Effect of bike weight on commuting time

Finbar Saunders

There's an interesting and light-hearted article in the British Medical Journal about one Doctor's experience of the effect of the weight of his bike on his commuting time. Abstract pasted below and the full article can be found here: http://www.bmj.com/content/341/bmj.c6801.full

Anyone else here interested in replicating his results next year? His study is an n=1 study so it could be interesting to see how others do.

Research
Christmas 2010: The Lives of Doctors
Bicycle weight and commuting time: randomised trial

J Groves, consultant in anaesthesia and intensive care

1Department of Anaesthetics, Chesterfield Royal Hospital, Calow, Chesterfield, UK

Objective To determine whether the author’s 20.9 lb (9.5 kg) carbon frame bicycle reduced commuting time compared with his 29.75 lb (13.5 kg) steel frame bicycle.

Design Randomised trial.

Setting Sheffield and Chesterfield, United Kingdom, between mid-January 2010 and mid-July 2010.

Participants One consultant in anaesthesia and intensive care.

Main outcome measure Total time to complete the 27 mile (43.5 kilometre) journey from Sheffield to Chesterfield Royal Hospital and back.

Results The total distance travelled on the steel frame bicycle during the study period was 809 miles (1302 km) and on the carbon frame bicycle was 711 miles (1144 km). The difference in the mean journey time between the steel and carbon bicycles was 00:00:32 (hr:min:sec; 95% CI –00:03:34 to 00:02:30; P=0.72).

Conclusions A lighter bicycle did not lead to a detectable difference in commuting time. Cyclists may find it more cost effective to reduce their own weight rather than to purchase a lighter bicycle.

ndru

This is utterly pointless, and getting a lighter, but less practical bike for anything except professional sport cycling is even more so. Is shaving off 5 minutes really worth the money? Bicycle is not only for commuting - it needs a rack to put your groceries in, you want it to be comfortable, keep you clean and so on.

Chiggy

20.9 lb Carbon bike!

Its 5 lb overweight!

My commute is ten minutes shorter ( 23 miles ) on my SWorks over my slicked up 33lb MTB.
The SWorks accelerates a damn sight quicker. Less weight,,, you see.

The Rookie

But acceleration only has an effect if you've slowed in the first place, I suspect on his journey there were few places were he was forced to slow and therefore subsequantly accelerate, once you are up to speed any acceleration (due to changing gradiant for example) is pretty slow anyway so the effect of mass minimal.

I suspect the bulk of your saving v the MTB is due to rolling resistance and the aero effects of the MTB tyres, plus a bit for gearing.

Downhill and coasting a heavier bike (with same aero drag) will roll faster.

Simon

gaz545

I find that RLJ saves you more time than losing weight. So those that don't want to stop eating the donuts, can risk their lives instead.

snailracer

Well here's a cycling calculator so you can estimate the effect of weight on any given journey:

http://bikecalculator.com/veloMetric.html

wgwarburton

ndru wrote:

This is utterly pointless, and getting a lighter, but less practical bike for anything except professional sport cycling is even more so. Is shaving off 5 minutes really worth the money? ....

I missed my train to work by under thirty seconds this morning.... and the route from home to the station is mostly uphill! :-)

Maybe it'd be worth spending a little bit more on my commuting bike(s). I don't think i've reached the point of diminishing returns quite yet.

Cheers,
W.

meanredspider

If nothing else, this BMJ article has been thought provoking - there's at least 5 threads discussing it.

The rolling resistance of tyres will definitely have a bigger effect assuming there aren't too many climbs. You don't get payback on the downhill sections as he argues because wind resistance saps more power (his own Vmax recording shows that).

As I read somewhere, there's no point buying a lighter bike until you can see your abs. That said, expensive bikes aren't only about weight.

Finbar Saunders

ndru wrote:

This is utterly pointless, and getting a lighter, but less practical bike for anything except professional sport cycling is even more so. Is shaving off 5 minutes really worth the money? Bicycle is not only for commuting - it needs a rack to put your groceries in, you want it to be comfortable, keep you clean and so on.

I wouldn't call it pointless. I think most people would assume a 4kg difference in weight would make more of a difference, but in this case, for a casual commuter it doesn't. The point is to highlight that you don't need a light bike, which I think is your point.

I'm not sure how you arrived at him buying a less practical bike. Did you read the article and look at the pics? Both are road bikes with drop bars, although one does have mudguards. And again in this case his bicycle is only for commuting. He bought a cheap £50 bike to try commuting, stuck to it and then used the cycle-to-work scheme for its intended purpose.

hfidgen

Option 1
Spend £2000 on carbon

Option 2
- Make sure you have a poo before setting out
- Don't drink so much beer

End result either way - you'll still miss your train because you got caught at a red light or pootled behind the fit lady roady admiring her lycra.

Chiggy

snailracer wrote:

Well here's a cycling calculator so you can estimate the effect of weight on any given journey:

http://bikecalculator.com/veloMetric.html

Hi Snailracer.

What that calculator is missing is what Gaz implied; How many times does the cyclist come to a stop?
A heavier bike accelerates slower for the same power application, so logic says the more stops the cyclist makes, the more seconds are lost.

turnerjohn

hfidgen wrote:

Option 1
Spend £2000 on carbon

Option 2
- Make sure you have a poo before setting out
- Don't drink so much beer

End result either way - you'll still miss your train because you got caught at a red light or pootled behind the fit lady roady admiring her lycra.

+1

redballoon

time is meant to be spent not saved

Pessable

I've been debating this point myself as my company is joining the C2W scheme in the New Year and I've got...

a. a shagged out hybrid bike; and
b. some money burning a hole in my pocket

My hybrid (with front suspension) is about 16kg plus about 4kg with my panniers to go to work. My weekend (cheap) alu road bike is about 11kg. During the week (well, 15 miles each way twice a week) I average about 15mph full laden on the hybrid. At the weekends on the same/ similar roads on the road bike I get about 17mph. I would expect the difference to be greater, especially since the drive-train on the hybrid is really worn out, the suspension is stuck fully on, I'm using a wheel/cassette from a different leisure bike, the chain is too short and can only use about 3 gears and even then intermittently. In fact necessity and curiosity have had me single-speeding for a couple of weeks. It's hard work but do-able and doesn't impact my average speed much. It does make me laugh when other cyclists comment on my Schwalbe Marathon Tyres being a bit heavy!

I think the novelty of a proper single-speed would soon wear off, but I've thought about an Alfine because the weight doesn't scare me. The reliabilty for year-round cycling does appeal. The way I look at it cycling a heavier bike during the week won't do my weekend performance any harm, and anyway I enjoy it so why would I want to make it last any less time?

snailracer

Chiggy wrote:

snailracer wrote:

Well here's a cycling calculator so you can estimate the effect of weight on any given journey:

http://bikecalculator.com/veloMetric.html

Hi Snailracer.

What that calculator is missing is what Gaz implied; How many times does the cyclist come to a stop?
A heavier bike accelerates slower for the same power application, so logic says the more stops the cyclist makes, the more seconds are lost.

If you assume a rider has fixed maximum power accelerating away from each stop, then yes, it will take longer to reach maximum speed. My rough calculations show that if a rider+carbon bike is 5% lighter than a heavier rider+bike, then at each stop the carbon rider will gain about 0.4s.
- however-
If you assume the rider expends the same amount of total energy for the journeys on each bike, then time is not lost. The extra kinetic energy that the rider needs to put into the heavier bike to get it up to speed is not "wasted", it is regained as the rider can ease off earlier before each stop i.e. the rider gains a longer rest coming up to a red light, so can ride slightly harder between lights.

In terms of bike acceleration seen in real life, fancy carbon bikes probably do not accelerate fast from a standstill because their gearing is race-oriented i.e. very high.

meanredspider

snailracer wrote:

Well here's a cycling calculator so you can estimate the effect of weight on any given journey:

http://bikecalculator.com/veloMetric.html

The bit I'm struggling with is the "Grade" section. My route is pretty undulating (about 1000ft of climbing & 1000ft of descent on average) but it's a round trip - so the grade is effectively zero. Despite what the good doctor says, elevation changes don't cancel themselves out because wind resistance is the cube of speed. A flat ride is always easier/faster than a hilly one.

snailracer

meanredspider wrote:

snailracer wrote:

Well here's a cycling calculator so you can estimate the effect of weight on any given journey:

http://bikecalculator.com/veloMetric.html

The bit I'm struggling with is the "Grade" section. My route is pretty undulating (about 1000ft of climbing & 1000ft of descent on average) but it's a round trip - so the grade is effectively zero. Despite what the good doctor says, elevation changes don't cancel themselves out because wind resistance is the cube of speed. A flat ride is always easier/faster than a hilly one.

I agree that elevation changes do not cancel out, the rule of thumb is that a lighter bike loses 1 second going downhill for every 3 seconds it gains going uphill. The point is, you won't gain much time even just going uphill.
Calculate the times for each one-way trip, then add the total time together for the round-trip. Repeat for each bike.

meanredspider

snailracer wrote:

I agree that elevation changes do not cancel out, the rule of thumb is that a lighter bike loses 1 second going downhill for every 3 seconds it gains going uphill. The point is, you won't gain much time even just going uphill.
Calculate the times for each one-way trip, then add the total time together for the round-trip. Repeat for each bike.

I actually know the time it takes me on my "8Kg" road bike versus my "11kg" MTB - I was just playing with the calculator to see how close it got but I found I was having to make very big changes in power to compensate for other things to get the times anything like realistic. What sort of power does a reasonably enthusiastic commuter generate? If it's important/helps, I'm 6' and about 90kg in my birthday suit and average about 18mph on my hilly Highland 30 mile round trip commute on the road bike.

snailracer

meanredspider wrote:

... What sort of power does a reasonably enthusiastic commuter generate? If it's important/helps, I'm 6' and about 90kg in my birthday suit and average about 18mph on my hilly Highland 30 mile round trip commute on the road bike.

125 watts?
I consider myself an "enthusiastic" commuter, however I am still rubbishly slow

meanredspider

To get something like the times I do, leaving grade at zero (which, as above, is a vast simplication), and wind at zero too (ditto) I need to put in 200W to get my times about right for both.