700c vs 26 inch

[frank9755]

Most electric bikes have 26-inch wheels. A few, such as the Ezee Torq and Kalkhoff range have 700c. The main reason for the prevelance of 26 inches seems to be that that is what is common on bikes in China. In the UK, these wheels are commonly found on mountain bikes (both genuine off-road and wannabe-type), while road /racing bikes, touring bikes and urban/city/hybrid bikes tend to have 700c.

Which size is better for electric bikes?

I have to declare my bias and say that I have long preferred the larger-sized wheels and would always choose them given an option. I've always felt they offer a smoother ride - as the bigger wheel is better able to bridge across small holes in the road - but had never thought about the performance difference.

I came across an article recently, written by a tandem manufacturer who makes bikes with both size wheels, which tried to analyse the way the wheels behaved and therefore which had which performance advantages. The conclusions he came to were that, while 26 inch wheels were stronger and also lighter, 700c offered not only a smoother ride but also higher speeds, of around 1 mph, for the same level of effort. The main reason seems to be that the bit of the tyre that is in contact with the road, while being the same size for any given tyre pressure, is a different shape - being longer and thinner for the larger wheel but more circular for the smaller wheel. The circular contact spot leads to significantly more tyre deformation and hence rolling resistance.

The article is here.

Thinking about what this means for electric bikes, 1mph additional speed for the same level of rider effort could translate into significantly lower demands on the battery and hence a marked improvement in range for no extra battery cost! Perhaps the larger wheels is another factor which helps to explain the very good ranges which Kalkhoff riders are reporting from what are only modest-sized batteries?

That's not all. The 11% wider contact which the 26 inch wheel makes with the road might be expected to result in 11% more punctures!

Frank

 

[flecc]

The 28" wheel that's quite common on the continent was also common here in the 1920s and before, but the 26" wheel became the dominant one in Britain on all classes of bike from the mid 1930s on though I've no idea why that was. The 700c was quite late appearing, well after WW2, and I don't remember seeing one in the first half of the 1950s. I have the impression that the 700c tended to grow at the same time as derailleurs, from the very late 1950s onward.

The contact patch difference is also commented on by tyre manufacturer Schwalbe, but they draw exactly the opposite conclusion!

It seems science depends on what's being sold.
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[Andy Day]

I agree with you completely Frank.

A 700c rim with a slim tyre has a lot less rolling resistance than a broad 26 inch tyre. Plus it's lighter, and and adding lightness means speed! Plus of course less expenditure of energy. There is also the energy required to accelerate the periphery of a heavy rim and tyre every time you increase speed. All these small increments in weight here and there kill performance.

Additionally the slightly larger diameter of a 700c rim raises the gear ratio, so increases the speed but may reduce hill climbing ability, but not in my limited experience of ebikes.

And a rant about modern bikes. I went down to Halfords today. Saw a bike called a Gryphon, £449, slim wheels and tyres, aluminium frame, looked really good. Felt the weight, it's heavier than my steel ( Renolds 501 tubed) 80s Falcon Camargue sports bike that I paid £40 for last year! Perhaps the alloy was solid?

 

[flecc]

It's not so cut and dried, like has to be compared with like.

A 26" wheel with a 1.25" smooth centre tread tyre has a lot less rolling resistance than a Torq with it's 1.95" Kenda tyres on the 700c size, again not comparing like with like so just as unfair.

If the correct tyre width relationship to diameter is used to give proper comparison, the difference is very slight, just fractionally in favour of the 700c due solely to the larger diameter being marginally more efficient on irregular road surfaces.
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[Miles]

Which size is better for electric bikes?

The smaller, the better, from the point of view of the motor

Miles

 

[flecc]

The smaller, the better, from the point of view of the motor

Miles

Indeed, it's 20" wheels a winning factor with my Q bike. Matches standard production e-bikes on speed and thrashes any of them on hill climbing.
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[frank9755]

Thanks for the comments. To elaborate on a couple of the points:

Depends on what is being marketed
I agree it is important to understand where people are coming from, but the article I referenced was written by a guy who sells both wheel sizes, so I don't think there is a commercial agenda there.

Comparing like with like
I also agree that no valid comparison can be made if other factors are different, so I am implicitly comparing 700c and 26" wheels with the same tyres. I think the Q-bike is a success for many reasons, with the size of wheel not being the main one (they are no smaller than the wheels on the original Quando!)

Smaller the better from the point of view of the motor
How so, Miles? Is that a gearing point? If so, the same impact can be achieved with a slower motor in a larger wheel. Or have I missed the point?

due solely to the larger diameter being marginally more efficient on irregular road surfaces.
This is undeniably a factor, but the main argument made by my tandem man was about the greater rolling resistance of a tyre (the same type and width) on 26 inch wheels - due to it having to deform more on making contact with the road. Conceptually I believe this has to have some impact, but don't know enough about the deformation of rubber to attempt to quantify it. Also the difference in size of the wheels - at 11% - is more than marginal.

There was another interesting conclusion he drew from the shape of this contact patch regarding handling - firstly that the wider footprint would make a 26 inch wheel more stable at lower speeds but less so at higher speeds. The related point about 26-inch wheels being more susceptible to punctures was from an article in this months CTC magazine.

the 26" wheel became the dominant one in Britain on all classes of bike from the mid 1930s
This was interesting. I had always thought of the 26-inch wheel as an American size and thought it might have come over with the GIs in the war. I know Raleigh 3-speeds were quite common in the US so perhaps it was an export product launched in the home market?
My perception nowadays (and I've not studied this) is that 700c wheels are far more common on most types of bikes, but that 26 inch is ubiquitous in two categories - mountain bikes and low-end cheapies (often styled to look like mountain bikes). I don't see much use of 26-inch outside these categories.

So what?
I can see that not everyone is convinced but, having read this article and combined with my own experience, I think the extra range which 700c wheels offer makes it hard to argue against them being the better choice for electrics. Even if the range advantage is tiny, it would still be worth having. Combined with fewer punctures in hard-to-remove wheels, it becomes a strong argument. If I were a designing an electric bike, I'd certainly put 700c wheels on it!

heavier than my steel
Not strictly to do with wheels but, Andy, I suspect that aluminium is mainly used as a marketing tactic. I had always understood that a quality steel alloy, such as Reynolds tubing, had a higher strength to weight ratio than aluminium. Of course if you can persuade someone to buy the aluminium frame, it is then easier to sell them the suspension forks they need because the frame gives such a harsh ride - but that is another point again!

 

[Miles]

Smaller the better from the point of view of the motor
How so, Miles? Is that a gearing point? If so, the same impact can be achieved with a slower motor in a larger wheel. Or have I missed the point?

A larger slower motor, sure.

I think the differences in rolling resistance between 26" and 700C sizes will be pretty marginal compared to other factors.

 

[frank9755]

A larger slower motor, sure.

I think the differences in rolling resistance between 26" and 700C sizes will be pretty marginal compared to other factors.

Miles, thanks for clarifying

Marginal rolling resistance + slight benefit from riding the bumps better + 11% fewer punctures ... starts to add up!

 

[john]

An very interesting article but the tests were on tandems where he says "internal tire friction is probably twice as critical". So on a single bike the difference could be expected to be about 0.5 mph.

Although I don't have any particular preference on wheel size, when I was looking at bikes to add a kit to it did occur to me that mountain bikes have stronger frames than road bikes. Since an electric bike can typically do more work than an non-electric, more miles and more weight carrying (including the electrics themselves) then this might be a better option.

 

[flecc]

Miles, thanks for clarifying

Marginal rolling resistance + slight benefit from riding the bumps better + 11% fewer punctures ... starts to add up!

I'd say very marginal lower rolling resistance. I don't understand where that 11% has come from. A 700c rim with a 28 mm wide tyre has an equivalent of a 26" wheel with a 30 mm tyre, approximating to 7% difference in diameter and tyre width.

Even accepting the premise of more punctures, that's 1 in 14 more, and for many cyclists who might only see around 14 punctures in very many cycling years, they're hardly likely to notice any difference.

The switch to 26" wheels had nothing to do with the USA to my knowledge, them being made by Raleigh, Hercules, BSA, Sunbeam and the like, all proud British manufacturers. The mountain bike wasn't involved in any way, the first of these arguably created in 1979 in the USA, almost half a century after our adoption of the 26" size. Even without the US example, our early mountain bikes would naturally have followed the standard wheel size on utility and earlier sports bikes.

Compactness might have been a factor in the gradual wheel size reduction. Britain supplied the world outside of Europe with bikes in those far off pre WW2 days, and the more compact the better for packing. Then as now, bikes were commonly packed with the front wheel off. Another factor is population stature. The 28" wheel remained popular and has done to this day in both Germany and the low countries where populations are famously often taller and notably long legged. Our home grown population in those days was generally shorter, and our exports were to countries where populations were generally much shorter as they are today, making 28" wheels an embarrassment.
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