i am getting a roadie bike for my sins. i ride (several) med mtbs (18in). what size frame should i be looking at? (in cm) i'm 5ft 8in tall

I'm about 5'7 and was supprised to be on a 50cm cannondale, i think you REALLY need to get measured up for road bikes

They are all slightly different. Go to a shop and throw your leg over a few. Sizing an MTB is easy but road bikes need sitting on at least.

It took me three bikes to get it right but it's well worth it. 20mph with little effort is a great feeling.

get measured and fitted properly at a shop
all the " i'm *ft**in and ride a **cm posts aren't worth a toss.

yep, road bikes are all over the place – compact geometries appear so much smaller than old school gemoetries… you want to be looking out for top tube length really, but then there is more to it than that too – head angles and seat angles make a difference, and as you're sat down a lot in the same position on a road bike it's important to get it right. get down to a bike shop!

oh, and the ratio of upper body length to leg length makes a huuuuge difference to what size you should be riding…!

ok, cheers fellas, i get the message but its only gonna be a hack/commute/couple of hours bike. not gonna drop a load of money or i would do it properly; more a case of s/hand make it fit and what can i get away with ball park

Try em.
I'm 5'8" and race a small Planet X when I should be on a medium.
Giants put me on medium, but I'm a small on them to.
Both my standard geometry bikes are 52cm
So deffo try. I prefer a smaller frame and longer stem/post.

Will depend on the type of geometry a bike has.
I'm 5'8 & ride a medium Giant TCR (50cm), but have also ridden a Felt F5 in a 54cm & a handmade steel frame in a 56cm, all of which fitted.
It varies hugely, depending on the geometry type, so you really do need to try first.

Look for top tube length (well, and stem and bar reach) – roughly 53 or 54 cm with a 100mm stem is probably about right if you are normally proportioned. The nominal seat tube size is irrelevant really – saddle go up and down.

5ft 10 on a 56cm Cervelo which compared to some folk on here sounds like a huge bike. Shows how important it is to actually try a few!

Still try them like the others have said.
I'd been out of road riding for a bit and bought a Giant to get me back into it. I went by all their suggestions but the end result was a good bike but a dreadful ride. Luckilly I had donkeys years worth of experience to tell me it was wrong, so no offence if you've not had one before you'll not know how far wrong you can go.
See mine are for racing, and UK racings pretty short so I buy a bike I can 'overpower' if that makes sense.
For day riding you nedd a good fit.

does the geometry on an 'old style' racer differ from a modernish one? and if so will this influence the size i need?

Yep, get a fitting. I just got something shiny and new and got computer measured and fitted in a good shop. Turns out ideally i would have a 54 seat tube and 58 top tube as i've got very long arms and short legs (5'10").
The position i have now is miles different from what i thought was right on my old bike. I got a medium long Lynskey in the end with a 120mm stem, as there was no way i could stretch to my perfect position with my current level of flexibility.
I had a trek previously, and although a 54 would have been good for seat height, i had to get a 56 as the reach was nowhere near enough on the 54. Turns out this would still not be long enough without a long stem.

Complicated these bike things aren't they?

jeez, did n't realise it was gonna be such a minefield

[does the geometry on an 'old style' racer differ from a modernish one? and if so will this influence the size i need?]

Not that much. I've got an 1989 Gios Evolution and it's exactly the same as modern standard geometry frames. Though we rode them a little bigger back then.

Two decades of racing apart.
I still think standard geometry is best for all day like the Gios, which will be up/down graded with modern kit.

old frames often have massive seat tubes and short top tubes, kinda the opposite of compact.

I was once told height in meters x 30 = road bike size. Which kinda works if your average, riding an average bike. I went up a size as I prefered the more stretched position.

so roughly. 1.75 x 30, i'm figuring 17.5 x 3 = 52.5 cm abouts? do they ride different with the top tube sloping down towards the stem rather than towards the seatpost?

That's right, they had longer seat tubes. That Gios is about 56cm but with the reach of a 52mm. Go back even more years and you had about 3" of post exposed.

my mates got a 'mick ives'- not sure the size and the frame is virtually all the way to the saddle. on the wrong size?

Quite shocking seeing the old and new next to each other. The Gios is so much neater and nicer, despite (to my eyes, sorry) dubious paint scheme.

Most likely the comfort of the Planet X would be transformed with a slimmer seat post.

This "height in metres x 30" rule of thumb… is that for the effective centre-to-top distance for a hypothetically horizontal top tube?

Think my 1989 frame is 21in c/t, but the rule of thumb suggests a good inch or 2 bigger (which I wouldn't dispute – bit more reach might be nice actually).

Mick Ives! I'm probably racing him next weekend.

Yeah the colour is very 80s

I'm 5'8" and ride bikes with 56.5 top tubes, so just go to a LBS and ask there. It'll be a lot simpler rather than trying to do it with someone's excel spreadsheet figures.

I'm 5'8" and ride bikes with 56.5 top tubes,

i'm 6ft1 and ride 56.5 effective top tubes too. which just reinforces my point that you can't really work off other peoples sizing.

Yes, which is why I wrote go to your LBS immediately afterwards…

Yeah aP I noticed the difference when the two crossers were leant together at the Rapha.

hehe! – that's a 54 just to confuse things…

Yes, which is why I wrote go to your LBS immediately afterwards…

i know, i was just reinforcing my earlier point about other peoples sizing.

I'm 5' 4" and I'm getting a 50cm road bike (although I can ride a 52cm comfortably if its the right one).
Definitely a case of try a few and see what you like 🙂

I'm 5' 8" and ride a 47cm Merckx, a 51cm Bianchi, a 21" Dacorrdi and a Small Giant

I've ridden everything from a 59cm to a 62cm (I'm 6'3") without a problem, they've all needed slight tweaks to saddle position/stem length etc to get them right but it just goes to show, that's a difference of 3cm between manufacturers as to what is "ideal" for me.

Even if you don't plan on buying new, at least go to a shop and see what's available, try out a few and get a feel for which brands would suit you.

And don't call it darkside…

seen a dolan, 47 cm. is there any concensus on dolan sizing? is it too small.(for 5ft 8)cant test ride it-its too far away. ok,if its not the darkside what should i call it (to mtb'ers)?