-14.

 

[Kingfish]

I ran through my options yesterday following up on the earlier lead and there are a few choices:

• Go with older cranks (5-10 years) where up to 56T is possible. Downside is that it is older equipment and parts will be harder to find.
• Outside-the-box solution 1: Tandem cranks offer unusual scales of gearing that might prove useful.
• Outside-the-box solution 2: Ditto with Recumbent – up to 60T available.

At present the top-of-the-line (or near-totl) are limited to these common ratios:

• TT: 45 to 55T single chainring
• Racing Doubles: 53-42T, 52-39T; potential up to 56-44T but these are custom and very $$$$$
• Racing Triples: Highest that I found < $400 were 52-39-30, and if you go back 1-4 years then the price drops even more and the selection grows (53-39-30 was not uncommon).

I am leaning towards the last option with a late model triple: I want to stay with the same Bottom Bracket. Lucky for me switching from MtB to Road crankset is not as critical if I stay within the last couple of years. It will still be $$, though not $$$$$.

Searching tips: I used “crankset 54” on eBay to worm my way up the stack looking for present options. I can guarantee the selection is skimpy, worn, and obscure. Type in “crankset 53” and the results pop from 10 to 870; this is a good place to mine for top-end (meaning speed) solutions.

The other tidbit I picked up from my local bike shop is that it pays to be attentive to the crankarm length. The Shimano FC-M770 (22-32-44) mounted on my P1 ebike is 175mm; I didn’t have a choice to change it when it was ordered 9-10 months ago; just didn’t think about it. The problem though is that when I’m pushing beyond 33 mph I am physically shifting my body left-right left-right on the seat; a wobbly ride noticeable in the headlights.

The local bike shop mechanic is about my size (a diminutive 5’5” counting the platform shoes and ankle-weights when hanging from a monkeybar) and suggested that at high-cadence it makes more sense to have a shorter crankarm. The long-length for MtB is required for strength and hill climbing, whereas for road racing, shorter is better, especially for us wee pixies, suggesting 165-170mm for my pint-size.

So there’s that into the mix of it all.
Budgeting, KF