geometry head scratcher

[jpw]

I seek the wisdom of The Forum.

It's a frame geometry thing.

Seat tube length 52cm
Seat tube angle 73 degrees

Top tube length 59cm
Top tube slope 6 degrees

Head tube angle 73.5 degrees

If a better person than i can grasp these figures what would the frame size 'be' with a horizontal top tube arrangement?

What size of rider is this frame for?

Any clues would be most welcome.

Yours hopefully.....dumbo.

[dbrk]

This is compact frame obviously and it will fit like a virutal 59, depending on the headtube length. Count on that. It has a longish toptube even for a 59, so it would be good to be long in the torso or reach. Sounds like Serotta numbers: more or less neutral, more or less "recreational" rather than too racy or French fit-like. Even-handed, from the numbers alone though rake/trail and ht length would make it that much clearer.

dbrk

[jpw]

I don't have any other numbers to hand.

From what you say this frame is too looooong in reach for me.

Another failed DKS possibility.

Disappointed.

[adavis]

Is the 59 the actual top tube length or virtual? If it is the actual length, the cooresponding virtual length would be longer than 59.

adavis

[jpw]

that's all the info i have. These compact frames really fox me.
Never yet ridden one, and i'm beginning to think they are not worth the trouble.

[Peter]

I'm no math genius, so I looked up info on how to "solve" a triangle, and with the information you've provided I performed the calculations; the "virtual" seat tube length would be 58.2cm.

My opinion is compact frames are a gimmick, unless you're too short to fit a conventional frame.

Your concern should be that you're provided with a sufficiently tall head tube to give you adequate handlebar height.

[Needs Help]

Well done! I'm getting something slightly different, though. Here is a picture:

http://www.pbase.com/image/26666226/original.jpg

First, you need to determine angles a, b, and c. There is a theorem that states: "when a line intersects two parallel lines, the alternate interior angles are equivalent", so if you examine the diagram you can see that:

angle a = 73º and
angle b = 6º

Since the angles of a triangle must add up to 180º, that means

angle c = 101º

Incorporating the calculated angles and the given TT length of 59 cm into the bicycle diagram gives you something like this:

http://www.pbase.com/image/26680205/original.jpg

Now, to determine the seat tube length and the top tube length on a traditional frame, you need to determine the lengths of sides A and B.

For a general triangle:

A / sin aº = B / sin bº = C / sin cº
http://www.pbase.com/image/26680167/original.jpg


Returning to the bicycle diagram:
http://www.pbase.com/image/26680205/original.jpg

and solving for side A first, you get:

A / sin 101º = 59 / sin 73º
A = (sin 101º)(59 / sin 73º)
= 60.56 cm

Solving for side B you get,

B / sin 6º = 59 / sin 73º
B = (sin 6º)(59 / sin 73º)
= 6.44 cm ===>>> adding that to the original ST of 52 cm gives 58.44 cm

So, by my calculations, the equivalent frame with traditional geometry will have a TT measurement of 60.6 cm and a ST measurement of 58.4 cm.

Note: all my calculations assume C-C measurements.

[Kevin]

nm.

Kevin

[Needs Help]

lol. You're in the wrong thread, Kevin.

[jpw]

I don't know what kind of help you think you might need, but it's not in the area of mathematics. I'm impressed with the energy and effort you have made in helping with my query, and i thank you.
From your calculations the frame in question is even larger than i had thought. Compact frames are such awkward things to get to grips with. I can see that they do address standover concerns, but that apart they seem to me to be too much trouble. Having said that, there's a compact Carl Strong picture in the bikefanclub Serotta gallery,......well, it's looks lovely. It's the one on the veranda. Exquisite. Nicer looking than...er, Serotta models. Did i say that? Here?
Thanks again.

[Peter]

Your calculations resulted in a different top tube measurement because you measured along the axis of the top tube rather than what I did, which was to assume the dimension given was a virtual/horizontal top tube. No matter; our numbers vary by only 2mm which in seat tube length is almost irrelevant.

[Needs Help]

...your welcome. I just did it to piss off flydhest. :)

[Andreu]

:banana: Well done! I'm getting something slightly different, though. .
At last I've found the science behind it all!
thanks for this....I can go away and ponder the figures and actually come up with some rational arguments for and against the compact bike instead of hearsay, and poppycock!
Cheers
:beer:
A

[Andreu]

Well done! I'm getting something slightly different, though. Here is a picture:

http://www.pbase.com/image/26666226/original.jpg
.
Just one question....
if I were to be foolhardy :rolleyes: enough to buy a compact frame does the mean I have to calculate the equivilent "straight top tube" dimensions to get a bike that would theoretically fit me?

Thanks
A :beer:

[DWF]

Just one question....
if I were to be foolhardy :rolleyes: enough to buy a compact frame does the mean I have to calculate the equivilent "straight top tube" dimensions to get a bike that would theoretically fit me?

Thanks
A :beer:

You shouldn't have to. Bicycles, compact and otherwise, do adhere, somewhat, to a sizing standard. Advertised top tube length for compacts is [almost] always given as the "effective" length, i.e., center of the TT/HT junction to center of the TT/ST junction as if the TT were parallel to the ground as it is for conventional frames.

Seat tube length gets a little fuzzier as manufactures state them as "center to center" (center of the TT/ST junction to center of the BB); "center to top" (center of the BB to the top of the TT at the ST junction); or as seattube length (center of the BB to top of the ST); or now with the advent of compacts, "effective length" which typically is referred to as "center to center".

Compacts are not a gimmick, it's just an option. If you're a traditional died-in-the-wool roadie, it's an insult to all that's holy and should be mocked. If you came to road bikes from MTB'ing, it's old home week. What matters is the fit and the rider's balance on the bike. How the tubes intersect makes little "real world", i.e., measurable by your butt, difference to most folks.

Personally, I prefer horizontal top tubes on road bikes, or at most, just a few degrees of slope. I can say that a compact road frame equipped with a long seatpost does impart some suspension effect. A seatpost designed to be flexible like the Moots can give them a "softride"/flat-rear-tire feel that some folks really like and some folks find disconcerting.

[Andreu]

Thanks for this. I am a bit of a "sloping top tube activist" at the moment (as you can probably tell by the number of posts I send on the subject!). I am learning alot about them through this forum. I have just bought a traditional frame (straight TT) but my next frame is definitely going to be a Serotta...I just love the look of them (Ottrott and Legend). I guess if I am going to spend a hefty percentage of my annual salary on a frame I want something that looks good but also fits! and I am seriously exploring the idea in my head of a slight slope if this means better performance and fit. I just want the facts and I am willing to eat humble pie.
Thanks again for the info....and I'll sleep on it.
A :beer: