Does sloping TT = smaller frame?

[scottcw2]

Thinking again in my idle time, I was wondering about my bike herd and one in particular. My ideal frame size is a 58 TT with an 11cm stem.

However, I have one frame that has a 56 TT and a 12cm stem that I am able to set up so all my contact points and set back are the same as on my 58 bikes. It's always puzzled me how this is so, but isn't this the same idea as a sloping TT?

So is a frame with a sloping TT effectively the same as a smaller non-sloping frame with a longer stem?

[Pete Serotta]

Is your seat to bar drop the same on both? are the stems the same angles and are the bars the same. (Keep in mind a cm is only 3/8 inch about.)


tt length (effective or actual) counts as does your st angle and ht angles - are they the same?

on a level tt serotta and a compact serotta with same angles bars and stem (as well as seat post and seat) .....It feels the same to me. But then I am old and slow ;)

[scottcw2]

Is your seat to bar drop the same on both? are the stems the same angles and are the bars the same. (Keep in mind a cm is only 3/8 inch about.)

Drops are close, but I never worry about seat to bar drop. I set the saddle height from c of BB calculated for ST angle, then adjust the set-back and call the drop good. Stems and bars are all similar.

[happycampyer]

Thinking again in my idle time, I was wondering about my bike herd and one in particular. My ideal frame size is a 58 TT with an 11cm stem.

However, I have one frame that has a 56 TT and a 12cm stem that I am able to set up so all my contact points and set back are the same as on my 58 bikes. It's always puzzled me how this is so, but isn't this the same idea as a sloping TT?

So is a frame with a sloping TT effectively the same as a smaller non-sloping frame with a longer stem?I don't think that the slope of the top tube is the factor. For example, Moots makes two road bikes: the Vamoots and the Compact. The geos are identical in each size (sta, hta, hth, bb drop, etc); the only difference is that the tt slopes on the Compact and is horizontal on the Vamoots.

Are the seat tube angles the same? A 55cm tt with a 74º sta could have a similar reach as, say, a 56cm tt with a 73º sta, for a given saddle position, hta, hth, bb drop, etc. The slope of the tt won't affect the other points in space.

[bironi]

I don't think that the slope of the top tube is the factor. For example, Moots makes two road bikes: the Vamoots and the Compact. The geos are identical in each size (sta, hta, hth, bb drop, etc); the only difference is that the tt slopes on the Compact and is horizontal on the Vamoots.

Are the seat tube angles the same? A 55cm tt with a 74º sta could have a similar reach as, say, a 56cm tt with a 73º sta, for a given saddle position, hta, hth, bb drop, etc. The slope of the tt won't affect the other points in space.

My Vamoots TT slopes, but not nearly as much as a Compact Moots. :beer:

[scottcw2]

Are the seat tube angles the same? A 55cm tt with a 74º sta could have a similar reach as, say, a 56cm tt with a 73º sta, for a given saddle position, hta, hth, bb drop, etc. The slope of the tt won't affect the other points in space.

73º sta on the 56 and one 58. 72.5º sta on the other 58.

[happycampyer]

My Vamoots TT slopes, but not nearly as much as a Compact Moots. :beer:Yeah, the TT of my Vamoots slopes a bit, too. Oh well, so much for that theory... :)

[giordana93]

head tube angle makes a difference. slacker will bring the bars further back, decreasing reach

sloping top tube does not make a difference, unless you forget that the actual top tube on a sloper is a longer "virtual" top tube, so it is the opposite of your original question: a sloping top tube 56 is actually bigger than a horizontal 56 both because the virtual top tube is longer and because it more likely attaches to a taller head tube. think of it as a 58 with increased stand over height. which has nothing to do stem length really, which is getting back to HTA.
also, the effective top tube length on the slacker sta will be about .6cm shorter between your 73 and 72.5 if your seat has the same set back in relation to the bb. it's kinda late here, so sorry if this isn't clear.

don't know why I had to add this, but the dave moulton site is always a good reference and link:
http://www.prodigalchild.net/Bicycle6.htm

see his notes on compact frames down on the page

[happycampyer]

That's exactly right--a slacker sta and/or steeper hta from one bike to the next will make the effective top tube longer, assuming other dimensions are held constant (e.g., head tube height, and esp where the tt meets the ht, as well as bb drop). You might like toying around with the bike CAD tool from bikeforest:

http://www.bikeforest.com/CAD/index.php#

[giordana93]

That's exactly right--a slacker sta and/or steeper hta from one bike to the next will make the effective top tube longer

http://www.bikeforest.com/CAD/index.php#
actually, it's kind of the opposite in practice: a 54 cm TT on a slacker STA is effectively shorter than on a steeper one, if the rider uses the same saddle set back. The only time the slacker STA yields a longer effective top tube is when a steeper STA prevents one from getting the seat far enough back (which happens much more often on smaller frames that also usually have steeper STA). this is because the set back gained by the slacker angle is effectively subtracted from the TT length. so on paper, it looks longer, until you put the rider contact points into the equation.

[Dave]

Reagarding the sloping TT length, every brand reports the horizontal TT length, but they might call it effective or virtual. It's all the same. I don't know why any geometry chart even bothers to list the length along the slope.

As for a smaller frame being the same a sloping one. Nope. The true vertical size of a frame is determined by the head tube length, with the headset (for a given BB drop). If there is a significant difference in the BB drop that must be accounted for. Although there are also some differences in the fork length between brands, they are most often 7mm or less at the extremes. Some brands list the fork length and others don't. The ones that don't are not providing all the info needed by the consumer.

Head tube angle makes very little difference in the fit - on the order of 1-2mm per degree compared to 8-10mm per degree for changes to the STA.

Steeper STAs make the frame reach longer, for a given TT length and saddle fore/aft position. If you've got two frames with a 53cm TT, but one has a 73 degree STA and the other a 74, the later will need about one size shorter stem to fit the same, with the saddle in the same position relative to the BB.

[happycampyer]

I agree with both of you--perhaps I used the wrong term when I said "effective" and should have said "virtual" or as giordana93 put it, "on paper." To the OP, I think that we are all saying that it's very possible for a bike with a "56"cm top tube with a 12cm stem to fit like a bike with a "58"cm top tube with an 11cm, and the slope of the top tube is not a factor.

I was referring to the horizontal line that connects the head tube and the seat tube. If you have one bike with parallel seat and head tubes (e.g., 73º sta/73º hta) and another with a slacker seat tube (e.g., 72.5º sta/73º hta), for a given head tube height and bottom bracket drop, etc., the measured top tube on the second bike will be longer, but if the saddle is in the same place relative to the bottom bracket (creating the same "effective" seat tube angles on both bikes), the reach should be the same using the same stem. OTOH, if another bike has the same seat tube angle but a steeper head tube angle (e.g., 73º sta/73.5º hta vs. 73º sta/73º hta), the bike with the steeper head tube angle will have a slightly longer top tube, and may need a slightly shorter stem for the same reach. A fourth bike with a 72.5º sta and 73.5º hta will have a longer measured top tube still, but will fit like bike #3. These relationships will hold regardless whether any of the bikes have a horizontal top tube, a sloping tope tube, or no top tube at all.

[Dave]

Happycamper...

Just keep in mind that HTA changes create a very small difference in the fit - quite often 1mm per degree - not enough to change stem length. It's STA that has the far greater impact.

The formula to calculate the change is the same one: length x (cosA-cosB) where A and B are the two angles. The length however, is the c-c frame size for STA differences and only the small distance from the intersection point of the HT and TT to the top of the stem in the case of HTA changes. On my bikes the c-c frame size is 51cm and the other length is about 9cm.