It Doesn't matters, It's not the bike/kit you have...

[bpsmith]

So why buy a MTB if you want to make it ride like a Road bike?

Assuming that the half a watt cost per kg is correct, are we actually suggesting that somebody weighing an additional 90kg would actually be less than 5 mins slower over 30km all other things being equal?

If that’s true, I am amazed.

 

[mjr]

So why buy a MTB if you want to make it ride like a Road bike?

Maybe there's no space for more bikes, maybe they need the frame strength, maybe it's something else. Who cares?

 

[mjr]

I was looking at my bike today and thinking. It's pretty fancy. I've spent a fair bit on it over the years. Some of the expenditure is hidden, in that I've tried and given up on things, some is quite evident. Frame is Reynolds 725 tubing (which might mean something to someone, but not to me). Good rigid altus rack, Altura rack pack with spare clothing. Hand built wheels, have been rebuilt multiple times. The last in a series of a zillion saddles that I've tried. Some of the expenditure is on small things, like the outsize drinking bottle that I prefer, or the BTwin handlebar bag, that I just don't think can be bettered. Wider handlebars that I found way more comfortable. Lots of small bits of fanciness aimed at making long rides more comfortable and convenient.

And now I find I've been wasting my time and money on tuning my fancy bike to how I want it. I could have been riding a random MTB as it would make no difference at all. What a fool I've been.

Look on the bright side: all those creature comforts haven't made you any slower, at least not directly.

 

[Vapin' Joe]

All else being equal a lighter bike will be faster than it's heavier counterpart, unless the law of physics has changed since I last looked.

 

[Ajax Bay]

Assuming that the half a watt cost per kg is correct, are we actually suggesting that somebody weighing an additional 90kg would actually be less than 5 mins slower over 30km all other things being equal?
If that’s true, I am amazed.

"All other things being equal" (includes aero drag co-efficient, remember, which it wouldn't be unless the extra 90kg was a bloody great weight under the saddle) 6 minutes slower.

All else being equal a lighter bike will be faster than it's heavier counterpart, unless the law of physics has changed since I last looked.

Yes, but only a very, very small amount - less than 0.1kph per kilo at 30kph [Conditions: Same aero drag; same quality of tyres (or even the same tyres); "aren't going up hills"; same route/roads/weather; same power output.] Had you a particular Law of Physics in mind?

Just ride and enjoy

+1

 

[bpsmith]

"All other things being equal" (includes aero drag co-efficient, remember, which it wouldn';t be unless the extra 90kg was a bloody great weight under the saddle) 6 minutes slower

You’ve finally arrived at the point. All things cannot be equal when comparing a MTB with a Road bike, or when comparing different weights.

 

[Ajax Bay]

You’ve finally arrived at the point.

No, I've been there all along: the premise articulated in the OP which was pointing out that bike weight per se doesn't matter much (see conditions above).

 

[bpsmith]

No, I've been there all along: the premise articulated in the OP which was pointing out that bike weight per se doesn't matter much (see conditions above).

It's like pulling teeth. Bike weight is one argument. Suggesting a MTB and a Road bike are exactly the same is something very different.

 

[Ming the Merciless]

All else being equal a lighter bike will be faster than it's heavier counterpart, unless the law of physics has changed since I last looked.

Remind me about the conservation of momentum and which bike has more momentum all other things being equal.

 

[Ajax Bay]

All else being equal a lighter bike will be faster than it's heavier counterpart, unless the law of physics has changed since I last looked.

Remind me about the conservation of momentum and which bike has more momentum all other things being equal.

All other things being equal (including power input) they will both have exactly the same momentum. But the lighter one will have greater velocity.

OK, include power input. Both twins (one on a 7kg bike, one on 15kg bike, ceteris paribus) start from rest (zero momentum) and accelerate (same power) to 30kph (8.3m/s). Relative acceleration will be inversely proportional to mass (say 80kg v 88kg) and the time taken to get to 30kph will be proportional - shall we say the power each twin generates allows them to get to 30kph in 10 seconds and 11 seconds respectively. As an aside please note the rider of the heavier bike now has more momentum: 11% more. Each twin cycles for an hour at the same power that, with equal aero drag, etc, etc (only difference is mass) achieves about 30kph. 30km later the twin on the lighter bike has to wait for about 30 seconds for her brother (who weighs the same as her btw). The second or two extra taken to accelerate to 30kph is negligible.
HTH (and @bpsmith - go to a dentist that pulls teeth with an anaesthetic)

 

[derrick]

OK, include power input. Both twins (one on a 7kg bike, one on 15kg bike, ceteris paribus) start from rest (zero momentum) and accelerate (same power) to 30kph (8.3m/s). Relative acceleration will be inversely proportional to mass (say 80kg v 88kg) and the time taken to get to 30kph will be proportional - shall we say the power each twin generates allows them to get to 30kph in 10 seconds and 11 seconds respectively. As an aside please note the rider of the heavier bike now has more momentum: 11% more. Each twin cycles for an hour at the same power that, with equal aero drag, etc, etc (only difference is mass) achieves about 30kph. 30km later the twin on the lighter bike has to wait for about 30 seconds for her brother (who weighs the same as her btw). The second or two extra taken to accelerate to 30kph is negligible.
HTH (and @bpsmith - go to a dentist that pulls teeth with an anaesthetic)

This is all good under perfect conditions, But in the real world. I ride with a guy who rides a S5 when he is on that bike we can ride 100 mile+ and stay together, but he also has an old steel bike that he rides not very often (Thank F****) i will drop him in no time at all.

Edit. it does not matter if he has a dump before hand either.

 

[mjr]

This is all good under perfect conditions, But in the real world. I ride with a guy who rides a S5 when he is on that bike we can ride 100 mile+ and stay together, but he also has an old steel bike that he rides not very often (Thank F****) i will drop him in no time at all.

I'm confused by that because it seems like he's not a friend (is only called "a guy who rides") but it also seems like you're upset by dropping him. If he's a friend then it doesn't seem hard to ride a bit slower and if not who cares? It seems like a contradiction.

I also suspect the electric motor and area recumbent position on his C5 matters more than the weight of the bike.

 

[derrick]

I'm confused by that because it seems like he's not a friend (is only called "a guy who rides") but it also seems like you're upset by dropping him. If he's a friend then it doesn't seem hard to ride a bit slower and if not who cares? It seems like a contradiction.

I also suspect the electric motor and area recumbent position on his C5 matters more than the weight of the bike.

It does not seem to take much to confuse you.

 

[Ajax Bay]

Well I read the S5 as SS ie single speed, to start with but suspect that @derrick is relying on chatters (exempt @mjr ) recognising that an S5 is made of carbon (so a '7kg supperbike' example).

 

[Milkfloat]

I hope he wasn't riding his supperbike at dinner time. That would be a terrible faux pas.

Does that not depend on how he is dressed?