I decided to try and quantify the speed difference between mountain bike and road bike. TLDR; the road bike is approximately 13% faster (equivalently the mountain bike is 15% slower). If you are commuting, buy a cyclocross bike.
I rode two 10-mile time trials on the same course. Road bike time 29.10; MTB time 33.20. I suspect that I could have done a better time on the road bike, but my pacing error probably just cancels out my superior form from that day.
It is worth noting that I spent much of the time on the MTB with my hands in the centre of the bars either side of the stem in order to reduce my cross-section and therefore drag, so the time difference for a less-aggressive rider would be higher.
The obvious question is: what are the causes of all this speed difference?
- My clothing and helmet was the same.
- My shoes and pedals were different. I used very stiff CFRP-soled XC shoes and XTR SPD pedals/cleats on the MTB and stiff GFRP-soled road shoes and Speedplay Zero pedals/cleats on the road bike. I do not think that this difference amounts to anything.
- The MTB is about 2.5kg heavier; general opinion is that 1kg costs around 3 seconds over this course: 7.5s lost there.
- The tubes on the MTB frame are wide and blunt-fronted, optimised for strength with no consideration for aerodynamics; the narrower circular-section tubes on the road bike are less draggy.
- The MTB fork legs are large-diameter circular-section tubes, whereas the road bike fork legs are much-narrower teardrop-section tubes which have less aerodynamic drag.
- The tyres are very different: knobbly, enormous, tubeless and low-pressure on the MTB and smooth, narrow, tubed and high-pressure on the road bike. According to this article I am spending at least 40W rolling the MTB tyres. According to this chart I am spending up to 60W rolling the road tyres. It’s quite possible that the MTB has the edge on rolling resistance if there is any difference at all (cf corroborating anecdote below). The width and shape of the MTB tyres/wheels certainly have increased aerodynamic drag compared to the road bike.
- The handlebars are very different: wide, flat, and level with the saddle on the MTB and narrow, curved and below the saddle on the road bike. This puts your body in a very different position if your hands are in the usual places. This means the aerodynamic drag on the MTB is much higher.
If I had to rank the factors in order of importance and guess at the contribution each makes I would say they are:
- Rider position aerodynamics 75%
- Tyre/wheel aerodynamics 10%
- Fork leg shape aerodynamics 7%
- Frame shape aerodynamics 5%
- Weight 3%
Sadly it is beyond my resources & resourcefulness to separately measure these factors.
When I was a student I commuted on a cross-country bike: I had the choice of a set of 37mm slick tyres and a set of 57mm trail tyres and found that I got home significantly quicker on the knobbly trail tyres even when the route was tarmac all the way. This may be because the roads in Britain are fairly rubbish, so the lower-pressure large tyre’s improved ability to flex around disturbances in the road surface is a big advantage. For this reason I don’t have a problem believing that the total rolling resistance on the MTB may have actually been lower than that on the road bike.
Given all of this, the best commuting bike for the UK is probably a cyclocross bike of some sort. It would give you the rider position, fork shape, frame shape and weight characteristics of a road bike combined with the superior comfort, traction and rolling-resistance of using large tyres. For this time trial I would expect the road bike to beat a cyclocross bike but it should be quite close (probably 20-40 seconds), and of course you can put road wheels on the CX bike and eliminate the wheel-aerodynamics deficit if you wish.
I hope to be able to configure my road bike into a time-trial geometry complete with aerobars for next week’s final TT of the season. I am hoping the difference will be as-large or larger than the MTB to road-bike difference: the difference in position is certainly larger.