Saturday, May 31, 2008

Thinking out loud.

An interesting thread on the randon Google group have given me cause to ponder bike weight. It's a topic which obsesses some people, who spend thousands shaving a as many grams as possible off their bikes. The group threw up some interesting answers - which were mainly for laden bikes including mudguards, racks, spares and in one case full water bottles - falling in a range between 11.4kg (25lbs) and 21kg (48lbs) with a cluster around the 16kg (36lb). It gave me cause to weigh the Surly. Ready for commuting/ century rides it comes in at just on 12.3kg (27lb). My long-distance audax load would bit a kilogram or two heavier.

Now I've been thinking about how to increase my speed on the bike. I manage to move along at a reasonable pace - somewhere around 22km/h (13.6mph) averaged over the course of a year - but of course I'd like to go a bit quicker. Is cutting the weight of my bike going to make any difference?

Happily, there is an excellent online tool for calculating just this. At my current (shameful) 103kg (227lb) travelling at 25.4km/h on the flat on a 12.3kg bike I appear to be putting out about 160 watts. It would take me about 3hours56seconds to ride a theoretical dead flat 100km with no wind. (Interestingly, my fastest ever metric century is 3hours47minutes.)

Now, if I bought myself a much lighter bike - say 7kg - I could expect to add a whopping 400m an hour to my speed and save about four minutes over 100km. Even the world's lightest bike, weighing in at 4.1kg top left, isn't going to help me go that much faster. Pretty though it is.

If I dropped my own weight to 90kg and bought a lighter bike I might I'd add 1.3km/h to my average speed and cut about 12 minutes off my time. If I just dropped the weight and stayed with my current bike the speed increase would be 0.7km/h or about six and a half minutes. Ish. Maths isn't my strong suit after a couple of beers.

Hills, of course are a different matter. On one of my commuting routes, there's a 10km climb with a grade of about six per cent. On my current bike I shuffle up at about 10km/h. At the same power output, a 7kg bike is going to lift my speed to 10.4km/h, while dropping my weight to 90kg will see me hit a giddy 11.2km/h. Doing both will get me to 11.7kmh, cutting my 'climb time' from one hour to 51 minutes.

Obviously losing weight is going to help me go faster, and it's a good thing in itself. But spending big dollars on a light bike? That isn't going to help that much at all, so I won't be rushing out to buy something made of carbon fibre any time soon.

Grant, at the engaging Rider Redux is probably on to something. I need to harden up. The secret, perhaps, is obvious to all who've ever ridden a bike. If I you want to go faster, be prepared to pedal a bigger gear, faster. I might just give it a try!

2,378km so far this year.

3 comments:

Grant said...

You're too kind.

cheers

grant

Adrian said...

I think your onto something. I went for a 240km ride a few months ago with a multi time Paris-Breast-Paris finisher and we discussed the various philosophes of bike weight and speed and the many mistakes people make when they get into bikes. Basically, his advise was that people spend way too much saving a few grams on flash looking carbon bits when they should focus their attention on getting the best fitting shoes and saddle. These contact points are going to make a far bigger difference to your speed, comfort and overall perfornance. Also, some of the heavier stuff lasts longer and is more reliable. That said, I would'nt trade my 8kg new Madonne for the 11kg Trek 1200 I used to race with.

Surly Dave said...

I guess the best thing is to ride the bike you like the best and works best for you. If that's a 12kg monster like mine and it makes you happy then that's the best bike for you. Some of the lighter carbon bikes are just lovely to look at and I'm sure they ride beautifully too. If I was a much richer man ...