Frame geometry/sizing question

Ordinarily, if the top tube length increases, so does the seat tube. This is more of a rule of thumb rather than some law. (It does make sense though, doesn't it?)

On the other hand, if it is a compact frame (like Smallyfurry mentioned) this is probably not the case.

The seat tube would be longer but so would the headtube. What would make it more/less than 2cm would be the difference between head and seat tube angles. If the seat and head tube were the same angle it would stay at 2cm (parallelagram)

Hi, let's say we have two frames with top tube lengths of 56cm and 58cm respectively.

That doesn't necessarily mean that the larger frame will only be 2cm longer along the top tube, does it.

If you mean seat tube here, then no the top tube may not vary exactly with the seat tube.

A lot of track frames have square geometry which means seat tube length = top tube length, though.

On most bike frames the top tube increase in length less than the seat tube. This because as peoples height increases, it is mainly through having longer legs. So the difference in top tube length of these two frames may only be 1cm*. Reducing your stem length by 1cm would fix this.

(*guesswork)

Hi, let's say we have two frames with ***top tube lengths ***of 56cm and 58cm respectively.

That doesn't necessarily mean that the larger frame will only be 2cm longer along the top tube, does it.

If you do mean top tube length here then 58 - 56 = 2 still stands.

Mmm, the thing is a 58cm frame will have a longer seat tube and so you'll be higher up off the ground and further away from the handlebars, and the top tube will be 2cm longer.

So, the overall resultant effective distance from arse to handlebars will be more than 2cm, won't it!

no

you see, the top tube length is measured from the (centre of) seat tube. so no matter how long the seat tube is the top tube will be just at song as you measure it!

plus. a long seat tube will not put you higher off the ground as you will compensate by sticking the seat pin further in to achieve the right saddle position (determined by the length of your legs)

If the TT is perpendicular to the ST, you should be able to determine the ST length using trigonometry... Do you have any specific measurements of the bike you're planning on buying ?

Sorry if I missed something.

If the TT is perpendicular to the ST, you should be able to determine the ST length using trigonometry... Do you have any specific measurements of the bike you're planning on buying ?

Sorry if I missed something.

Methinks you missed something. TT is very seldom perpendicular to ST - angle is usually 72-75º. Perpendicularity is just weird-looking:

Of course, that doesn't stop you from using trigonometry - you can split an iscosceles or scalene triangle in to two right-angle triangles and combine trigonometry and algebra to figure things out.

2cm. Everything else means nothing.

If the TT is perpendicular to the ST, you should be able to determine the ST length using trigonometry... Do you have any specific measurements of the bike you're planning on buying ?

Sorry if I missed something.

Methinks you missed something. TT is very seldom perpendicular to ST - angle is usually 72-75º. Perpendicularity is just weird-looking:

Of course, that doesn't stop you from using trigonometry - you can split an iscosceles or scalene triangle in to two right-angle triangles and combine trigonometry and algebra to figure things out.

er you can use the cosine and sine rule in trigonometry which apply to non right angled triangles. the problem here is that:

a) a bike frame is not a triangle, at least not enough for using trig to be a better way of measuring the tube lengths than a ruler
b) you don't know the relevant angles, and you might as well just use a ruler.

what is wrong with the ruler?

^ +1 Ruler
^^ +1 Everything else means nothing

I've been riding the same 52 x 54 frame for a while now, was using an 80mm stem, just moved down to what we all guess is 50mm (not arsed to measure) and it still feels a bit too long for me. I was able to try a 52 x 52 frame a few weeks back and it felt lovely, but it was a track frame and I can't remember what the toe overlap was like. I'm considering track but wonder if the toe overlap would be awful...already have a little bit of trouble with that but it was helped out by going clipless.

So I was wondering, am I likely to find a 52 square frame other than a track frame? I'm trying to figure out how to do eBay searches, though that's not going well for me. How do you guys who get all these intricate searches on eBay do it??

do you want to look at/try my bob jackson?

its 49 st and 52 tt.

my langster is a similar size, teeny bit bigger and less toe overlap.

I've tried the 49 steamroller and that's too short a frame for me....hm. maybe the langster, though.

So I was wondering, am I likely to find a 52 square frame other than a track frame? I'm trying to figure out how to do eBay searches, though that's not going well for me. How do you guys who get all these intricate searches on eBay do it??

Searching for specific sizes on EBay is'nt very effective as people dont always the same standard to qoute size.

If you fit a 52 top tube, then a non compact geometry road frame will likely have a 49/50 cm seat tube. If you go with compact geometry, it could be as low as 44 cm. But the seat tube length is pretty irrelevent TBH. You should'nt really experiance more toe overlap with a smaller frame, as this is compensated for by increasing the seat tube angle and slackening the head tube angle (can sometimes be a problem though).

Just to confuse things further, I wonder if your bars might be a bit too low. Raise your bars and they will also move closer to your saddle.

Giving sizing advice over the internet is not easy. It's like telling someone how to trim a hedge over the phone.

Yeah, I know what you mean. I like bullhorns. I'm just not a fan of risers except for the odd go on someone else's bike.

Suppose the question should have been "How do I search for a frame on ebay that's 52 square?" : )

Suppose the question should have been "How do I search for a frame on ebay that's 52 square?" : )

You cant really. 52cm might be advertised as 20 1/2 inches, or as a small. You just need to narrow it down by frame materials etc.

Not sure why you'd specifically want a 52/52 frame though, when the 52cm effective top tube is the more important factor. Unless you have long legs and dont like the look of a lot of seat post showing.

Well, bc I've ridden 49s, which are far too short, and 53s, which are a bit big. had a go on a 52 x 52 and it was great!

Well, bc I've ridden 49s, which are far too short, and 53s, which are a bit big. had a go on a 52 x 52 and it was great!

I was'nt questioning the size. Preferred top tube length can only be determined by a long ride, as it depends on build (of your body not the bike), and flexibility, as much as body dimensions.

What I meant was that the seat tube need'nt be 52 for it to fit as well as the one you tried.

Ah, I getcha. The seat tube on this frame feels about right, I haven't been fit again since I went clipless. It's all a bit of a muddle once you start getting into the more specific aspects of frame sizing, and tbh, I'm not racing so it's not like it's do-or-die, just that I'd like to sort out some niggling issues with frame fit.

With the shorter stem I'd say I'm effectively at a 53 top tube and for some riding on the commute, it's quite nice, I'll go out on where the hoods would be, a bit of a stretch for going faster. I'd say I need to ride with people who are used to doing longer rides with variable terrain and see what they think about frame geometry for my uses. But when I get my road bike over from the States, all the questions will start up again. Heh...

Depends on how you use the bike too. I mentioned a long ride as I find my reach only settles in after a few km, as my aging back needs time to stretch. Also as mentioned by Chris667, bar height is important.

I have a track frame with a short top tube, and a large drop to the bars;
a cyclocross frame with a medium top tube, and a small drop to the bars;
and a 29er with a long top tube, and zero drop to the bars.

Its all a bit trial-and-error for me. But I'd be careful not to be fooled by a short test ride on someone elses bike. A short reach set-up can feel great at first but become cramped as the body warms up.

Anyway back too the original question about searching EBay. The only tip I can offer is to get used to the look of a 52cm frame and browse the pics. Its often easier to look at the head tubes, to judge size. I've become quite good at spotting 53's myself ;)

I'd also be wary of a short stem, i find that long stems give nice stable handling.

As i've said before you are welcome to try my bob which has a 52 top tube that i run with a 12cm stem. welcome to borrow it for a day or two as long as it is locked properly, should give you a chance to take it for a proper spin/try some different bars etc.

Very good point^

I think this bike riding thing is going to be a lifelong learning experience. Basically, I'd like two bikes, a bike that I enjoy for the day-to-day rides of less than 15 miles, jaunts around town, etc. and one for daily commute up and down medium-sized hills. One will prob be fixed, one prob geared. I do find that with my shorter stem, regular riding around is fine on the flats but then I go farther out and stretch out a bit when I'm warmed up and wanting to go faster. The problem with the longer stem was that I felt stretched out even on the flats, to the point where I was having problems with shoulder stability on the right. As a massage therapist, I sometimes overuse and overreach with my right shoulder, since I feel stronger on that side, so my muscles are a bit less there, in response. Maybe I should be fitted again, but fundz aren't what they could be at the moment.