effective top tube length

[richpur]

I stumbled on a good price on a frame that has a shorter top tube than I generally use. My current frames have a top tube length, horizontal, of 600mm-610mm with seat tube angles in the 72 degree range. The frame I'm looking at has a horizontal top tube thats 570mm, but the seat tube angle is 76.5. Would appreciate any help in comparing the differences. It seems like the higher seat tube angle would effectively lengthen the top tube mesurement. Hope this all makes sense. Help appreciated. :beer:

[David Kirk]

76.5° ???

That is very steep. Very Steep. I can't think of a way you'd be able to side the saddle back far enough on most any post to replicate your position.

A very rough rule of thumb is that 1° equals about 1 cm at the saddle on bigger bikes. This means you'd need to slide the saddle back on the post by about 4.5 cm. That's lots of cm.

Dave

[DfCas]

The problem would be getting a seatpost with enough setback to get your saddle position the same as your 72* bikes, unless you ride far forward on those bikes.

The 76.5* seat tube would move your saddle forward about 6.5 cm compared to a 72* seat angle.

[salem]

Without knowing your seat tube length, one can't say just how much the steep angle will effectively lengthen the TT, but you can use the scientific calculator function that comes with Windows to figure this out. Sine=opposite angle/hypotenuse.

If you don't remember trig: subtract seat angle from 90. Multiply this by the c-c seat tube length for a horizontal top tube. Compare this answer to those for other angles and you'll find the effective difference. I ride roughly 56cm frames, c-c, so one degree of seat angle is about 1cm of TT for me.

As for seatposts, this one and an old FSA model have some of the most set back I've seen.
http://www.amazon.com/Seat-Post-TruVativ-27-2-Bolt/dp/B0013J726M
Also, check VeloOrange, as they made one for the Brooks saddles, which have short rails that limit setback.

[ahumblecycler]

As stated above, FSA has the most dramatic setback I know.

Two questions:
1) What is the head tube angle of your current ride and the potential purchase?

2) Is it a TT bike? I ask b/c TT bikes have steeper seat tube angles.

[jds108]

Here is a seatpost with plenty 'o setback:

http://www.rivbike.com/products/show/nitto-wayback-seat-post-272-x-250mm/11-048

[jlwdm]

You could probably get a tri post and reverse it to have a lot of setback but do you really want to? If you ride a 72 degree sta buying a 76.5 makes no sense.

It will also be a tough sell when you want to get rid of it. That is why the price seems good, but might not be good.

Jeff

[mnoble485]

76.5° ???

That is very steep. Very Steep. I can't think of a way you'd be able to side the saddle back far enough on most any post to replicate your position.

A very rough rule of thumb is that 1° equals about 1 cm at the saddle on bigger bikes. This means you'd need to slide the saddle back on the post by about 4.5 cm. That's lots of cm.

Dave

'nuff said. Listen to the master.

Mike

[Dave]

Obviously the wrong frame for the intended purpose. That STA is for a TT or Tri bike, not a road bike.

Although you do need the center to center frame size to figure out the exact change in reach, it's certainly going to be in the 8-10mm per degree range. The exact amount is (cosA-cosB) times the center to center frame size, or .076 times the c-c frame size.

[ultraman6970]

I think the OP should start indicating the brand and models of both bikes (and maybe pictures) because clearly one is a track or a TT bike and the second one is a touring bike.

Secondly nobody knows if the one he is using is fitted right, since the OP wants to bike a bike maybe 5 sizes bigger that's a bad indication, or one bike is fitted really wrong, or he is looking at the wrong frame (a 61)

[veloduffer]

I stumbled on a good price on a frame that has a shorter top tube than I generally use. My current frames have a top tube length, horizontal, of 600mm-610mm with seat tube angles in the 72 degree range. The frame I'm looking at has a horizontal top tube thats 570mm, but the seat tube angle is 76.5. Would appreciate any help in comparing the differences. It seems like the higher seat tube angle would effectively lengthen the top tube mesurement. Hope this all makes sense. Help appreciated. :beer:

That frame doesn't fit!! It's not a good deal if the frame doesn't fit you, and there is no shortage of good used bikes these days. You'd be much better off waiting for a bike that fits you better.

[richpur]

Thanks for all the information. As usual the forum has proven to be a valuable source of learned opinions. I'll keep looking. :beer: