Chainring Reccomendation for High-Speed Pedal Assist

[SHARKBITEATTACK]

So I really like pedal assist and the ratios are almost perfect with my current setup. 36v, 9C 2808r with a 9 speed freewheel (11T smallest sprocket and 44T largest chainring) on a 26 x 2.14 tire. However, in the near future I will be doubling the voltage to 72v and will be slipping on a larger tire (Cyclops Pro 25 x 2.4). From what I've read on the forums this setup should give me a 40mph top speed. I only plan on making short sprints at full speed and I think I'll spend the majority of time cruising between 25 - 30mph. Anyone have experience or advice on what size chainring I could use so that I can pedal assist at these speeds at a reasonable rate?

I've never really payed attention to my cadence but I did play around with an online gearing/speed calculator and it looks like I generally cruise at 70 rpm. Although I'm not certain. Based on the calculator it seems that a 60T would suite my needs but I'd still like to hear from others.

Also wondering if any other problems would arise, derailer issues, too wide of a jump from sprockets etc.

 

[Chalo]

You can switch to a road bike crank, with 53-39 or 52-42-30 rings. But I wouldn't bother spending the money to take it beyond that. Here's why.

At 40mph, assuming a typical sit-up-straight e-bike rider position, it takes about 2700W of mechanical power to maintain a steady speed on flat ground with no wind, with no pedaling. Adding 200W to that, which is a hard but sustainable effort for most cyclists, would add only 1.0mph to your speed.

BUT-- to add this much power at the pedals, you have to turn them. That costs 1.2mph. That's right; the drag from stirring the air with your legs costs more power at 40mph than you can sustain at the pedals. And that isn't even taking into account the efficiency benefit of crouching, pulling your knees in to the sides of the bike, and generally becoming streamlined in a way that you can't do while pedaling.

At 40mph, it's not a bicycle any more. You can pedal all you like if it makes you feel good, but it actually costs you motor power to do it. Being able to pedal along at 30mph is more or less a wash. Pedaling along at speeds much above that is a net loss. So don't bother having gears for speeds over 30. They don't help. 53/11 gearing with your big fat tire results in 34mph if you pedal at 90rpm. That's more than enough gear.

The relentlessly exponential power requirement is one of the reasons I don't think it's a good idea to have an electric "bicycle" that is actually a light motorcycle. The other reason is that according to the law, you'll need to get your bike registered, inspected, insured, and have a special license to ride it. Maybe you can slide by under the radar without hassles, but if you get in an accident, the other party's lawyer can make you sorely wish you'd gone to all that trouble. Likewise if a cop wants to mess with you for whatever reason.

 

[SHARKBITEATTACK]

Thanks for the info, I had no idea about what happens to pedal efficiency at high speeds. As I mentioned, the gearing I'm looking for is one that would allow me to pedal assist comfortably at 25-30mph. At 2700 watts my nimh battery pack is gonna have a hard time delivering the current required for 40mph, thus I will probably only only see those speeds for a minute or two at most.

 

[friendly1uk]

What speed are you comfortable pedaling at now, and what speed do you want to be comfortable at.

The percentage of speed difference wants to be a percentage of gearing alteration.

I put on a 10% bigger crank, now my comfortable speed is 10% faster.

What are your goals?

 

[SHARKBITEATTACK]

I guess I want to go from pedaling at 17mph to 27mph

 

[cal3thousand]

How much you willing to spend? There's the Hammerschmidt, that will give you a greater range of gearing but costs a pretty penny

 

[SHARKBITEATTACK]

How much you willing to spend? There's the Hammerschmidt, that will give you a greater range of gearing but costs a pretty penny

Ughh I was hoping to spend like 30-40 bux for a single chainring...

 

[speedmd]

I also think you could go to a standard road crank with 53/39 rings. 55t can be a relatively common option for a standard 130mm bolt circle and also possible to find one slightly used at a good price.

 

[slacker]

you can get a 60 tooth vuelta chainring from amazon for the price you are hoping to spend.

 

[SHARKBITEATTACK]

I also think you could go to a standard road crank with 53/39 rings. 55t can be a relatively common option for a standard 130mm bolt circle and also possible to find one slightly used at a good price.

Do roadbikes usually have 2 or 3 chainring sprockets up front?

 

[neptronix]

vuelta USA sells up to 60T chainrings for decent $.

Very good quality, i had a 60T on this bad boy; i could pedal up to 35mph or so.

 

[bee]

So don't bother having gears for speeds over 30. They don't help.

I would recommend it for folks in cold climates. One of the main reasons I got into this hobby was because my scooter is uncomfortable to ride in the winter. With proper gearing, I can exert a lot of energy while commuting at a high speed which keeps me reasonably comfortable when it's -30 outside. For me being comfortable is way more important than efficiency, my electricity is free anyway

 

[SHARKBITEATTACK]

Neptronix, what size wheels do you have in those pics? I'd imagine with my 26" wheel I could pedal even faster.

Bee, I'm not as far north as you are but it still gets chilly in Chicago and I plan on riding through all of winter. Part of the reason I got into this hobby was to get outside and exercise as well!

Ideally I'd still like to retain 3 Chainring sprockets up front. I'm just wondering if I were to slap on a 60T if I'd have problems shifting with the bigger "gap".

 

[Chalo]

I'm just wondering if I were to slap on a 60T if I'd have problems shifting with the bigger "gap".

Yep. You'd need to switch to a road front derailleur to better match the curvature of the front rings, and you'd need to observe the derailleur's overall range limits. Use a road double and the rings should be 60-46t; use a road triple derailleur and the crank should be set up 60-50-38t.

If you switch from an indexed front shifter to a continuous sweep "friction" or "microclick" shifter, you may be able to fool around with the intervals between rings, but the derailleur itself is designed for specific intervals.

 

[mrzed]

http://www.machars.net/bikecalc.htm

Use that to figure out your gearing needs. If your normal cadence is 70, I'd aim that for a little under the 25mph mark so you still feel like you are pushing. I ran 48/11 top gearing on a 26" wheel and only started to wish for more well over 30mph. That calculator shows that as 80rpm with a 26x2.0 tire which fits my experience.

 

[neptronix]

Neptronix, what size wheels do you have in those pics? I'd imagine with my 26" wheel I could pedal even faster.

That's a 20" or 21" size wheel.

26" with a 60T would be insane. But would look hilarious as you pedal along at what, 40mph in traffic? lol

 

[chisixer6]

Sharkbiteattack;
Hostel Shoppe has your 60t in 110mm and 130 mm for $38. you'll get it in a few days, they are about 200 miles north of Chicago.

http://www.hostelshoppe.com/cgi-bin/readitem.pl?Accessory=1016049820

 

[speedmd]

I would not go bigger than a 56T on a 26" wheel. Learn to spin a bit. No sense pushing a gear at 70 rpm. Your just asking for knee problems if you push at all. The 60 will require a bunch of extra chain that the R. derailleur will not be able to deal with. I would forget a triple with this much range. If you need a low gear also possibly go to a compact crank set up which you may have now and they come commonly stock with a 34/50 and swap out to a 54 or 55 so you would have a 34 /54 that will work relatively well depending of the front derailleur.

Lots of talk about the pedaling causing drag. In a wind tunnel possibly. I know that when I start pedaling when my ebike is going 30 mph range with my feet still, it picks up a few mph when I start spinning and adding a few watts. Not major amounts of energy, but more of a 60k steady pace. It may be the right hand is twisting a bit inadvertently but I have tried this many times now with the same results. When I get my CA I will be able to see this more definitively. Coasting down hills I get the same results on the pedal only bike. Maybe my feet are smaller and legs are thinner than the folks in the wind tunnel. Certainly my legs are hairier.

 

[lbz5mc12]

I bought a set of 58, 48 road bike cranks on amazon for less than $30.00 plus shipping. They worked great until I broke my other bike so now I don't have anything to use them on.

 

[cal3thousand]

Just make sure your frame will accept the larger chainring.

On my DH rig, the chainstay's flair near the suspension pivot limits the chainring size to about 46T. Some frames won't be at all affected except for ground clearance.

 

[neptronix]

You can run a wider bottom bracket to get around this. It will negatively affect your chain angle though, which may mean that you will want to limit yourself to the gears closest to the dropouts.

 

[SHARKBITEATTACK]

I bought a set of 58, 48 road bike cranks on amazon for less than $30.00 plus shipping. They worked great until I broke my other bike so now I don't have anything to use them on.

Did they work ok with the stock deurailler?

 

[bee]

I would not go bigger than a 56T on a 26" wheel. Learn to spin a bit. No sense pushing a gear at 70 rpm. Your just asking for knee problems if you push at all. The 60 will require a bunch of extra chain that the R. derailleur will not be able to deal with. I would forget a triple with this much range. If you need a low gear also possibly go to a compact crank set up which you may have now and they come commonly stock with a 34/50 and swap out to a 54 or 55 so you would have a 34 /54 that will work relatively well depending of the front derailleur.

Lots of talk about the pedaling causing drag. In a wind tunnel possibly. I know that when I start pedaling when my ebike is going 30 mph range with my feet still, it picks up a few mph when I start spinning and adding a few watts. Not major amounts of energy, but more of a 60k steady pace. It may be the right hand is twisting a bit inadvertently but I have tried this many times now with the same results. When I get my CA I will be able to see this more definitively. Coasting down hills I get the same results on the pedal only bike. Maybe my feet are smaller and legs are thinner than the folks in the wind tunnel. Certainly my legs are hairier.

A good tuck really makes a difference:

[youtube]QsPwK6dZYyU[/youtube]

 

[speedmd]

Nuts! He would be faster if he pointed his toes!

 

[fechter]

vuelta USA sells up to 60T chainrings for decent $.
Very good quality, i had a 60T on this bad boy; i could pedal up to 35mph or so.

That looks like what I need for my A2B. The stock 46T gets me up to about 20mph on those 20" wheels. I'd like to be able to pedal up to about 27mph. The A2B uses a 4 bolt ring though, so it looks like I'd have to change the whole crank.

What's the biggest you can get in a 4 bolt pattern?