corrado33

I ride a 54-56 cm road bike. I ride an 18-19 inch mountain bike. Converting both to a reasonable system of measurements... that's 54-56 vs. 45-48 cm. I have a couple of questions.

Are top tube lengths of road bikes relatively shorter (compared to the seat tube length) than mountain bikes? So Road bike TT/ST < Mountain TT/ST. They would have to be to fit the same person, right? Is the reason seat tubes on mountain bikes shorter because we like to get the saddle out of the way when we're riding downhill?

I mean, when we're pedaling to get the optimum efficiency our saddle to bb distance is going to be pretty much the same on both bikes. So, if you don't move the saddle out of the way (XC bikes?) could you simply convert a road frame to a mountain bike? (Theoretically, not practically, I'm not really interested in doing this.)

I'm asking this because I recently found a bike that's super cool, it's a 54 cm fully chrome bike, but it has mountain bike tires (wider 26s). My next thought would be to measure the TT, but then the thought after that was, can you compare TT lengths between road bikes and mountain bikes since road bikes use a bar that puts the hands 4-5 inches in front of the stem, and mountain bars put the hands pretty much at the end of the stem? The bike is from the 80s, where the line between road and mountain bike was blurred, so I'm really not sure how to tell if this bike would work for me or not. It feels fine, but most bikes do feel fine until you ride them a couple of hundred miles.

fietsbob

Size ... measure ... a frame has measurements.

Top tube length* & seat tube length ..

match them?

* a horizontal virtual line if the top tube slopes.


Old Coaching book for Rome olympics used a plumbline thru BB axis
and divided up top tube fore and aft of that .

leaping off or dabbing with a foot downslope of the MTB track is not anticipated road riding is it?

Spoonrobot 

IME*, which may not apply, yes you can but it depends on if you're road riding or mountain biking on the bike.

If you want to ride your mtb on the road then you're going to be fine just getting close to the same toptube as your comfortable road position and then fine tuning the stem length.

If you want to ride your mtb off-road you need to be more careful as fine tuning the stem can only really be done by decreasing length. Too long of a stem upfront on a mtb destined for offroad puts your hands too far in front of the wheel and makes off-chamber and downhill riding significantly slower and potentially more prone to crashing and washing out the front wheel.

This all sort of depends on the year and style of the mtb. Current mtb geo is very long tts designed for short stems, hard to compare these to road geo. Usually I will assume a 2016 mtb in medium is about the same as a large road bike for tt purposes. Meaning that if you were riding the mtb as an mtb you get the medium. If you wanted to ride the mtb as a road bike, you would get the small.

So I guess if you're going to ride that mtb on the road, you're good to go.

Sorry if this is confusing, there have been several waves of different mtb geometry trends so it's a lot of info to try to parse out.

*My preferred reach for drop bars is 655mm. This is tt+stem so 545mm + 110mm and is generally a medium in regular road geo. I've ridden 3 road bikes with this geometry +/-10mm several thousand miles over the past 4 years with no issues. I recently converted an MTB that I had previously been racing and riding heavily in regular xc style riding with a reach of 715mm with flat bars, this is a 605mm tt and 110mm stem. I converted this bike to drop bars using the preferred reach mentioned above but slightly longer since I couldn't easily find a shorter stem. This MTB has a reach of 665 using drop bars, 605mm tt and 60mm stem. I've ridden it a few hundred miles on road and gravel and it's been great. Feels the same as my road bike in position but obviously rides different due to being an MTB.

unterhausen

can't really compare top tube sizes between a drop bar road bike and a upright position mtb. On an MTB, standover height is a lot more important. Road bikes are bigger because of tradition and because it's nice to have a place to put a full-size water bottle.

NYMXer

I think that the handling of two totally different types of rider needs would require the rider's body position to also be so much different that the top tube would not be similar in lengths.
Mtb riders have their head over the bars more so than a roadie and a Mtber also needs more precise and faster turning, all helped with a shorter top tube.
I would think ground clearance would also change frame geometry making the two unlikely for comparison. The differences mention do seem a bit more than I would have thought though. I guess it can be easiest explained by slow speed handling versus high speed stability.

BobbyG

I think mountain bike top tubes are lower so one can stand over them more easily, which would happen more often in our road racing than in road racing.

gsa103

Yes, after you account for a particular stem/bar combination. Mountain bikes also run significantly wider bars so the reach is even shorter than a road bike.

Most people actually have their road and mountain bikes set for a very similar riding position, since that's what's comfortable for their back. The distance from saddle to BB is essentially identical, maybe 1-2 cm lower for mountain bike.

canklecat

This.

When I first saw what was called competitive mountain biking, it was on converted road bikes with rigid forks and horizontal top tubes. It looked terrifying, not fun.

The origins of the downhill bombers made from old Schwinns or whatever was handy looked fun. The converted road bikes did not. It looked like ice skating on stilts. And the few times I tried riding just ordinary gravel roads with only modest hills, it felt like it too.

I'd assumed compact frame bikes were designed to minimize neutering riders who climbed out of the saddle for rough riding. Some folks say it's just to make one-size-fits-most bikes that sell more cheaply. I dunno. I like my simple heavy compact frame bike for noodling around off road. I wouldn't enjoy that on a conventional bike with a horizontal top tube. It's not just for climbing out of the saddle over bumps. If I skid or start to fall I can put a foot or both feet down and if I slide off the saddle the boys don't get crushed.

I don't know much about geometry, as Sam Cooke sang. My compact bike has a shorter cockpit (stem to seat post), while the road bike's is about an inch longer. But the compact bike's wheelbase is two inches longer, so I guess that's mostly in the rear. Whatever or however it's designed, it works. Feels very stable off road, while the road bike feels skittish on gravel. And while the road bike feels faster on pavement, my actual riding speed isn't much faster. But I'm a slowpoke anyway.

corrado33

I don't know if this is true. There is definite merit to being able to get out of your saddle and LOW over the back end of the bike while descending REALLY steep stuff. Actually, just while descending in general. That's simply not possible on a flat TT bike. (Unless you really want to crush the jewels.)

In all honesty I think the bike in question (in the OP) was built as a road bike initially, then converted to a mountain bike.

smarkinson

Going by some geometry charts it looks like the measure different things. A road bike size is the length of the seat tube from the centre of the bottom bracket to the centre of the top tube if the top tube was horizontal. Not all road bikes have horizontal TT but they still use a virtual horizontal TT to give the bike size. It will be called something like the effective bike size or effective seat tube length.

Mountain bikes look like they just use the physical seat tube length which is shorter due to the sloping TT. The virtual horizontal TT is used for TT length though.

Just Google images for MTB geometry chart and compare it with road bike geometry charts. The charts will indicate where they measure from and in MTB bikes it appears it is just the seat tube length. In road charts there might be two measurements down the seat tube, actual and virtual/effective.

Darth Lefty 

Because road bikes are from the Alps and mountain bikes are from the Coast Range

Slash5

Top tubes on modern mountain bikes are much longer than road bikes. A modern mountain bike will have a very short stem and bars that put the reach very close to the length of the top tube - so all the reach needs to be in the top tube.
Road bikes have longish stems and the bars themselves add to the reach.
My mountain bike shows an effective top tube of 622 mm. Road bikes usually are more or less square - a 58 mm seat tube will have a 58 cm effective top tube. If you want to put drop bars on a mountain bike not designed for them you will usually need to use a smaller frame and short stem and lots of rise to get the reach reasonable.

Seat tubes are shorter on mountain bikes to provide top tube clearance on dismounts - they have sloping top tubes. Early mountain bikes had horizontal top tubes and could be painful if you had to dismount when the wheels were on a rise or you stepped off into a low spot.

AlexCyclistRoch

Yes.
no, it's because you need to maneuver more.
MTB's often make jumps, when you want to be on your feet and a god distance from the saddle, for impact.

If it feels fine, go for it. I have one bike that is made from an old touring frame (59cm, I think, just a tad small for my road bike fit), which has bullhorn bars, and wide 700c tire/wheels (29'er in MTB terminology), which I take on trails pretty often. Yeah, it's a bit too big for the heavy stuff, but works well otherwise.

cyccommute 

The reason that the seat tube is shorter should be obvious or, rather, it becomes obvious the minute you have to bail off a mountain bike in the middle of a trail. You need more space between the frame and the dangly bits. You usually don't have to bail off a road bike in quite the same manner.

The longer top tube is a bit less obvious. Part of it is for the reasons you give below. But a more modern reason is for the longer top tube is rider position. While it is true that old mountain bikes had shorter top tubes, it wasn't because they shared much with road bikes. Their geometry came from the old klunker days when guys were using Schwinn Excelsiors as mountain bikes.

The Excelsior is a cruiser bike through and through. It was meant for cruising down the street in a full upright manner on a bike that was completely stable. It had a slack head tube...68° or less..., a slack seat tube and a long wheelbase but a short top tube. The first production bikes mimicked this geometry, although they also raised the bottom bracket siginificantly.

These were great bikes for downhill. They were almost stable enough to ride no handed off-road so you could really fly down a hill. The problems started to develop when you needed to pedal to the top of the hill. That slack head tube and seat tube worked against the rider. The front wheel wanted to flop from side-to-side and the rider was so far back that it was difficult to keep the front wheel on the ground on even moderate climbs. You could try to stand but the long wheelbase meant that you lost traction on the rear wheel as it unloaded. All in all, riding up hill on an early mountain bike was tricky and required lots of finesse.

Mountain bikes quickly dumped the long wheelbase, steepened the head tube, lengthen the top tube and shortened the wheelbase. This puts the rider in a more centered position so that the rear wheel doesn't lose traction as easily and the front wheel stays on the ground.

The steeper head tube, especially on the middle year mountain bikes, came at a price, however. The bikes lost their downhill stability. Downhills could be white knuckle affairs on a 73°/73° head angle/seat angle rigid bike. Riders really had to exaggerate movement behind the saddle to keep from endoing.

Not really. Especially not now (probably not since the late 90s either) with all frame changes to accommodate the suspension. The bikes have gone back to a slack head angle with long travel forks and the rider position has been tweaked further to accommodate those slack angles so that the wheel doesn't flop on uphills nor does the rider have to move too far forward to keep the front end on the ground. You can climb in a rather neutral position where you used to have to crouch more and work differently (harder) to climb trails that are simple to climb now.

Probably not. The line between mountain bikes and road bikes wasn't that blurred in the 80s. If anything it was less blurred because nearly all bikes then had horizontal top tubes. A 21" mountain bike is a very tall mountain bike. While you won't have any problems with the standover height, it will have a top tube that is proportional for someone who is 6'2" to 6'6". It's closer to a 65cm road frame than a 54cm road frame. Those bars are going to be a long way away with a flat bar. They are going to be in the next state with a drop bar.

Cruising around town with a flat bar or a bar that sweeps back a lot might work but putting drops on that bike probably won't be comfortable. I"m not saying that this kind of conversion isn't possible but you have to start with the right frame. For your size, that's going to be a 17" or 18" mountain bike frame. Go 2" to 3" shorter than your road frame and you'll have the proper top tube length.