TRaffic Jammer

I usually only ride my MTB commuting in the winter, having not dirted it in quite sometime .... since it got SS treatment, it's not geared for trails. I run a Schwalbe(sp) CX Pro in the winter... it a thin 'cross style tire with leeetle tiny cross knobbies.... so cute. On the roads they are surprisingly nice and quick, and have the grab that lets me rip across the park, unlike the slicks I would ride, and good in the the sloppy crap O winter city streets. So much of your speed does come from aerodynamics, so for speed, getting low in the front is key. I've flipped my stem over to get lower and that made quite a difference I'll tell ya. Next I got rid of the ollllld front suspension, and put on an aluminum fork from a period correct Cadex... saved a pound and half right there, and dropped the front end slightly more than half an inch more. I'm poking around looking for a super short stem to use on it so I can slap road bars on it with some long travel Road style levers for V brakes. To me it seems that the lower I get in the front I engage my hips and upper legs differently. Basically, a MTB can be made PDQ if you need some toughness, and no fear lock ups for commutes. I might even see if the old crank off my SS tarck biek will fit. I want to get a set of big honking slicks like Big Apples or some such thing for poking around on in the city.

Loose Chain

Isn't it amazing that people learn that an aerodynamic profile and proper bike positioning are important to speed and performance. Each generation has to learn over and over. There is a reason that pro riders are down in a tuck, tons of seat post exposed and drop bars way down there, speed and muscle engagement.

Leiniesred

Nope. Not as fast given the same level of effort. Simple matter of frontal area.
Mtb (especially with rider) has more frontal area than a road bike = more wind resistance = slower speed.

mustachiod

consider the ride

are you bouncing over potholes, traversing dirt/rock roads, or is it smooth the whole way?

do you have alot of stopsigns and red lights or can you you hammer it and get up to a good speed?

neil

I would think it obvious, but I guess it's worth mentioning that the impact of wheel diameter on speed can be entirely offset by using different gear ratios in the drive train. Thus why the Moultons can still run perfectly fast on tiny wheels.

Larger wheels do feel different (the gyroscopic forces and centre of gravity are noticeably different when you change wheel size), but this has no impact on speed.

agarose2000

For commuters, the aero penalty on an upright mtn bike vs a road bike low position is huge.

However, it's more than the aero - the bike tires, weight, and gearing give just as big a penalty. I know this because I've ridden my hardtail with roadie pacelines more than a few time . In a tight paceline, the aero penalty of the mtn bike is greatly decreased due to the draft effect. It's still extremely hard to hold 22+mph on the mtn bike, even in the draftline, and a lot of this has to do with the fat tires, fork, and weight (in addition to a still significant aero penalty).

neil

This depends a lot on your commute, I would think. Many commuters are constantly stopping, so can never get up to speeds where the aerodynamics have major effects.

noisebeam

One can also set up a mtb with a lot of drop, or a road bike with bars higher than saddle. Sure there are still width differences, but the full range of height difference has more of an effect.

himespau 

or you can throw drop bars on your bike along with the slicks. my commuter still heavy as all get out and those panniers still add width, but it gives you more options.

PaulRivers

Yeah, exactly - aerodynamics only have a "huge" effect at "really high" speeds.

The Golden Boy 

I'm not sure I agree with that.

I'm old and slow. A few weeks ago I rode into a gusting headwind that kicked my tail. We're talking getting up to 10-12 mph and hitting a gust to almost a stop. Downhill. Pulling myself in tighter and shorter and smaller would have made a big difference.

cccorlew

Not really. But I guess kind of. I commute on several different bikes. My main commuter is a mountain bike with slicks. I have two grocery panniers. They act like parachutes even at slower speeds, and in a headwind (which I guess equals more airspeed) I really get slowed down.

If I ride my road bike I run about 3-4 mph faster, which feels like a huge difference when I doing it.

Metzinger

I have a ratty old MTB with 1.25" tires that I occasionally use for group road rides when my road bike is unavailable. A low stem, and a short travel fork mean that my position on the bar-ends is almost identical to that on my racer. Panaracer Paselas at high pressure mean rolling resistance isn't a factor.

I leave the Zefal rear fender and enormous rear blinky on for extra Fred points. They can try to drop me but they won't.

noisebeam

What really matters is effective air speed, not ground speed. Wind resistance is always there even at low speeds, but because the power needed to overcome it increases with air speed cubed the effects become quickly more noticeable the faster one is moving relative to the air. Traveling at 20mph vs. 10mph on a windless day takes 8x the watts to overcome the wind resistance.

PaulRivers

You introduced a whole different dynamic by adding in panniers. And I was very specific when I talked about a "huge" difference. I get passed on the MUP several times by guys riding upright mountain bikes while I'm riding my road bike with spandex, clipless, etc. Their secret? They bike everywhere, every day, while I don't bike nearly as much. Riding more makes a much bigger difference than the aerodynamics between my road bike and their hybrid/mountain bike.

I have the same problem with panniers, myself. Don't know that the bike frame style makes a difference in that regard though.

I have to admit, I did forget about the headwind thing. I would actually agree - in a headwind the aerodynamic difference starts to noticeably matter. It's kinda weird, usually I'm the one making that point. Huh. Still - not something I encounter regularly on my commuter at least.

AlmostTrick

I love it. Rock on!

alan s 

Actually, you would square the power, rather than cube the power to obtain the increased output, but otherwise you are correct.

dahut

+1 on all the comments. Im considering the same thing with my old MTB and its always interested to watch these threads progress.

Some things seem obvious: higher pressure slicks on the MTB frame are bound to increase speeds. Some others things like aerodynamic "tuck," made possible by installing drop bars make sense, too.

But it is a compromise, after all. Not a road bike, not a mountain bike, but something in between. Stands to reason the results will be in the middle, too.

As for the tires themselves, Id like to see what you get and what the others might recommend.

tjspiel

FWIW runners debate the effects of aerodynamics too. Or, more specifically, the value of drafting.

Most competitive runners agree that drafting during a headwind is beneficial. There's some controversy over whether or not it's worth it when it's calm. Some feel that the cooling effect of having the air move by you is more beneficial than any small difference in air resistance. One guy who's actually quoted some studies settled on speed of 11 mph.

In other words, if you're running at 11 mph or faster in calm weather, then you're better off drafting. If you're going slower than 11 mph, the benefits are negligible.

The only reason I bring this up is that I often hear in cycling circles than unless you're going 20 mph or faster aerodynamics don't matter. The above would seem to contradict that.

On the other hand, I'm talking about racing where times are measured in fractions of seconds. Commuting is a different animal. Better aerodynamics may or may not have much impact on how fast you get to work. For some of us though feeling faster might be as or more important than getting there faster.

furballi

Air drag is always a factor when riding. Tuck-in closer if you want to benefit from the draft at speed below 10 mph.

PaulRivers

I think that's for racing.

For commuting, I'm not in the camp of "I'm commuting, so time no longer has any hold on me, la-de-da-de-da" but at the same time, I'm not doing an aero tuck to get to work 15 seconds faster either.

For Lance Armstrong, a $3,000 wheel that shaves 15 seconds off an hour long race is worth it. For most of us, it certainly is not. For the OP, a road bike would likely be faster than a mountain bike with slicks. But "how much faster" is the big question - is a 45 second improvement in commute time worth spending $1,000 on a new bike rather than just using the one he has? What if it's 1:30? What if it's 5 minutes? 10 minutes?

I have no idea. I just have some rough guidelines in my head.

The other thing is that this isn't a comparison between some wind resistance and no wind resistance, it's a comparison between the wind resistance on a road bike vs the wind resistance on a mountain bike. And I don't think it's going to be terribly noticeable unless as someone else pointed out he/she's biking into a strong wind. On the other hand, I personally was willing to spend an extra grand on a road bike to shave 5 or 10 minutes off a half hour to hour commute vs sticking with my winter bike, so who am I to say! :-D (To be fair, it wasn't the only reason - road bike also fit way better in my car)

alan s 

If you're drafting at speeds below 10 mph, it's a sign that you need to get on the front and pull for a while!

whitecat

No.

The Golden Boy 

So how much of a difference does tire width make beyond tread?

In other words, I've got a set of Panaracer Paselas in 1.75- how much difference would the same tires in 1.5 make?

Sixty Fiver

Those skinny little 18's on Moultons are very aerodynamic and with the brilliant suspension design you can roll these high psi slicks over some pretty craptastic roads and barely feel a bump... if you ever get a chance see if you can't take JC's Moulton Speed out for a test ride.

It is a pretty amazing bike.