PDA

View Full Version : So how come...



KelOnWheels
06-04-2012, 08:06 AM
... (in a HIGHLY scientific experiment conducted this weekend on the streets of DC and fueled by very large chocolate milkshakes), does my friend's road bike coast downhill considerably faster than my MTB?

Is it the tires? (700Cx32 vs 26x1.5)

(Well, I just looked up rolling resistance and I think I answered my own question, but I was surprised at HOW MUCH FASTER he could go without touching the pedals :p )

DismalScientist
06-04-2012, 08:32 AM
Yes. Imagine how much faster it would be with 700 x 23 pumped up to 120 psi.

TwoWheelsDC
06-04-2012, 08:37 AM
I commute on a cross bike that is basically a slightly more upright road bike, but with wider knobby wheels, and my road bike is 2-3mph faster under just about every circumstance (even more so when climbing, due to weight), so I think that tires and seating position (road bikes generally give a more aero position) are the biggest factors...but depending on your friend's bike components, he/she could have less friction in the drivetrain. Also, the combined masses of each of you and your bikes will be a small factor, with the whole gravity thing and all.

KelOnWheels
06-04-2012, 08:39 AM
I was hoping the mighty tank-like weight of the MTB would magically overcome the resistance of the tires. Kinda like how I could pass people on downhills in my 1976 Pontiac Bonneville. :D

I guess that doesn't work when the mighty tank-like weight is mashing all that extra rubber into the road, huh?

Tim Kelley
06-04-2012, 08:53 AM
I was hoping the mighty tank-like weight of the MTB would magically overcome the resistance of the tires. Kinda like how I could pass people on downhills in my 1976 Pontiac Bonneville. :D?

I've noticed that the additional weight from a baby trailer does not make up for the extra wind and rolling resistance on lengthy downhills.

KelOnWheels
06-04-2012, 08:55 AM
I've noticed that the additional weight from a baby trailer does not make up for the extra wind and rolling resistance on lengthy downhills.

Darn babies, lazing around in their fancy trailers and not pulling their weight!

brendan
06-04-2012, 01:57 PM
Is the total weight of the other rider+bike significantly higher than yourself+bike? If so, then that may swamp all of the other "speedier" variables that you have on downhills and the other rider+bike will go faster.

http://www.newton.dep.anl.gov/askasci/eng99/eng99681.htm

Note: yes, in a vacuum, gravity accelerates different masses the same, but this is a balancing forces problem.

Brendan

KelOnWheels
06-04-2012, 02:59 PM
Nope, my bike's a good 15 pounds heavier... not counting the chocolate milkshakes. :D

If I'm riding in a vacuum I'll probably have other things to worry about than who can roll downhill faster ;)

vvill
06-04-2012, 03:59 PM
I'm going to guess it was a combination of momentum (how fast were each of you going before you started coasting?), aerodynamics/position (probably poor on a MTB) and rolling resistance (wide knobby tires).

mstone
06-04-2012, 06:30 PM
Less the size of the tire than the design, probably. Knobs are slow, it's like going up a little hill each time you hit one. :) Pick out a 42mm tire optimized for on-road and it'll be much faster.

off2ride
06-05-2012, 08:31 AM
The answers are endless on that one. I used to hear "How come my RX Hybrid gets less gas mileage than what I see online? Or "It gets less gas mileage than my neighbors RX".

KelOnWheels
06-05-2012, 08:54 AM
I blame the chocolate milkshakes. Next time I'll try not to spill half of mine on the table :P Clearly that is what slowed me down.

PotomacCyclist
06-05-2012, 01:17 PM
The upright riding position on a MTB/hybrid can slow you down quite a bit. When you sit up on a bike, the body surface area that hits the wind can be more than twice the area when riding in an aerodynamic position. The faster you go (i.e., downhill), the more that aerodynamics play a role. On slow and steep climbs, aerodynamics are much less important.

KelOnWheels
06-06-2012, 07:07 AM
All my climbs are slow ;)