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Old 12-28-09, 01:25 PM   #1
bitterken
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MTB TT length compared to road bike TT length (for XC)

Hey folks.

I'm planning on getting my 1st mountain bike and wanted to get a better feel for how sizing works. I'm usually focused on TT length as my primary size indicator on road and CX frames, and I wanted to see how this translates to an XC bike.

Is there a decent enough formula for road bike TT + Xcm = mtb TT that I can use as a starting for looking at used MTB's?

Thx!

-Ken
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Old 12-28-09, 03:03 PM   #2
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Hey folks.


Is there a decent enough formula for road bike TT + Xcm = mtb TT that I can use as a starting for looking at used MTB's?



-Ken
Doubt it as it is the norm on Mountain bikes isto go for a smaller frame than you would normally use. On top of that most now use riser bars to stop riding with your head between your knees and to bring the bars back towards the saddle.

Bit difficult to compare sizes really but I have noticed that on the road bike I use a 535 mm top tube on a 51cm frame and on the MTB I have a 15" frame with riser bars. Never bothered to measure the top tube length on it.
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Old 12-28-09, 04:19 PM   #3
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Originally Posted by bitterken View Post
Hey folks.

I'm planning on getting my 1st mountain bike and wanted to get a better feel for how sizing works. I'm usually focused on TT length as my primary size indicator on road and CX frames, and I wanted to see how this translates to an XC bike.

Is there a decent enough formula for road bike TT + Xcm = mtb TT that I can use as a starting for looking at used MTB's?

Thx!

-Ken
My top tubes between road and mountain bikes are fairly close for ETT, my road bike is actually a tad shorter than most of my mountain bikes. Depends on the mountain bike as well as your preferences. Go test ride some in the type of mountain bike you're interested in and note top tube/stem/bar setups for an idea...
What kind of riding do you want to do? Good luck on having used mountain bike ads tell you what the ETT (or even manufacturer's measurment) was....
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Old 12-28-09, 06:04 PM   #4
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What size Road/CX do you ride?

Typically, you have a longer toptube, shorter stem with MTB, but overall the same, or slightly shorter TT+Stem.
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Old 12-28-09, 07:56 PM   #5
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Thanks for the replies folks.

Both my current road and cross bikes have 56.5cm ETT's with 11cm stems. Both are on the shorter side of my fitted reach range, but I find it to be great for the way I ride.

There's about 12cm between the tops of my drop bars and the hoods, so the overall reach is approx 67.5cm to 79.5cm (there are angles and overlaps from the HT and ST angles, but I figure it's pretty minor.) So compared to the road bike, where do you suppose the mountain flat bar should sit - in between somewhere?

This would be for XC riding...and I kinda got talked into trying a few XC races by some friends during the last cross season. It wasn't hard...I've been getting sorta turned off on road racing the last couple of years and if you read the road racing subforum here, you may see why.

And as far as getting an accurate ETT measurement, I typically don't buy a used frame unless the owner can at least tell me the year and frame size so I can look up the geometry. It's doesn't always work out though...

Thanks!
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Old 12-28-09, 08:49 PM   #6
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And as far as getting an accurate ETT measurement, I typically don't buy a used frame unless the owner can at least tell me the year and frame size so I can look up the geometry. It's doesn't always work out though...
Good luck! Having just finished the Craig's List dance in search of a hard-tail, I can tell you that in my experience it was hard to get detailed info out of the folks selling bikes. Poor or no pictures were the norm, and most ads left out the size of the frame and model year.

Hope you have better fortune than me.

As a full-time road rider, it seems that mtb's are a bit more "compact" on the tt. It seemed that every bike I tested that was my size felt a little cramped in the cockpit as compared to my road bikes. Bikes that were comparable to the road bikes tended to juggle "the boys" a bit too much. The compactness seems to work out for me really well on the dirt, though - different game than riding on the pavement, and being more forward (in general) seems to keep the bike moving on steep ascents.

But what do I know - I'm a complete noob at this.
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Old 12-28-09, 10:36 PM   #7
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I have been experimenting with mountain and road bike fit for years and I have ended up with longer effective top tubes on the mountain bike than on the road bike. I use the same stem length on both but the mountain bikes have zero setback on the seatpost. I think if you compare the same manufacturer's medium road bike to their medium mountain bike you will find that the ETT is much longer on the mountain bike.

I would suggest going to your LBS and riding a few different mountain bikes to get a better feel for the correct size.
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Old 12-28-09, 10:57 PM   #8
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I made my comment about good luck on getting top tubes lengths because you said it was your first time....heck, lots of mountain bikers don't know their top tube length (or could measure it) compared to a dedicated roadie. Fortunately the internet makes research a good option, if it's not too old a model...
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Old 12-29-09, 01:59 AM   #9
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My road bike was set up by a specialist. Mentioned I do the odd bit of mtb'ing also and he sent me comparitve mtb setup values. My road bike TT is 545mm with 100 stem. He advised me to get a mtb with TT (effective) of 56-57cm - so longer than roadbike.

Just got mine - and both myself and the guy at the shop measured the TT at 56, feels great. But the website said 59. Then I realised this morning we measured it at stem height, not end of headtube. With the converging angles and the spacers betwwen headtube and stem - it made the diffs. So there's another thought... I think the smaller size, which has a 57TT at frame would be more compact when spacers are added (to not have excessive drop) and thus going under what is good - at "true reach" level of the handlebars.

Hope it makes sense...
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Old 12-29-09, 02:36 AM   #10
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Keep the big picture in mind too. I went to compare my XC race bike to my road-racing bike, and what I measured was center of saddle to the hoods/grips. Think about it: a road bike's bars and lever hoods add massive forward extension beyond the stem. People don't ride a road bike holding onto the bar tops too much, at least I don't.

In my case, the road bike is about 34.5 inches and the XC-race bike is about 32.5 inches.
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Old 12-29-09, 01:18 PM   #11
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Keep the big picture in mind too. I went to compare my XC race bike to my road-racing bike, and what I measured was center of saddle to the hoods/grips. Think about it: a road bike's bars and lever hoods add massive forward extension beyond the stem. People don't ride a road bike holding onto the bar tops too much, at least I don't.
I think you have something here. This thread prompted me to go measure my bikes. As I stated earlier, I seem to feel a little more compacted on the mtb's as opposed to my road bikes.

First, I measured center of seat post to center of bars. The road bikes were at about 64cm, the mtb's at around 69cm. The TT's were longer on the mtbs. Then, I measured the road bike from center of seat tube to center of hoods, where I usually place my hands when I ride. That was about 76cm.

So - in my case the mtb top tubes are indeed longer, but where hands end up are closer in than on the road bike by about 7cm. That's why I feel the compactness.
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Old 12-29-09, 02:28 PM   #12
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Personally I don't think comparing your road bike fit numbers to a mountain bike mean much unless you ride them in the same position, which I certainly wouldn't want to do...make for horrible mountain biking.
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Old 12-29-09, 02:35 PM   #13
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Personally I don't think comparing your road bike fit numbers to a mountain bike mean much unless you ride them in the same position, which I certainly wouldn't want to do...make for horrible mountain biking.
Good observation. Probably why I never bothered to measure and compare the two until now, when this thread piqued my interest.
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