Comparing gears and the impact of wheel size

[quixoticgeek]

Been idly ogling the Genesis Croix de Fer 10 in the LBS, and I noticed that it's got an 11-34 on the back, and a 50/34 on the front. Running 700x35C tyres. I was surprised, as this struck me as quite high gearing.

I've come home and run the numbers through for this gearing. The lowest gear on this works out at 2.2m. (Highest is 9.9).

My current stead is a Brompton with an 8 speed hub, 20t on the back and 33 on the front, this gives me a smallest gear of 2.19m.

In this gear, with any luggage It's a struggle to get the Brompton up even the slightest of hills, and I end up pushing it up most. With the same fitness, would having a gear that is essentially the same, but on a bigger wheel make much difference? How much is this governed by wheel size, how much is down to the geometry? If I got the Croix de fer would I be likely to be pushing it up the same hills as I do the Brompton?

Please forgive my naïveté in this, road bikes are new to me.

J

J

[hatler]

Sheldon Brown often helps out here.

[quixoticgeek]

Sheldon Brown often helps out here.

Really? Does he? Do you want to read my question again?

J

[hatler]

The SB calculator takes into account different wheel sizes. I interpreted your post as being unsure about what difference the wheel size made to the gearing you experience. The SB site should be able to remove the wheel size variable.

If you're familiar with Sheldon Brown then apologies for diverting you off there.

[quixoticgeek]

The SB calculator takes into account different wheel sizes. I interpreted your post as being unsure about what difference the wheel size made to the gearing you experience. The SB site should be able to remove the wheel size variable.

No. I've done the maths, as mentioned in my original post. The gear size I give you takes into account the wheel size. A 2.2m gear on the cdf should be near as to the 2.19m on the Brompton. Mathematically. Turn the pedals one complete revolution, and the bike goes forward 2.2m.

So the theory says that bottom gear on the Brompton is the same as the Bottom gear on the cdf. So the amount of effort required should be the same. But I don't know what impact geometry and the wheel size here makes. Meaning I can't work out if I'll still be struggling up the hills on a cdf just as much as I do on the Brompton...

J

[ElyDave]

I would assume the "rolling resistance" argument will be along soon on the smaller vs larger wheels.

A 34/34 doesn't seem that high a gearing to me.  You could always fit yourself a triple though to get lower ranges - mine is a 52/39/30, but I've never used the granny ring (will eventually get moved to a recumbent where it might get some use)

[quixoticgeek]

I would assume the "rolling resistance" argument will be along soon on the smaller vs larger wheels.

I'm also wondering if the squidgy suspension block on the Brompton has an impact here as well?

A 34/34 doesn't seem that high a gearing to me.  You could always fit yourself a triple though to get lower ranges - mine is a 52/39/30, but I've never used the granny ring (will eventually get moved to a recumbent where it might get some use)

I come from a touring/hybrid/brompton background, so a 34/34 is quite high as bottom gears go. Fitting a triple is something I've pondered, but it would require changing the derailure, shifter, cranks, and chainset... which is not a cheap upgrade...

J

[Karla]

I don't own a brompton, but 'everyone knows' that they don't climb well, so a bike with a fullsize frame and wheels should handle much better and should be much easier going uphill.  34x34 is quite low on a roadgoing bike, so don't be put off by it.

[Kim]

Bromptons are certainly less efficient than road bikes or hybrids, and small wheels can be noticeably harder work on certain flavours of rough tarmac, but at the end of day gears are gears, and most bikes that aren't tourers or mountain bikes have gears that are IMHO far too high.  A 2.2m gear won't get you up hills (where the cosine of weight dominates over these other factors) any better on a road bike than it will on a Brompton.


(As for comparing gears, Gain Ratio is the One True Measurement, both for its intuitive elegance, and for the fact that it takes both wheel diameter *and* crank length into account.)

[Kim]

I don't own a brompton, but 'everyone knows' that they don't climb well, so a bike with a fullsize frame and wheels should handle much better and should be much easier going uphill.

I do own a Brompton, and find that while the handling's distinctly dodgy, it goes up hills about as well as any other bike of equivalent weight and gearing.  On a DF bike it's straightforward to have better weight and gearing than a Brommie, which is presumably the reason they 'don't climb well'.

Where the Brompton loses out is on the flat and particularly downhill, which at times is like cycling through very lumpy treacle.


(Assuming a tarmac road surface where small wheels aren't a problem for traction.)

[Highlander]

First off could I persuade you to quote gears in the accepted fashion ie wheel diameter in inches?

The gearing on most modern bikes is way too high, even these days. Very few people have the legs to turn 50x11 unless it's downhill with the wind but unless you're racing why bother.

But mainly you ask about comparisons of wheel sizes.

There is a huge difference between 700c wheels and the tiny 16" wheels on the Brompton.  Basically you have more rolling resistance to overcome on a smaller wheel.  Now, some folk reckon this is offset by the lighter weight of a small wheel.  It isn't.  Even on wheels weight is not that important, what matters is rolling resistance.  Ask any trike rider.   

Alex Moulton tried to address this by introducing suspension to improve the rolling efficiency of small wheels.  It works on the flat but once the road points upwards the power loses through the suspension come into play.  Moultons are well known for not being fast uphill. 

One of the reasons 29er MTB's are catching on fast is the big wheels roll better, but in the off road world it's complicated by the handling issue, some like the more responsive handling of smaller wheels.

So you will find an equivalent gear on the Genesis easier to get round.  Actually I'd say much easier.  And a 34x34 on a 700c bike ought to get you up anything, even loaded.

[Kim]

And a 34x34 on a 700c bike ought to get you up anything, even loaded.

Hmm.  That's 2.0 (or 26.8"@170mm).

Yeah, I could get up most things on that, but I'd prefer a couple of lower gears if there's a going to be a load or a risk that I might be having a bad knee day.

Pedalling style comes into it too; gears that force you to mash aren't good if you want to spin (and presumably vice-versa, but few off-the-shelf geared bikes have that problem).

[rogerzilla]

I gear my Brompton a little lower than the equivalent 700c bike with the same type of hub, going for a 56" middle gear rather than 60".  This feels about right to compensate for the extra rolling resistance; the cruising speed is 7% lower for a given rpm.  Likewise, bottom gear with an AM hub is 48" rather than 52".  Yes, I know it's fairly high but it gets me over most things.

[Cudzoziemiec]

Riding style too, as distinct from pedalling. If you want to get up hills faster or become a stronger climber, higher bottom gears will help you in that. If you're riding with people who all use granny gears, it forces you to either grind uncomfortably or wait at the top of the hill for them. All of which is assuming no load beyond a saddle and/or bar bag.

[cycleman]

It sounds quite low enough for a light racing bike. On a tourer I would like a lower gear 

[Kim]

Riding style too, as distinct from pedalling. If you want to get up hills faster or become a stronger climber, higher bottom gears will help you in that.

That's a fair point, though I reckon such shenanigans is best kept to the deliberate masochism of single-speed / fixed.

[Somnolent]

It sounds quite low enough for a light racing bike. On a tourer I would like a lower gear 

But the Croix de Fer is neither.

[mzjo]

I would avoid metres development as a measure in gear calculators (unless you really want to pass the time of day with a frenchman). The reason is that calculators all tend to work only to one or two places of decimals, with the disadvantage that regardless of tyre size they all turn up with 2.1m as the circumference (whereas in inches there is a difference because two decimal places of an inch is a lot smaller than two decimal places of a metre). Even a lot of modern french cyclists don't understand metres développement and prefer to talk in terms of "braquet" (which is funny when they don't understand the difference between 700c and 650c - cf Bernard Thevenet a few years ago when Virenque had a tt bike with 650 wheels for a hilly tt stage in the tour).

Contrary to Kim my (limited) experience of a Brompton was that the frame was not at all suited to going uphill, at least with me on it, but pootled along very nicely on the flat. It felt ideally suited for a bloke in a bowler going between train and office! The only point at which the tyres felt really at a disadvantage was going up the Oxford canal towpath (the bit after the Sustrans route leaves it) - they don't like mud and negative camber. I think any "normally" shaped bike is going to climb more easily. A 1:1 ratio (30/30, 28/28, 34/34 etc) would normally have been considered as the biggest bottom gear that you could get away with on a touring bike (luggage but not necessarily camping) in the days before mountain bike gearing became common on tourers. My (elderly) french clubmates would still gear a tourer around that as a bottom gear. Lower would be nice - you might not need it but, as has been said before, you play a better hand of poker with two aces stuck under the table!

[Ningishzidda]

Gear length in inches is Ring/sprocket x TYRE diameter.

NOT what an online calculator says.

eg. a 26 x 1.125" Conti slick is 24.1 inches diameter, so 42 ring and 18 sprocket gives a 56" gear.

A 23-662 tyre is 26.2" diameter, so use this.

To find this figure, stretch a tape meaure across the tyre when its inflated, and then take off 2 x the tire drop you get.

Using 27" as the diameter to calc a 23-622, will give a higher figure than reality.

For development, use the height of the rear hub when you're sat on the bike, x 2 x pi. In metres.

[ElyDave]

Gear length in inches is Ring/sprocket x TYRE diameter.

NOT what an online calculator says.

eg. a 26 x 1.125" Conti slick is 24.1 inches diameter, so 42 ring and 18 sprocket gives a 56" gear.

A 23-662 tyre is 26.2" diameter, so use this.

To find this figure, stretch a tape meaure across the tyre when its inflated, and then take off 2 x the tire drop you get.

Using 27" as the diameter to calc a 23-622, will give a higher figure than reality.

For development, use the height of the rear hub when you're sat on the bike, x 2 x pi. In metres.

I'd nevee realised gear inches was such a nonsense measure. 

Surely the only rational measure is gear ratio (ring/sprocket) x tyre circumference (PixD) = how far per pedal rev

[Ningishzidda]

Crank length is merely a torque multipier.

[Ningishzidda]

Gear length in inches is Ring/sprocket x TYRE diameter.

NOT what an online calculator says.

eg. a 26 x 1.125" Conti slick is 24.1 inches diameter, so 42 ring and 18 sprocket gives a 56" gear.

A 23-662 tyre is 26.2" diameter, so use this.

To find this figure, stretch a tape meaure across the tyre when its inflated, and then take off 2 x the tire drop you get.

Using 27" as the diameter to calc a 23-622, will give a higher figure than reality.

For development, use the height of the rear hub when you're sat on the bike, x 2 x pi. In metres.

I'd nevee realised gear inches was such a nonsense measure. 

Surely the only rational measure is gear ratio (ring/sprocket) x tyre circumference (PixD) = how far per pedal rev

Gear length in inches harks from the days of the highbike ordinary, where you went to the shop and sat on several sizes. On those bikes, the size of bike you bought was not frame size, it was drive wheel diameter.
They were direct drive fixed , like a kiddy's tricycle, so it was 1:1 x wheel diameter.

The size of a highbike ordinary was close to the height of your sturnal notch. Mine is 56". The saddle has a bit of adjustment for fine-tuning. On more expensive bikes, the cranks had three pairs of holes to adjust pedal distance from the axle. That was the 'torque multiplier' on a direct drive 56" wheel ordinary.
When Starley and Bianchi introduced their 'Safety' bikes, the crank ring and rear sprocket could be sized so with a 26" diameter rear wheel, the bike could simulate a highbike ordinary to suit the size and strength of its rider.
Then when 'Safety' bikes started racing, higher gears than a 60" ordinary were built. Then Sturmey and Archer introduced a multi ratio hub gearbox. Then Campagnolo et al, gave us the derailleur.
Then Shimano copied the lot, not that fishing tackle was going out of fashion. They were just greedy.

If you go into a LBS and say your bike has a 24" low, everyone will know what you're talking about. If you state the development value of your lowest gear, they will shout "Bonjour monsieur!"

[ElyDave]

well I did start logging training distances in km a few years ago - pass the crozes hermitage

[Biggsy]

I haven't noticed it being harder to climb on a Brompton when aproprately geared, but that might just be me not noticing it as I rarely try as hard as I can.  Stiffness of the bike is a factor when climbing and accelerating.

[Highlander]

[
If you go into a LBS and say your bike has a 24" low, everyone will know what you're talking about. If you state the development value of your lowest gear, they will shout "Bonjour monsieur!"

Well there was a time when everyone knew what you meant by 108 top or 63 fixed but I'm exasperated by the number of cyclists I meet these days who have no idea.   

The continental tendency to express gears as "52 by 15" or whatever started becoming popular I think when the Tour de France began being televised over here but for crying out loud we have a perfectly good method, easy to understand, that has been used in Great Britain for generations why on earth do people have such difficulty with it?


Another factor involved in the small wheels/large wheels  issue is the lack of really good quality tyres in the smaller sizes.  The poorer rolling resistance of small wheels is exacerbated by the lack of quality tyres compared to the choice you have with 700c, or even 26".

Alex Moulton IIRC had to have a special tyre developed by Wolber to get the most out of his design.  I believe many of the current breed of small wheel bikes would fare much better if there was something similar available today but most of the tyres on offer assume that these bikes will be used in a "utility" fashion.