MTB geometry Vs road bike geometry

[Rik O'Shea]

I ride a road bike with a 56 cm seat tube and 56 cm top tube (centre to centre). Most frames tend to
be sold based on the seat tube length. Using the generic rule that an MTB frame should be approx 4
inch or 10 cm smaller than a road frame I arrive at a 46/45 cm MTB frame. However I've noticed most
46/45 cm MTB frames have 59/58 cm top tube.

Is this correct, are MTB frames deliberately sized this way ? Should I be riding a MTB frame with a
59/58 cm top tube or should I be going for a smaller frame with a shorter top tube ?

Thanks & regards R

 

[Bruni]

Road bars have "reach" ,somewhat analogous to barends except that many of us live on our hoods/reach
section. In addition, road bikes have short TT/long stems to aid in drafting as it tucks in the
front wheel allowing bodies to draft closer. No such requirement for mtb. I think most mtbs have it
pretty good for many folks. Tom
--
Bruni Bicycles "Where art meets science" brunibicycles.com
410.426.3420 Rik O'Shea <[email protected]> wrote in message
news:[email protected]...
> I ride a road bike with a 56 cm seat tube and 56 cm top tube (centre to centre). Most frames tend
> to be sold based on the seat tube length. Using the generic rule that an MTB frame should be
> approx 4 inch or 10 cm smaller than a road frame I arrive at a 46/45 cm MTB frame. However I've
> noticed most 46/45 cm MTB frames have 59/58 cm top tube.
>
> Is this correct, are MTB frames deliberately sized this way ? Should I be riding a MTB frame with
> a 59/58 cm top tube or should I be going for a smaller frame with a shorter top tube ?
>
> Thanks & regards R

 

[Jim Edgar]

Rik O'Shea at [email protected] wrote on 9/18/03 8:50 AM:
> I ride a road bike with a 56 cm seat tube and 56 cm top tube (centre to centre). Most frames tend
> to be sold based on the seat tube length. Using the generic rule that an MTB frame should be
> approx 4 inch or 10 cm smaller than a road frame I arrive at a 46/45 cm MTB frame.

Never heard a generic rule of that magnitude - maybe a couple inches in the days of horizontal
top tubes. Probably semi-valid these days, given the way that mtb frames are growing ever shorter
while their seatposts grow ever longer... Still, generic standover guidelines = generally bad.
The important measurements are cockpit dimensions. BB spindle center to saddle, saddle nose to
bars, that sort of thing. On an mtb, the standover height is likely to be plenty - pay attention
to the reach.

> However I've noticed most 46/45 cm MTB frames have 59/58 cm top tube.

Are you sure you are measuring correctly? Sounds like you measured actual tube length, which is
misleading because of the more extreme angles used in current mtb's (and "compact" road frames). The
valid comparison would more than likely be "effective" top tube length - measured along the
horizontal from headset center to seat tube center.

>
> Is this correct, are MTB frames deliberately sized this way ? Should I be riding a MTB frame with
> a 59/58 cm top tube or should I be going for a smaller frame with a shorter top tube ?

You might wander into your LBS with your road bike and line 'em up - that would give you a good idea
of the actual saddle-bars-pedal dimensions. Nothing wrong with being a touch shorter and more
upright on the mtb, particularly if you ride tricky trails.

Hope that helps,

-- Jim

 

[Phil Brown]

>In addition, road bikes have short TT/long stems to aid in drafting as it tucks in the front wheel
>allowing bodies to draft closer.

Huh? Since when? In 45 years of riding and 30 building I've never heard that. Phil Brown

 

[Bruni]

Phil Brown may not have heard the contention that road bikes have short TTs and long stems for
drafting but we all have seen and ridden such bikes. A 120mm stem,100mm of reach and 30mm of hood
produces quite a lot of tiller effect on road bikes. Ten years agoMTBs had similar control
leverages..Not any more. Notably, Trek added about 30mm to their TTs and lost the same from stems.
Now it is hard to find those 135mm stems of yesteryear. My contention is that racing shapes our
bikes and that the geometry that makes for a fast peloton, can be less than optimal for the rest of
us. Jobst Brandt wrote years ago about team time trial bikes being the fastest/shortest and that it
is a tradeoff. MTBs rarely draft and could use the greater stability of frame length. At the time
MTB geometry was changing, some worried about front washout from kicking front ends outward.
Stability seems to have won that debate. Drafting friendly short road bikes rule the 4% road bike
market as it is a race replica niche. If and when road bikes again dominate, there will be many
choices other than crit bikes. See Rivendell thread. Tom

Bruni Bicycles "Where art meets science" brunibicycles.com
410.426.3420 Phil Brown <[email protected]> wrote in message
news:[email protected]...
> >In addition, road bikes have short TT/long stems to aid in drafting as it tucks in the front
> >wheel allowing bodies to draft closer.
>
> Huh? Since when? In 45 years of riding and 30 building I've never heard
that.
> Phil Brown

 

[Rik O'Shea]

"Bruni" <[email protected]> wrote in message news:<[email protected]>...
> Phil Brown may not have heard the contention that road bikes have short TTs and long stems for
> drafting but we all have seen and ridden such bikes. A 120mm stem,100mm of reach and 30mm of hood
> produces quite a lot of tiller effect on road bikes. Ten years agoMTBs had similar control
> leverages..Not any more. Notably, Trek added about 30mm to their TTs and lost the same from stems.
> Now it is hard to find those 135mm stems of yesteryear. My contention is that racing shapes our
> bikes and that the geometry that makes for a fast peloton, can be less than optimal for the rest
> of us. Jobst Brandt wrote years ago about team time trial bikes being the fastest/shortest and
> that it is a tradeoff. MTBs rarely draft and could use the greater stability of frame length. At
> the time MTB geometry was changing, some worried about front washout from kicking front ends
> outward. Stability seems to have won that debate. Drafting friendly short road bikes rule the 4%
> road bike market as it is a race replica niche. If and when road bikes again dominate, there will
> be many choices other than crit bikes. See Rivendell thread. Tom
>
> Bruni Bicycles "Where art meets science" brunibicycles.com
> 410.426.3420 Phil Brown <[email protected]> wrote in message
> news:[email protected]...
> > >In addition, road bikes have short TT/long stems to aid in drafting as it tucks in the front
> > >wheel allowing bodies to draft closer.
> >
> > Huh? Since when? In 45 years of riding and 30 building I've never heard
> that.
> > Phil Brown
> >

If I can summarise what I think is being said:

An MTB of the same comparative size to a road bike will generally have a longer top tube. This takes
into account that MTBs dont include the extra reach associated with brakehoods on a road bike and
the MTB will generally have a shorter stem (horizontal length) than a road bike. The longer top tube
on the MTB also encourages more stability in the handling of the bike.

So in my case with a road frame top tube of 56cm it would not be unusal to have an MTB with a top
tube of 58cm.