Question for anyone who owns/rides a folding bike with small wheels like a Brompton - can you realistically average 20mph+ on these things on flats? I seem to have read in places that you will spin out in the 20-21mph range, just curious, as I always thought it would be fun to own one of these things.
I guess you could always modify the drivetrain of the Brompton with a larger cranksize, but I think there’s a limit on how big you can go up front.
Watched that video, unfortunately doesn’t give any info as to how realistic it is to ride 20mph without spinning out like a madman in a non-race situation.
It’s hard to say from a quick glance since Brompton doesn’t appear to actually list their gearing specifications, but purported gear charts for their options seem to suggest that the gearing would not be a significant limiting factor for cruising at ~20mph. At least not in the 6-speed options.
The standard 6-speed supposedly reaches about 100 gear inches in its top gear, which is about what you get on a 25mm 700c road tire in a 53-14 gear ratio. That’s around 25mph at 85rpm, and it’s a bit higher than the gearing that Eddy Merckx used to ride 30.7 miles in one hour.
Question for anyone who owns/rides a folding bike with small wheels like a Brompton - can you realistically average 20mph+ on these things on flats? I seem to have read in places that you will spin out in the 20-21mph range, just curious, as I always thought it would be fun to own one of these things.
I guess you could always modify the drivetrain of the Brompton with a larger cranksize, but I think there’s a limit on how big you can go up front.
It is counter intuitive by a shorter crank is effectively a higher gear !!! So 150 mm cranks are way larger gearing than 200mm cranks!!!
To your question, you want a massive pizza plate large chainring in the front but it is doable. Massive large chainring, tiny rear smallest cassette and shortest cranks and you can get to a big number of gear inches on small wheels.
Just my impressions here - I used to own a dahon folding bike with small wheels (not the tiniest). Never rode it with any kind of bike computer/gps so I don’t have any numbers but it just wasn’t fast on the flats, or the downhills. I don’t know when it started to spin out but even compared to other commuter bikes I’ve ridden, it felt slow there.
Good little climbing bike, though. Very responsive.
I have commuted on a Brompton for years with the standard Sturmey Archer 3-speed internal. The front ring is a 50 but you can go up to a 54 with the rear sprocket normally being either 13 or 16 (a 6-speed Brompton is the same 3-speed internal but with both the 13 and 16 external sprockets). The hardest gear combo Brompton will sell as standard is a 54 x 12. In terms of gear inches this isn’t bad but you are only turning a 16 inch wheel at back which limits the top speed. 20mph is doable but that would be like 100 rpm on a standard setup.
There are a couple of other factors that go against you in terms of trying to average 20 mph on a Brompton. The default rider position is not good for pedalling efficiency. You really need to customize the bars or spend a lot of time on a Brompton to be able to put out big power. The default position is also extraordinarily upright and un-aero so you need to be generating a lot more power than you might think to go 20mph. Finally the weight distribution is unlike any other bike you will ride making for a twitchy ride further complicating your ability to get aero and/or put down big power. The thing to remember is that Brompton’s only come in one size and while the seat post goes up and down their is no adjustability in the bars. A lot of how it rides and feels is dependent on your personal physiology because the frame doesn’t scale up or down with changes in rider size.
I used a folding bike for work in London for a few years. I wasn’t too worried about top speed, rather intervals between the lights. But you notice other bikes and there were a few that look proper fast to me. Airnimal Chameleon and Tern Verge I think both use regular groupsets so give you lots of options.
…Massive large chainring, tiny rear smallest cassette and shortest cranks and you can get to a big number of gear inches on small wheels.
The cranks do not impact “gear inches”
Gear inches are simply a way to combine gear ratio and wheel circumference (in an intuitively flawed way and using the wrong units ;p). Cranks play no part in this.
…In terms of gear inches this isn’t bad but you are only turning a 16 inch wheel at back which limits the top speed. 20mph is doable but that would be like 100 rpm on a standard setup…
Gear inches already factor in the wheel diameter. I’m pretty sure “gear inches” is defined as:
wheel diameter * gear ratio
In which case you just multiply that by pi to get rollling distance per revolution of the chainring. Thus rpm required for a given speed will be the same on bikes with the same gear inches regardless of the wheel diameter. I suspect rolling resistance may differ quite a bit due to wheel diameter, but that’s a different matter.
ETA: Actually if wheel diameter doesn’t factor in the specific tyre there’ll be a fair amount of error in this but I’m not sure of the convention for choosing the “wheel size” figure. Do you use the actual diameter including tyre used, or just the nominal diameter. I think the concept is pointless and never use it, so not sure what the convention is.
You are right and I was messy with my text. Gear inches does include wheel size which is why big gears don’t mean fast speeds. You already have a 54x12 but the wheel size means that is only an ok gear. Even if you put 60+ tooth front ring on you are limited by the rear wheel. I never played around with rear cog size but you can’t go much smaller than 12 because of the hub diameter.
…Massive large chainring, tiny rear smallest cassette and shortest cranks and you can get to a big number of gear inches on small wheels.
The cranks do not impact “gear inches”
Gear inches are simply a way to combine gear ratio and wheel circumference (in an intuitively flawed way and using the wrong units ;p). Cranks play no part in this.
Crank size is part of the lever that turns Fxdxrpm = power
crank size, then large ring, then rear casette than wheel size all work to convert pedal force to distance on the road.
A longer crank is like using an easier gear at the same RPM (but usually people cannot keep up their RPM), so they go to longer crank and RPM drops
…Massive large chainring, tiny rear smallest cassette and shortest cranks and you can get to a big number of gear inches on small wheels.
The cranks do not impact “gear inches”
Gear inches are simply a way to combine gear ratio and wheel circumference (in an intuitively flawed way and using the wrong units ;p). Cranks play no part in this.
Crank size is part of the lever that turns Fxdxrpm = power
crank size, then large ring, then rear casette than wheel size all work to convert pedal force to distance on the road.
A longer crank is like using an easier gear at the same RPM (but usually people cannot keep up their RPM), so they go to longer crank and RPM drops
I understand the mechanics, however gearing specifically relates to the ratio of revolutions one shaft produces in the other, and “gear inches” is an odd derivative of this which attempts to incorporate the impact of wheel size in a single number. Neither are dependent on rpm, cranks, power, or pedal force. Power is a product of torque and angular velocity, Torque is a product of crank length and pedal force, but these are outside the scope of “gear inches”. My comment was simply that the cranks have no bearing on “gear inches”.
Question for anyone who owns/rides a folding bike with small wheels like a Brompton - can you realistically average 20mph+ on these things on flats? I seem to have read in places that you will spin out in the 20-21mph range, just curious, as I always thought it would be fun to own one of these things.
I guess you could always modify the drivetrain of the Brompton with a larger cranksize, but I think there’s a limit on how big you can go up front.
A friend of mine won the National 24 (as in “how far can you go in 24 hours?”) many years ago in the 35-39 age group (and second overall) on a folding bike, going 445 miles. That’s 19mph, but I’m pretty sure he managed to go well over 20mph for long stretches.
Question for anyone who owns/rides a folding bike with small wheels like a Brompton - can you realistically average 20mph+ on these things on flats? I seem to have read in places that you will spin out in the 20-21mph range, just curious, as I always thought it would be fun to own one of these things.
I guess you could always modify the drivetrain of the Brompton with a larger cranksize, but I think there’s a limit on how big you can go up front.
Simple physic
https://www.bikecalc.com/gear_speed
Using a 53 x 11 gearing… and 90 rpm (assuming you have no issues with watts)
700c = 55km/h
650c = 50.8km/h
17’’ = 34.3km/h
Depend what you call ‘fast’ and what 34.3km/h is for you 
Crank arm size will not change anything (except comfort)… but ring size will (only so many options if you want to go pass 53)
Simple physic
https://www.bikecalc.com/gear_speed
Using a 53 x 11 gearing… and 90 rpm (assuming you have no issues with watts)
700c = 55km/h
650c = 50.8km/h
17’’ = 34.3km/h
Depend what you call ‘fast’ and what 34.3km/h is for you
Crank arm size will not change anything (except comfort)… but ring size will (only so many options if you want to go pass 53)
Folding bikes often use unusual gearing mechanisms due to their small wheel circumference, low ground clearance, and need to avoid damage when stored folded. For example, the 6-speed Bromptons use a low-profile 2-speed rear derailleur and a 3-speed internal gear hub.
Thinking about what happens if you throw a road racing drivetrain on it doesn’t give an accurate picture of the possibilities.
Interesting…
Now you are down to a 50t x 13t…
https://www.bikefolded.com/brompton-gear-inches/
Interesting…
Now you are down to a 50t x 13t…
https://www.bikefolded.com/brompton-gear-inches/
My main point was that there’s also a hub gear in there which can multiply the gearing up, so it’s not as simple as (wheel diameter)*(chainring/cog).
Question for anyone who owns/rides a folding bike with small wheels like a Brompton - can you realistically average 20mph+ on these things on flats? I seem to have read in places that you will spin out in the 20-21mph range, just curious, as I always thought it would be fun to own one of these things.
I guess you could always modify the drivetrain of the Brompton with a larger cranksize, but I think there’s a limit on how big you can go up front.
Simple physic
https://www.bikecalc.com/gear_speed
Using a 53 x 11 gearing… and 90 rpm (assuming you have no issues with watts)
700c = 55km/h
650c = 50.8km/h
17’’ = 34.3km/h
Depend what you call ‘fast’ and what 34.3km/h is for you
Crank arm size will not change anything (except comfort)… but ring size will (only so many options if you want to go pass 53)
Per those calculations, any MOP triathlete would literally be spinning out the Brompton on even the slightest decline, where it’s easy to go 22+mph.