MTB vs Road Bike Sizing

[Limitseeker]

Hi guys. I’m an absolute noob when it comes to bikes with skinny wheels, but I have decided to give the June half iron man a go, and see how I feel about this whole triathlon thing. So I need to start shopping for a road bike. But I want to buy something relatively cheap, from the classifieds, but I have no idea what sizes to put into the search bar.

I have always ridden and felt more comfortable on Large frame MTBs, even thought I am borderline between L and M according to most brands sizing. 

I am 1.78 but have relatively long legs, and that is probably why I prefer large MTBs. So what is the comparable size range in Road Bikes, and how specific is the sizing. can i look 1 cm up or down, and then make up the difference with seat position and stem length or doesn’t it work that way on road bikes. The simple fact that there are so many very specific frame sizes, makes me think it is pretty important to get it just right. Compared to the generic S-M-L-XL gaps with MTBs

 

Looking forward to advice/pointers.

 

I feel dirty going to a bike shop and getting measured up, even if I know I’m not even remotely considering buying from them… 

[Limitseeker]

Because the feed got spammed by the event bot, I’m just giving this a little bump

[gerriemtb]

1 hour ago, Limitseeker said:

Hi guys. I’m an absolute noob when it comes to bikes with skinny wheels, but I have decided to give the June half iron man a go, and see how I feel about this whole triathlon thing. So I need to start shopping for a road bike. But I want to buy something relatively cheap, from the classifieds, but I have no idea what sizes to put into the search bar.

I have always ridden and felt more comfortable on Large frame MTBs, even thought I am borderline between L and M according to most brands sizing. 

I am 1.78 but have relatively long legs, and that is probably why I prefer large MTBs. So what is the comparable size range in Road Bikes, and how specific is the sizing. can i look 1 cm up or down, and then make up the difference with seat position and stem length or doesn’t it work that way on road bikes. The simple fact that there are so many very specific frame sizes, makes me think it is pretty important to get it just right. Compared to the generic S-M-L-XL gaps with MTBs

 

Looking forward to advice/pointers.

 

I feel dirty going to a bike shop and getting measured up, even if I know I’m not even remotely considering buying from them… 

Great question, @Limitseeker. I am also interested to hear the answer as my cycling has been limited to MTB. I am also considering getting a road bike.

[Longbarn Killer]

You will probably be borderline between a M and a L as with your mtb. 

Measure your inside leg and x 0.65 to get an approximate size. 

52 = S

54 = M

56 = L

The above is classic sizing. Lots of road bikes now days have sloping top tubes which may have different sizing. 

But if you aim for the S, M, L, you will at least know what to aim for. 

[Dappere]

Best advice I can give you is to go a try a few at a bike shop. 
 

Road bike sizing, like most things in the bike industry isn’t standard. Some manufactures 54 might be like other brands 56. Once you get a good idea of the geometry fits you, it’s relatively easy to compare those numbers to the various manufactures geometry charts and see which size fits best. 
 

There is also an element of riding style. How aggressive you’d like the bike to be or not, and most importantly - comfort. 
 

I am a few centimetres  taller than you are. All the mtbs I have owned are large. One of my road bikes are 54 and the other is 56, both very similar in geometry numbers. 
 

If you’d like to start a search on the hub, maybe search for 54, 55 and 56. Go an ride this bikes and see what is more comfortable. 

[DieselnDust]

Road bike sizing is different to Mtb, especially for the discipline you want to enter , namely triathlon. 
genrally you will likely fit in most brands Medium to medium large. Maybe select a bike first then it’s easier to assist because it’s strips out the difference between compact , sub compact and conventional , endurance, climbing and aero 

[Limitseeker]

36 minutes ago, DieselnDust said:

Road bike sizing is different to Mtb, especially for the discipline you want to enter , namely triathlon. 
genrally you will likely fit in most brands Medium to medium large. Maybe select a bike first then it’s easier to assist because it’s strips out the difference between compact , sub compact and conventional , endurance, climbing and aero 

And here I thought road bikes were gonna be simpler than mtb

[Frosty]

Have you had a bike fit? Even if on the MTB.

I see lots of people on the wrong size bike, mostly too big. They end up moving the saddle forward  to shorten the reach, or back if the bike is too small. Long story short, they have a number of aches and pains, like back, knees, fingers, shoulder and more.

If you can try borrow a bike from a friend, that is similar sized. Explain to a good bike fitter what you’re planning and then ask them to measure you. They will give you a list of key dimensions which you can then measure in store. 

* Centre of BB to top of saddle;
* Saddle setback
* Saddle to bar reach
* Stem length

and much more. You can then search for bikes that “fit” you and you will be comfortable.

[Frosty]

Examples of the dimensions from a bike fit.

[Eldron]

Pay someone to measure you up.

Most fitting systems have frame a sizing software guide based on your measurements and has the last few years geometry in the database. The fitter should be able to nail it every time.

Small investment to ensure you get the right geometry and size.

[King_Crispy]

5 hours ago, Limitseeker said:

Hi guys. I’m an absolute noob when it comes to bikes with skinny wheels, but I have decided to give the June half iron man a go, and see how I feel about this whole triathlon thing. So I need to start shopping for a road bike. But I want to buy something relatively cheap, from the classifieds, but I have no idea what sizes to put into the search bar.

I have always ridden and felt more comfortable on Large frame MTBs, even thought I am borderline between L and M according to most brands sizing. 

I am 1.78 but have relatively long legs, and that is probably why I prefer large MTBs. So what is the comparable size range in Road Bikes, and how specific is the sizing. can i look 1 cm up or down, and then make up the difference with seat position and stem length or doesn’t it work that way on road bikes. The simple fact that there are so many very specific frame sizes, makes me think it is pretty important to get it just right. Compared to the generic S-M-L-XL gaps with MTBs

 

Looking forward to advice/pointers.

 

I feel dirty going to a bike shop and getting measured up, even if I know I’m not even remotely considering buying from them… 

Where you based?

[DieselnDust]

14 minutes ago, Frosty said:

Examples of the dimensions from a bike fit.

the problem is the reach, stack saddle set back are not transferable from the mtb to the road bike unless the seated position is the same and in most people's case it is not.

measuring the riders and then making recommendations is still not going to be an exact science, especially for triathlon where the rider is resting the elbows on tri bars in a more prone position vs an mtb where the rider sits more upright.

Even if not using tribars, triathletes tend to use the drops more and therefore this needs to be considered when sizing and fitting the bike especially if the rider is between sizes

[Frosty]

1 minute ago, DieselnDust said:

the problem is the reach, stack saddle set back are not transferable from the mtb to the road bike unless the seated position is the same and in most people's case it is not.

measuring the riders and then making recommendations is still not going to be an exact science, especially for triathlon where the rider is resting the elbows on tri bars in a more prone position vs an mtb where the rider sits more upright.

Even if not using tribars, triathletes tend to use the drops more and therefore this needs to be considered when sizing and fitting the bike especially if the rider is between sizes

The OP said he/she wanted to get a road bike, and I specifically said to try get a friend's bike to get measured on... I did fail to say road bike.

I am aware of the differences between road and MTB.

[DieselnDust]

9 hours ago, Frosty said:

The OP said he/she wanted to get a road bike, and I specifically said to try get a friend's bike to get measured on... I did fail to say road bike.

I am aware of the differences between road and MTB.

I hear you . What I am suggesting is that road bike geometry is not as uniform as Mtb. Most Mtb are within a cm of each other in terms of reach and stack for the same size. There outliers but they’re fewer than for road bikes.

example one brands endurance bike may be close to another brands race bike in geo but be two different sizes , S vs M.

so sizing on a friends bike is more hit and miss than oicking the frame or bike you want and Getting sized on that. There are a lot of variables that can influence sizing like stem length handlebar reach, saddle post set back, seat tube angle and often 

hence it’s probably best to get a generic bike fit for measurements and then apply that to available bikes in the classifieds or on the shop floor.

 

 

[shaper]

At 1.78 I would be inclined to say you are a 54 or medium, but best going to get a frame size fit done.  Some bike shops will be able to do this and there are some fitment centres you can go to like cyclefit in JHB? You do not say where you are located.

As for the 70.3 in June, as a triathlon coach, you only have 15 weeks until the race, so you need to be already training all 3 disciplines as well as clocking up long kms on a bike, whether a MTB or once you have a road bike

[Job]

Your case is definitely for a bike fitter you can't just take any bike size. But at a ballpark a size 55 might be ideal for your height.