Author Topic: Brompton [In]Efficiency  (Read 7374 times)

Brompton [In]Efficiency
« on: 13 January, 2017, 06:25:54 pm »
So I was reading the "Just Bought a Brompton.... " thread below, and noticed a few mentioned the Brompton was rather inefficient.  I was wondering if anyone else has measured for, and/or identified reason of, these inefficiencies?

My seat-of-the-pants estimate originally put my M6R pace ~15% slower than my CX/gravel bike (30 spd, 5 lbs lighter, tires of identical inflation and cross section width/height).  So I was surprised to find that a few runs with a GPS on my usually exercise loops put that differential at half that (~7%, or ~1 mph).  Not a bad sacrifice for the folding advantages, I think.

I'm guessing rolling resistance is pretty even between my two samples and I saw a driveline test putting the SA internal hub as efficient as a Shimano derailleur.  I calculate the 5 lb weight differential to only account for ~1% on an average ride (equal parts uphill/downhill/flat).  Clipless/30 spd vs half clips/6 spd may be worth a little bit too.

I think this leaves aerodynamic drag as my big difference, and so rigged an aero drop bar position for my M bars.  Unfortunately, it was too late in the season for objective testing, but it feels significant based upon my ability to hold a higher gear longer.  Some very dirty calculations (based on a wind tunnel article) may explain ~4% of my 7% differential.  I'll need to wait til Spring for more objective side-by-side testing, but I'd really love to get within ~3% efficiency between these two bikes since they are my primary touring options. 

Anyone else measure/analyze theses inefficiencies? 

(Forgive me for the long and nerdy post, but it's winter, I've got cycling cabin fever, and just needed something to obsess over.)


LittleWheelsandBig

  • Whimsy Rider
Re: Brompton [In]Efficiency
« Reply #1 on: 13 January, 2017, 06:35:15 pm »
Rolling resistance isn't a wash between your bikes but some small tyres don't give up too much speed to their bigger cousins.
Wheel meet again, don't know where, don't know when...

Re: Brompton [In]Efficiency
« Reply #2 on: 13 January, 2017, 06:39:48 pm »
I assume that smaller wheels have slightly more rolling resistance, probably due to distortion of the tyre the occurs because the contact patch is shorter.

If it were the other way round, competitive cyclist on track or road would surely have smaller wheels.

There may be some loss of efficiency due to the fact that the gearing has to have a larger gear ratio to get the same overall distance / turn of the wheels.

Rider position may be more compromised on a folder.

I have also found little difference in speed between a Brompton and the other bike, but that is full suspension mountain bike (with road tyres)
Quote from: Kim
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Kim

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Re: Brompton [In]Efficiency
« Reply #3 on: 13 January, 2017, 06:44:51 pm »
It's not the gearing.  My Brompton is rubbish at freewheeling, too.  How much of that is made up of:

a) aerodynamic drag
b) tyre rolling resistance (anecdotally I find small wheels are much more sensitive to the type of road surface)
c) other linear losses (eg. the underwhelming Shimano dynamo hub)

I'll leave as an exercise for the reader.

FS mountain bike sounds about right as a comparison.

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton [In]Efficiency
« Reply #4 on: 13 January, 2017, 06:52:12 pm »
Small tyres are more sensitive to casing flexibility and road surface texture and roughness.
Wheel meet again, don't know where, don't know when...

hellymedic

  • Just do it!
Re: Brompton [In]Efficiency
« Reply #5 on: 13 January, 2017, 06:54:59 pm »
I never had a dynohub on my Brompton so that didn't apply to me.

Aerodynamic drag is a factor.
Small wheels do seem less efficient. I suspect energy is wasted dropping and climbing out of road surface irregularities over which lager wheels will ride.
I suspect hubs have more friction (or possibly the same friction per wheel revolution but more rotations are needed.)

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton [In]Efficiency
« Reply #6 on: 13 January, 2017, 06:57:59 pm »
Moulton suspension helps reduce rolling resistance but Brompton front wheels can only rely on pneumatic suspension, another reason for flexible sidewalls and lower pressures than might otherwise be the case. Aero drag is the biggest factor for most folk.
Wheel meet again, don't know where, don't know when...

Re: Brompton [In]Efficiency
« Reply #7 on: 13 January, 2017, 08:33:44 pm »
I assume that smaller wheels have slightly more rolling resistance, probably due to distortion of the tyre the occurs because the contact patch is shorter.

If it were the other way round, competitive cyclist on track or road would surely have smaller wheels....

As I understand it small-wheeled Moultons were banned from racing by the UCI for having an unfair advantage (primarily allowing for closer drafting), but are still used in time trial and ultra-distance events.  IIRC, a small wheel Moulton still holds the faired/upright bicycle land speed record too. 

Anyways, whether smaller tires increase rolling resistance, or short spokes reduce aerodynamic wheel churn, I think the wheel size issues are relatively immaterial in the scheme of things - and even less than the difference weight seems to make.

I'm thinking the vast majority of the difference to simply be this:

.....Aero drag is the biggest factor for most folk.

And it is an easy fix.

Kim

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Re: Brompton [In]Efficiency
« Reply #8 on: 13 January, 2017, 08:43:53 pm »
Wheel size is however a major part of the Brompton experience.  Good acceleration and nippy handling come into their own as soon as the traffic builds up a bit - around the same time that overall efficiency goes out the window.  In its intended use, the main disadvantage of small wheels is that hitting a non-trivial pothole is likely to end in disaster.

Phil W

Re: Brompton [In]Efficiency
« Reply #9 on: 14 January, 2017, 04:18:57 pm »
Smaller wheels are generally  lighter and accelerate faster and are more aero.  You can now get the Schwalbe Pro One Tubeless in 406 (20") size, haven't seen tubeless for 16" yet.  If you fitted the same make and model of tyre to your race bike as your Brompton that would be an interesting comparison where it came to rolling resistance.

You'll certainly notice the more upright position of a Brompton in a headwind and it definitely climbs slower than my road bike. Around town though, they are great for stop / start and being nimble.

Kim

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Re: Brompton [In]Efficiency
« Reply #10 on: 14 January, 2017, 05:18:35 pm »
Climbing is the one thing where I find the Brompton doesn't lose out (compared to my DF bikes), in spite of it bending like a very bendy thing.  Similar all-up weight, similar gear ratio[1], same engine.  All the other inefficiencies are lost in the noise at climbing speeds.

When it's warmer and my ankle's fixed, I'm tempted to try some of those Schwalbe Ones on The Red Baron, which could do with some new tyres at some point.


[1] Not stock Brompton gearing.

rogerzilla

  • When n+1 gets out of hand
Re: Brompton [In]Efficiency
« Reply #11 on: 14 January, 2017, 05:41:50 pm »
I was going to do the 2010 Dunwich Dynamo on a Brompton but, on the afternoon of the ride, I did a rolldown test to compare it to my touring bike.  I didn't take the Brompton.  That was with Kojak folding tyres, too.

The Brompton is fine for long rides at a modest pace, but not if you are going to completely exhaust yourself on a normal bike.
Hard work sometimes pays off in the end, but laziness ALWAYS pays off NOW.

Re: Brompton [In]Efficiency
« Reply #12 on: 14 January, 2017, 06:49:37 pm »
I was going to do the 2010 Dunwich Dynamo on a Brompton but, on the afternoon of the ride, I did a rolldown test to compare it to my touring bike.  I didn't take the Brompton.  That was with Kojak folding tyres, too.

The Brompton is fine for long rides at a modest pace, but not if you are going to completely exhaust yourself on a normal bike.

Curious what you'd estimate the peak and avg speeds were on your roll-down test, and if there was any difference in riding positions.  This sounds like a good test I'd like to try. 


zigzag

  • unfuckwithable
Re: Brompton [In]Efficiency
« Reply #13 on: 15 January, 2017, 12:22:17 am »
in my view there are energy losses in the chain tensioner (about the same as in a rear derailleur), internal gear hub (if there is one), rear suspension block, average quality chainset, flexy frame/handlebar assembly, flat folding pedal, small size wheels/tyres, upright position. these all add up to quite a bit, if compared to an equivalent full size hybrid bike.
once i finish building my 20" wheeled folding bike, i'll put a power meter on and do a few test runs on my local loop to see how it compares to racing bikes.

rogerzilla

  • When n+1 gets out of hand
Re: Brompton [In]Efficiency
« Reply #14 on: 15 January, 2017, 09:41:39 am »
It's mostly about the wheel size on rough surfaces, although my rolldown test was on quite smooth tarmac.  Peak speed probably no more than 12mph in the test.  The M-type is very aerodynamically challenged at higher speeds; you can get a better position with the S-type.  The chain tensioner and small sprocket make the drivetrain a lot stickier than a full size bike with an SA hub and horizontal dtopouts for chain tensioning.
Hard work sometimes pays off in the end, but laziness ALWAYS pays off NOW.

LEE

  • "Shut Up Jens" - Legs.
Re: Brompton [In]Efficiency
« Reply #15 on: 15 January, 2017, 05:47:31 pm »
Strava does a reasonable job of estimating power.

In the absence of a real power-meter take your Brompton around a known circuit as fast as you can and then do the same on another bike.

You should get some idea of efficicency when you look at the times and power output required to achieve them.
Some people say I'm self-obsessed but that's enough about them.

Kim

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Re: Brompton [In]Efficiency
« Reply #16 on: 15 January, 2017, 05:50:25 pm »
Strava does a reasonable job of estimating power.

*giggle*


Quote
In the absence of a real power-meter take your Brompton around a known circuit as fast as you can and then do the same on another bike.

So I go round slower on the less-efficient Brompton, and naturally Strava infers I'm putting out less power...

pdm

  • Sheffield hills? Nah... Just potholes.
Re: Brompton [In]Efficiency
« Reply #17 on: 15 January, 2017, 08:50:32 pm »
I guess what he means is:
All things being "equal"...
If the strava power indication for a circuit averages, say, 200W on one bike and 180W on another for the same effort, you are losing 20W or about 10%. You would need quite a few samples to achieve some significance, though.
I suspect you could use equal average heart rate measurements as a gauge of effort equality?


Kim

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Re: Brompton [In]Efficiency
« Reply #18 on: 15 January, 2017, 09:01:49 pm »
I guess what he means is:
All things being "equal"...
If the strava power indication for a circuit averages, say, 200W on one bike and 180W on another for the same effort, you are losing 20W or about 10%. You would need quite a few samples to achieve some significance, though.
I suspect you could use equal average heart rate measurements as a gauge of effort equality?

A fair point.  I suppose it works if you can ride at a controlled effort level.  The roads around here are a bit too urban for that.

Re: Brompton [In]Efficiency
« Reply #19 on: 16 January, 2017, 03:01:28 am »
.....In the absence of a real power-meter take your Brompton around a known circuit as fast as you can and then do the same on another bike.

You should get some idea of efficicency when you look at the times and power output required to achieve them.

Which is what I've done, a few times... and was actually surprised the differential was only ~1 mph or 7% for me.  Think I might have fixed ~3% just by rigging an aero drop bar position - which also happens to be more comfortable, just by offering an alternative riding position.  Another ~1% is the 5 lb weight differential - but to be fair, the locks required on the CX bike to even remotely match the Brompton's security, would tip the scale the other way ;)

Anyways, if I've gotten the efficiency within ~3%... IMHO, I would consider the Brompton to be rather... "efficient" (well, for what it is anyway).

Re: Brompton [In]Efficiency
« Reply #20 on: 16 January, 2017, 01:22:39 pm »
When I replaced my greased NIG SA 5 sprinter with an old oiled non-NIG AW 3 speed (14T), my Brompton seemed to freewheel better/ be zippier. I have since fitted a15T sprocket.