Mtb v Road bike - The main difference?

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2009-07-31 9:22 AM

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Subject: Mtb v Road bike - The main difference?
This is my 1st year of Tri's, so am using my mtb. For my 3rd and last sprint this year I have bought some road tyres, so hopefully that will help. My question is, "What is the main difference between the two bikes." Roadies are more streamlined, lighter probably. But is the main difference the larger diameter wheels? 700mm compared to 26"(660mm) If we say that the cranks are 1 to 1 ratio to the wheels for this argument. For every pedal revolution on the roadie you travel 2.199m and on the mtb 2.073m. So on the roadie 9095 revolutions & on the mtb 9647 revolutions (552 rev more) for a 20k sprint...  Is that the main difference?  And if it is, is there a limit to the wheel diameter allowed? Just pondering on a slow Friday morning...

2009-07-31 9:39 AM
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Subject: RE: Mtb v Road bike - The main difference?
Aerodynamics and weight are the biggest differences I am think of.  Mountain bikes are designed to be taken off road they have stronger frame, stronger wheels, suspension, all these things add weight.  You also sit up straighter on a mtb so more wind hits you.  Last is gearing, mountain bikes are geared very low to get up steep hills on the trail. 
2009-07-31 11:55 AM
in reply to: #2319195

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Subject: RE: Mtb v Road bike - The main difference?
Guarantee those slicks are better for tri than typical knobbies.  On my old hardtail I'm 1.5-2 mph faster riding roads on slicks vs knobbies.

Main bike differences (assuming slick tires on both) are:

1. Body position- roadie is much more aero, and tri bike more aero than roadie
2.  Weight- roadies are much lighter (except for some highest-end HT MTBs)
3.  Gearing- roadies have taller gearing for going faster at same cadence. Think 50-53 tooth big ring vs 44 on MTB.

Wheel diameter isn't a major factor since overall diameter (wheel + tire)  isn't hugely different. MTB tire height sort of cancels out larger road rim diameter.

Edited by Oldteen 2009-07-31 11:57 AM
2009-07-31 12:07 PM
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Subject: RE: Mtb v Road bike - The main difference?
Friction is another factor, even road tires on a MTB have considerably wider road surface area contact than a road bike.

Bob
2009-07-31 2:08 PM
in reply to: #2319195

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Subject: RE: Mtb v Road bike - The main difference?
dtrimmer - 2009-07-31 7:22 AM
700mm compared to 26"(660mm)


Just FYI, a 700c wheel isn't 700mm dia.  It's 622mm, which is about 29".  OD of a road tire is about 668mm depending on the tire itself.  OD of a MTB 26 x 1.9 is about 650mm.  The difference is about 1/2 inch.  All things being equal, the difference in wheel diameter will gain you about 2% in speed.

The big differences between MTB and Road bikes are resistance (rolling and wind) and gearing.

Gearing:  Going from a standard MTB (44/32/22) to a standard road (52/39) will gain you about 16% in the big ring if the rear cassette was the same, which they aren't.

Resistance, rolling:  Road bikes usually weigh a lot less, and most less than 20lbs.  Thats a fairly linear change in rolling friction.  Road tires have much less friction than knobby MTB tires and less friction that 26" road tires - a 700c x 21 has a much, much smaller contact patch and is of slipperier rubber.

Resistance, air:  well known, but road and tri bikes are much more aero than MTBs
2009-07-31 2:14 PM
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Subject: RE: Mtb v Road bike - The main difference?
to get a shorter answer - question may be 'what are the similarities?" ----they are quite different and feel way different when you ride them.  as other have mentioned, fit/gearing/wheel width are the main ones.

2009-07-31 2:35 PM
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Subject: RE: Mtb v Road bike - The main difference?
Don't forget that most road (and Tri) bikes are also designed to be much much stiffer than you regular mountain bike for greater power transfer.
2009-07-31 4:18 PM
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Subject: RE: Mtb v Road bike - The main difference?

If you want to lower your tri time, a road bike is def faster. Others here have explained why. I don't think anyone would disagree. I think the math to consider is simpler than revolutions/20K or whatever:  
On a road bike, I usu do around 17-18 mph ave on my fav 30 mile (1.5hr) training ride. On a mtn bike ridden on the road, I usu do ballpark 12-14 mph ave (never tried with slicks which will help) for an hour or so training ride. Doesn't sound like much but, 17-18mph (17.5) / 12-14mph (13) = 34% faster which is WAY faster.  And that is same effort/power in. 

2009-07-31 5:22 PM
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Subject: RE: Mtb v Road bike - The main difference?

A few things on the mtn bike will KILL your speed compared to a road bike:

1. Lack of big gears equivalent to road bike top-end gears. Less of an issue if you're not a fast cyclist, but a lot of mtn bikes have gearing that max out around 22mph, which on a downhill, is pretty slow.

2. Heavier weight. 30lbs is common for a mtn bike, whereas 20-22lbs is roadie range.

3. Thicker tires on the mtn bike = bigger rolling resistance.

4. Upright aero position. At the same time but I'll bet I could ride my road bike in full upright aero faster than I'd ride virtually any mtn bike in aggressive TT aerobar position, so the aero thing isn't as big a deal at this level.

Once you get used to a road bike, mtn bikes are PAINFULLY slow to ride on roads. 

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