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  1. #1
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    how do mtb toptube lengths compare to road?

    mostly a roadie, started mountain biking last year during offseason and would like to try some mtb races next season - so thinking about a new frame. Was wondering if anyone could explain the large variances in toptube lengths on mountain bikes - I'm looking either at 20 or 21 inch frames (or L or XL). Even in the same size I've seen up to a 4 cm difference in toptube lengths. Some of these, like the Specialized, seem really long - 61.5cm on a 19inch stumpjumper and 64 on their 21inch. Other 21inch frames have toptubes of only 60 - would appreciate any reasoning behind the long toptubes and whether there's any guidelines for estimating a mtb toptube length from your road bike fit. thanks!

  2. #2
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    You have to ride them . Its more about body type , long leg or long torso or both . So many variables that without already knowing what you need you have to go ride a bunch of them until you find what fits . Chances are if your road bike has a longer top tube your going to want the same on an mtb .

  3. #3
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    Front centre distance and wheelbase will be longer on an MTB and stem length will likely be shorter so the longer top tubes gives you more room to climb out of the saddle without whacking your knees on the bars. HTT length is the key factor in getting fit properly.
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  4. #4
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    Quote Originally Posted by stevesbike
    whether there's any guidelines for estimating a mtb toptube length from your road bike fit. thanks!
    Nope...different principles entirely.

    Road is all about your body's dimension. You're aiming for a particular position or angle.

    Mountain bike is all about your body's weight. You're aiming for a particular weight distribution.

    Heavier upper body and you'll likely be more upright. More downhill riding will be more upright too. XC should be about 40% of weight on front wheel, FR 35% and DH 30%, give or take.

    You'll need to gain some experience and develop your handling to be able to feel what is right. As long as you're average proportions and looking at a bike of the right style + class, you should be able to be right in the ballpark by what feels comfortable. Test ride.

  5. #5
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    Since you're coming off the road I'm guessing you're going to be mostly XC riding/racing. And if you're a size Large on a road frame, there's a very good chance you're going to be on a large on a MTB. If you're between sizes, I've always thought it was good advice to err on the smallish side for MTB. You need room to move and for your bike to move under you. Once you have the size of the frame in the ballpark, you can usually dial it in by trying different stems, or seatposts with more or less layback. FWIW, I see a lot more people riding with MTBs that are too big for them than too small. Also, since the late 90s, TT lengths on MTBs have definitely gotten longer. So while an XL frame might have fit you in 1991, now an XL by the same MFR will have you stretched out to a less than optimal degree. Hope that helps.

  6. #6
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    thanks for the comments!

  7. #7
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    Quote Originally Posted by stevesbike
    thanks for the comments!
    I honestly haven't read a solid answer to your question so will weight in. Answer is mtb's are generally 4-6cm longer in the top tube. It isn't too hard to deduce why. With me my mountain bike is sized about 7cm longer with the same stem length. The reason is a mountain bike has backsweeep to the bars albeit in some instances slight. Many that ride a roadbike aggressively also like an aggressive fit on a mountain bike with perhaps a bit less bar drop. If you visualized a roadbike nominal position, this is on the hoods. This places your hands about 60-80mm in front of the stem clamp. On a mountain bike the hands are generally behind the stem clamp even on aggressive flat bars with 3-5 degrees of backsweep. Now things change if you like the dark side of alernative bars on your XC rig which don't require a flatish bar to brace your body due to technical stuff. Handlebars that are ergonomically comfortable for high speed XC riding generally require a longer cockpit. This creates even a longer top tube or stem requirement. Some alt bars do have forward sweep to help maintain cockpit length however. Above doesn't even address seat tube angle which also affects top tube length but will leave that for a different time.
    Hope that helps.

  8. #8
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    Quote Originally Posted by dirtrider7
    I honestly haven't read a solid answer to your question so will weight in. Answer is mtb's are generally 4-6cm longer in the top tube. It isn't too hard to deduce why. With me my mountain bike is sized about 7cm longer with the same stem length. The reason is a mountain bike has backsweeep to the bars albeit in some instances slight. Many that ride a roadbike aggressively also like an aggressive fit on a mountain bike with perhaps a bit less bar drop. If you visualized a roadbike nominal position, this is on the hoods. This places your hands about 60-80mm in front of the stem clamp. On a mountain bike the hands are generally behind the stem clamp even on aggressive flat bars with 3-5 degrees of backsweep. Now things change if you like the dark side of alernative bars on your XC rig which don't require a flatish bar to brace your body due to technical stuff. Handlebars that are ergonomically comfortable for high speed XC riding generally require a longer cockpit. This creates even a longer top tube or stem requirement. Some alt bars do have forward sweep to help maintain cockpit length however. Above doesn't even address seat tube angle which also affects top tube length but will leave that for a different time.
    Hope that helps.

    Yea, what he said!
    As a roadie first (then mtbiker) I found the 4-6cm toptube rule to work well for me also (54cm road, 59.5cm mtb). Just remember that the measurement is taken from EFFECTIVE top tube on both bikes (google it if you are unsure of what that is). Sure you can get even more specific with sizing a mtbike but just get the basics right first time round, its all an evolution of experience.

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