eMTBs why the big chainrings ?

Most I look at come with 34T or even 36T. The speed limiter is hit before you run out of gears, even on a lazy cadence.

Speed ranges at a cadence of 60 (calculator), with a 10-52 cassette, and a 29×2.4 tyre:

36T 3.6 – 18.8mph

34T 3.4 – 17.7

32T 3.2 – 16.7

30T 3.0 – 15.6

It seems to me there would be many advantages to fitting a normal 32T. You can use sensible cadence for control on slow technical climbs. Some of the motors like a high cadence so a smaller ring would help that too. You won’t wear out that aluminium cassette cog so soon. And it’s more feasible to pedal the bike if you’ve ran out of battery.

I guess it must be easier to fit everything in without the chain fouling the frame with a big ring, so on many frames you can’t actually fit anything smaller than the standard ring. And riders might like the ability to pedal at a lazy cadence.

I spun out pedaling at 32 mph this weekend on my MTB. Down a very big hill.

Gearing is 30 front with a 11-51 cassette.

I think you don’t need the lower gears due to electrical assistance

On a Bosch system I’m running 29″, 34T and 10-42 cassette. The 42 is very rarely used; a 10-36 range would be fine, so low gearing isn’t a requirement. The advantage of a bigger chainring is the ‘power’ gears that get the most abuse, 16T, 14T and 12T could be bigger and hence last longer. These three gears always wear out first. The 10T isn’t an issue because you are above the speed limiter.

Some countries they are limited to 32kmh, around 20mph, so maybe they just fit the same stuff.

The smaller cogs also don’t last long if you put a lot of power through them

I think you don’t need the lower gears due to electrical assistance

This is true but neither do you need the higher ones due to the limiter. It would be nice to be able to thread your way up a technical climb at slow speed without needing to pedal in slow motion.

The advantage of a bigger chainring is the ‘power’ gears that get the most abuse, 16T, 14T and 12T could be bigger and hence last longer.

Makes sense, although these would vary by riding location and style.

Even I don’t need lower.  It will climb anything at low speed in turbo nutter bastard setting on existing gears.  I think my lowest is 38 chainring 42 rear.  Pedal out in top downhill to 30 ish.

“The speed limiter is hit before you run out of gears, even on a lazy cadence.”

When you’re going downhill it doesn’t matter that you’re above the speed limiter and that’s when you use the bigger gears.

If you find yourself heading downhill in a group, on one of those unfortunate fireroad/road downhill moments (such a waste of elevation gained) then you’ll have noticed that the heavier riders tend to roll faster and the lighter ones slower. Swap onto an ebike and the same is true – they’re damned fast downhill, especially as the suspension works better too. Just because the motor is off, it doesn’t stop you pedalling to good effect!

Replies above seem to nail it. But gearing is very personal if you feel you’ll benefit from a smaller chain ring just fit one. I’ve probably adjusted the gearing to suit myself on every bike I’ve ever bought.

60rpm! RIP knees. When needed for tech then fair enough, but wow that’s low

[GeForceJunky]

@GeForceJunky

What chainring are you running?  I’m looking to get a 32t or smaller for Bosch and I am struggling to find any.

Rather than change the chain ring i changed the cassette on mine from a 52t down to a 44. Paired with a 34t chainring I’ve never had an issue climbing anything.

Thinking about it, i actually bent the alloy 52t ring slightly on the cassette the bike came with which was why I changed the cassette.

Our Turbo Levos came with SRAM 32 T chain rings.

The only compelling point I’ve read so far is to minimise cassette wear.

l if you feel you’ll benefit from a smaller chain ring just fit one

If the frame even allows it!

60rpm! RIP knees. When needed for tech then fair enough, but wow that’s low

Just picked a low number to illustrate that for top speed, there’s no need for a big ring.

I’m looking to get a 32t or smaller for Bosch and I am struggling to find any.

If it’s for 104BCD then iirc 32 is the smallest possible except Wolf Tooth do a special one that’s 30T. But it’s aluminum.

it’s not a ‘speed limiter’ as such.  It’s a motor cut-out.

If you’re doing 20+mph downhill, there’s nothing to stop you pedalling to get a bit more speed up, it’s just that the motor won’t help.

I could do with a faster gear on the Blue near me, quite often get past my comfortable cadence on 30/10 29 and 32/10 27.5 on my non eeb, around 18mph . And the 51/50 gearing is quite easy enough for me to climb everything i ride.

On an Ebike id certainly want a 34 or 36, if i could keep the speed past cut off

Just because the engine cuts out doesn’t mean you never want to go above 15mph.

I’m going to guess at a combination of:

While a 500%+ range is fairly pointless on an ebike, its cheaper for the OEM product managers to spec mtb 12 speed than it is to try to be clever, using high end gravel, or mtb 11 or even 10 speed groupsets. I know you want to reduce wear on your 50t alu cog but at the other end putting lots of use through the 10t is going to wear out even quicker.

Spec unchanged from North American market where speed limit is 20mph/they are basically expecting people to chip their bikes.

60rpm is low cadence by our (proper mtber) standards – although look at yesterday’s Olympic gold medalist, she is probably in that range – and possibly casuing you problems with your low speed tech climbs; but have you seen how half these things get ridden – dropper down, sat down pedalling at 30 rpm in turbo.

but have you seen how half these things get ridden – dropper down, sat down pedalling at 30 rpm in turbo.

This is definitely a thing I’ve noticed around trail centre stuff, and as previously mentioned a lot of people just stop pedalling as soon as they hit the cut off going downhill, which boils my piss when stuck behind them on the non electric bike

thecaptain
Just because the engine cuts out doesn’t mean you never want to go above 15mph.

This. I don’t even know what size chainring mine has, but when I’m pedalling fast downhill on a bashtrack or something, I keep pedalling even if the motor has cut out.

Wolftooth make rings in Stainless too…

The smaller cogs also don’t last long if you put a lot of power through them

Because motors will merrily churn out more torque constantly than feeble most human legs alone can. Put that same torque through a smaller sprocket and you’ll knacker it faster, more load over a smaller surface area basically.

The speed limiter is hit before you run out of gears

I do like how it keeps getting called a “limiter” the only thing limiting my non-eeeb is the tubby sack sat on top of it 😉

Because motors will merrily churn out more torque constantly than feeble most human legs alone can. Put that same torque through a smaller sprocket and you’ll knacker it faster, more load over a smaller surface area basically.

also remember that the torque is at the chain ring. that then becomes an axial force in the chain, then applied to the rear cog as a torque again. The force in the chain x the lever arm to the rear axle gives you that driving torque. The smaller the cogs the higher the tension in the chain.

I think in reality, only the first few teeth of the cog are actually under load at any given time (even if the chain is wrapped around half of it). But obviously it then, as the wheel rotates, it is constantly changing which of the teeth are engaged. So having more spreads the load over time.

I tried a aluminium Rotor 32T on my Bosch but it didn’t last long (6 months), second chain was noisy on it. Currently using a steel E13. There are very few 32T options. I’ll probably go up to a 36T when I next need to fit a new ring. I tend to do 1 chainring + 1 cassette and 3 chains over 2 years before replacing the lot.

I put a larger chainring on just to try and eek out more cassette / chain life after switching to Linkglide.  Running 38t 11-48 I think and very very rarely use the low cassette gears.

“The only compelling point I’ve read so far is to minimise cassette wear.”

Have you read any of the posts?!!

I’ve been riding an ebike for over five years, with a hardtail (now singlespeed), as my other bike. With gravity on your side (ie downhill) an ebike is faster than a normal bike. Almost every day I pedal my ebike past 30mph downhill. And many days (because I’m often late) I pedal it past the limiter on the flat.

And that same bike that I commute on gets used for lapping the steep local trails or taking to gnarlier trails further afield where I want more suspension than my hardtail.

If anything, with a motor you don’t need the lower gears – I hardly use them when I’ve got the power on but they’re nice to have when riding with people without motors or for when you run out of battery. I recently went from 11 to 10 speed LinkGlide to have a cheaper and lighter cassette.

To add a broader perspective to that, I was the first of my riding group to get an eMTB (I spotted the fun commuting opportunity) but now it’s mostly the gnarlier riders I know who’ve bought ebikes, for lapping the steep and/or jumpy stuff. I don’t do big jumps but even I’m pedalling past the limiter at times to hit jumps the size I ride.

Others I ride with do go large, and are taking their e-bikes down tracks like Full Moto at Dirt Farm, and they’re going so fast to clear the 40’ plus doubles. (When you’re riding the twisty bermy tracks alongside it sounds like a motorbike coming past!)

When you’re going downhill it doesn’t matter that you’re above the speed limiter and that’s when you use the bigger gears.

I rarely feel like pedalling faster once I’ve topped out my 30T chainring and 10T cog at high cadence on my normal bike, but of course that’s personal preference.

it’s not a ‘speed limiter’ as such.  It’s a motor cut-out.

Oops, wrong term. I know how it works.

60rpm is low cadence by our (proper mtber) standards – although look at yesterday’s Olympic gold medalist, she is probably in that range – and possibly casuing you problems with your low speed tech climbs; but have you seen how half these things get ridden – dropper down, sat down pedalling at 30 rpm in turbo.

Yeah the difficulty was having to pedal really slowly to thread your way through things, worked much better to point the eMTB in the right general direction and keep momentum up.

Wolftooth make rings in Stainless too…

Ah didn’t realise they did the special 30T in stainless, thanks.

“The only compelling point I’ve read so far is to minimise cassette wear.”

Have you read any of the posts?!!

Of course! I was saying that of the supposed advantages that were posted, none of them were things I cared about other than the wear aspect. We all use bikes in different ways etc.

“Of course! I was saying that of the supposed advantages that were posted, none of them were things I cared about other than the wear aspect.”

Your post suggested it was your view on how e-bikes should generally be, not your own specific use case.

“I rarely feel like pedalling faster once I’ve topped out my 30T chainring and 10T cog at high cadence on my normal bike”

A lot of e-bikes are running 27.5 rear wheels and 11t smallest sprockets. And when you’re going downhill fast on techy or jumpy stuff, you don’t want to be doing twiddly spinning, you want to be able to stomp a few hard strokes at critical moments.

On a Bosch system I’m running 29″, 34T and 10-42 cassette. The 42 is very rarely used; a 10-36 range would be fine, so low gearing isn’t a requirement.

Whilst I suspect you are more typical of newer riders coming to eMTB’s without much or any of an MTB history before eBikes, I personally couldn’t disagree more… I ride with the same high cadence style on eMTB as I do on MTB, and I want to get up any climb that presents itself, so first thing I did was ditch the 34T ring on my Orbea Wild for a 32T (which was the smallest I could find, nobody seems to do a 30T for a Bosch Gen 4, friends on Levo SL’s have got 30T rings on their bikes) and swapped the 10-51 12spd setup out for an 11-51 11spd setup… I still haven’t spun out on 32-11 yet (anything over the 25km/h assist limit and pumping for speed is more effective than pedalling an eBike with no assist usually!), but I do use the 32-51 bottom gear for steep climbs quite a lot.

But then I can sit at 90-95rpm cadence all day on a bike… Which is a LOT more efficient than stomping a low cadence style, if not quite as explosive!

There are very few 32T options

 I’m looking to get a 32t or smaller for Bosch and I am struggling to find any.

I had to get a Wolf Tooth specially imported (UK distributor Saddleback don’t keep them in stock, took about 5-6 weeks, no demand for them apparently, though will order them in as required)… Was worth the hassle though. Much better quality ring than the stock E13 too.

https://www.wolftoothcomponents.com/products/direct-mount-chainrings-bosch-e-bike-motor?variant=39966845042723

Quite the opposite, I’ve ridden MTBs for decades, but mostly ebikes since the 2018. Last year I was using an E8000 Shimano motor which was much better at high cadence, so used 32T with that and much more use of the 42T ring. The Bosch CX I find feels inefficient when spinning, it’s tuned for more like 70 than the 90 rpm the SX/Shimano motors are tuned for. Another option is use one of the many 104BCD spiders available and any old 104 chainring. Doesn’t look as clean though.

The Bosch CX I find feels inefficient when spinning, it’s tuned for more like 70 than the 90 rpm the SX/Shimano motors are tuned for.

Don’t confuse the level of assistance it gives with efficiency…

The CX motor is more efficient at higher cadences… All the motor systems are. But it does noticeably punch harder at 70rpm or below than the Shimano motors do for sure… The the EP8 I had previously, you didn’t really get any assistance at all below about 70rpm and were only getting anything meaningful in the 80-100rpm range.