Mountain Bike Reviews Forum banner

1 - 19 of 19 Posts

·
Rock and/or Roll
Joined
·
489 Posts
Discussion Starter · #1 ·
It may be a silly question to some, but I have to ask...

I have a superlight carbon hardtail mtb and a steel frame road bike that have similar weights, but the road bike is just soooo much faster on my work commutes. I've even swapped skinny slicks onto the mtb, but it still take a ton of effort to get it rolling up to speed and maintain it, compared to the road bike. Does the aero postion really make that huge of a difference? The wheels don't even weigh that much different.
I can't imagine trying to push a 53t big ring on my mtb. So, is it the riding postion?
 

·
Content from my avatar
Joined
·
4,355 Posts
Mostly. Aero drag increases exponentially with speed.
 

·
mtbr noob
Joined
·
7 Posts
I presume the frame shape would make a difference as well, suspension forks on the mtb vs the thin streamlined fork and more streamlined road bike frame.
 

·
Registered
Joined
·
989 Posts
It may be a silly question to some, but I have to ask...

Does the aero postion really make that huge of a difference? The wheels don't even weigh that much different.
I can't imagine trying to push a 53t big ring on my mtb. So, is it the riding postion?
Yes, largely. The faster you go the more aero makes a diff, and it ramps up quickly. You can feel it by comparing sitting up straight and then tucking down at road speed. I put 1 inch slicks on my steel fully rigid and pumped up to 100 psi and itll go pretty darn fast, but need to tuck in.
Wheel diameter makes a little difference, but its mostly aero.
 

·
Registered
Joined
·
5,435 Posts
The main difference is down to aerodynamics.:)

"Aero drag is 80-90% of the overall resistance affecting a rider. Roughly 80% of this aero drag is the rider, the remainder is the bike. Aerodynamics are important!" Cervelo

http://www.cervelo.com/en/engineering/thinking-and-processes/aerodynamics.html

If you have a more upright riding position with wide bars and outstretched arms that adds a lot of drag. The faster you go the more drag this upright position creates.

"Aerodynamic drag increases as the square of velocity; if you double your speed, you will expend 4 times the energy to overcome drag. Expressed another way, for every increment of extra power you apply at the pedals, your increase in speed becomes progressively smaller. Eg, it requires 7.2 watts of power per mph to sustain 19mph on the flat; to sustain 30mph requires no less than 13.1 watts per mph!" Joe Beer

http://www.pponline.co.uk/encyc/cyc...g-will-improve-your-cycling-performance-41267

.
 

·
Registered
Joined
·
393 Posts
Ooooh you're trying to get me to get a road bike for my commutes.. Nicely done.

I have been thinking aboyt a "city bike" with roadie features, but if road bikes are actually that much faster, maybe I will just go full speed.
 

·
Professional Crastinator
Joined
·
6,120 Posts
Real life experiment on hilly 3 mile circuit for 15 miles:
MTB with 2.1 Ignitors on pavment - avg. speed = 15.6mph
MTB with 32mm slicks on road (same bike, same rims, same gears, same everything) = 16.5mph

Tires make a small difference.

Real life experiment on motorcycle, constant throttle position, full fairings:
Head up @ 60 mph
Head down (behind the fairing) @ 65mph

Check yourself on some climbs and there prob'ly won't be such a disparity.

-F
 

·
Premium Member
Joined
·
48,238 Posts
Real life experiment on hilly 3 mile circuit for 15 miles:
MTB with 2.1 Ignitors on pavment - avg. speed = 15.6mph
MTB with 32mm slicks on road (same bike, same rims, same gears, same everything) = 16.5mph

Tires make a small difference.

Real life experiment on motorcycle, constant throttle position, full fairings:
Head up @ 60 mph
Head down (behind the fairing) @ 65mph

Check yourself on some climbs and there prob'ly won't be such a disparity.

-F
The different tires changed your final gearing.
 

·
Ride Instigator
Joined
·
3,220 Posts
Road bikes are magic, they somehow work with your internal body mechanics to transform you into a lean, mean, pedlin' machine. I actually choose to wear bagge MTB attire when I ride my road bike to govern the speed because I get going scary fast if I wear aero, lycra duds. That's how I figure it anyway;).
 

·
Content from my avatar
Joined
·
4,355 Posts
Check yourself on some climbs and there prob'ly won't be such a disparity.

-F
On climbs, aero drag doesn't factor much (if at all) because you're going so slowly. The main difference-maker there is just weight. On flats and descents, though, a proper road bike will be faster than a mountain bike.
 

·
Professional Crastinator
Joined
·
6,120 Posts
The different tires changed your final gearing.
Yes, but not the motor.

I could ride that course in the big ring, 4th or 5th cog with the knobbies. With the road tires I could push 6th or maybe 7th.

-F
 

·
Registered
Joined
·
101 Posts
You'd be shocked what a difference your body position can make on power output.

Aero is obvious, but the position you're in on a road bike let's you get more power to the cranks.

Ever grind up a hill and notice you slide forward and hunch down to generate power? That's the spot you're always at on a roadie (steeper seat tube, higher seat, lower bars, generally speaking. Your glutes come more into play, plain and simple. )
 

·
Registered
Joined
·
1,069 Posts
Mostly. Aero drag increases exponentially with speed.
That!!

Ever ridden either bike on flat, even pavement with a strong tail wind? It feels great. If you match the wind speed, let's say 18 mph, then you get the feel for what effort is needed to just overcome the mechanical friction -- drivetrain and wheel-to-road. It's very little on a road bike and only a small bit more on a mountain bike due to the tires. Most of your energy is normally dissipated on aero drag. That's why most of road racing is a team sport where racers take turns being in the front and in the instances when it is not (time trials, triathlons) the bikes and bikers become extremely aero.
 

·
It's about showing up.
Joined
·
12,738 Posts
That!!

Ever ridden either bike on flat, even pavement with a strong tail wind? It feels great. If you match the wind speed, let's say 18 mph, then you get the feel for what effort is needed to just overcome the mechanical friction -- drivetrain and wheel-to-road. It's very little on a road bike and only a small bit more on a mountain bike due to the tires. Most of your energy is normally dissipated on aero drag. That's why most of road racing is a team sport where racers take turns being in the front and in the instances when it is not (time trials, triathlons) the bikes and bikers become extremely aero.

Excellent. People seem to miss how much more general resistance there is in a mtb than a road bike.
 

·
Registered
Joined
·
175 Posts
Combination of things really. Road bike=less rolling resistance, better aerodynamics, plus no suspension means minimal loss of power absorbed by the movement of the suspension.

I have no idea why people commute on mountain bikes quite honestly.
 

·
It's about showing up.
Joined
·
12,738 Posts
I gave worked Bike To Work events for years. In the last 5 I have noticed that late 80s rigid mtgs are one of the most ubiquitous commuters. Those bikes were set up with race geometry, flat bars, and long stems which meant fe rode them for long when new. So they sat around in garages and their major components had little wear. Available, inexpensive and strong, they are non the road now. Few econo-machines are are a better bang-for-buck than the one you own outright.
 

·
Registered
Joined
·
356 Posts
It may be a silly question to some, but I have to ask...

I have a superlight carbon hardtail mtb and a steel frame road bike that have similar weights, but the road bike is just soooo much faster on my work commutes. I've even swapped skinny slicks onto the mtb, but it still take a ton of effort to get it rolling up to speed and maintain it, compared to the road bike. Does the aero postion really make that huge of a difference? The wheels don't even weigh that much different.
I can't imagine trying to push a 53t big ring on my mtb. So, is it the riding postion?
Don't worry about it, passing one road biker on the road while riding your mountain bike makes up for it all.
 

·
Registered
Joined
·
175 Posts
Don't worry about it, passing one road biker on the road while riding your mountain bike makes up for it all.
I passed a road biker on my 29er hard tail a few weeks ago on the paved portion of a ride I do regularly. I must be ready for the Tour Day France!
 
1 - 19 of 19 Posts
Top