Sloping top tube

[wc1934]

Hi all,
I am looking for as much info as possible about sloping top tubes. For example, is it possible to replicate/duplicate the same reach and fit as a bike with a non sloping tt - If so, is there a formula? My current bike has pretty much standard geometry - How would I transfer those numbers to a sloping tt bike, or am i trying to compare apples to oranges? What are the pros/cons of a sloping tt?
thanks

[Louis]

From a user's perspective a sloping TT affects you in only two ways:

1) Reduces stand-over height
2) Aesthetics

There are a few other things like seatpost length, but IMO those are the two biggies.

Edit: Be sure to compare apple to apples when it comes to TT length:
"virtual" for a sloping TT frame = "normal" on a horizontal TT frame

[tuscanyswe]

Hi all,
I am looking for as much info as possible about sloping top tubes. For example, is it possible to replicate/duplicate the same reach and fit as a bike with a non sloping tt - If so, is there a formula? My current bike has pretty much standard geometry - How would I transfer those numbers to a sloping tt bike, or am i trying to compare apples to oranges? What are the pros/cons of a sloping tt?
thanks

Yes its possible and very easy even to replicate/duplicate the same geometry regardless of sloping or level top tube.

There is really no "formula" but its very easy. Draw a few identical frames with no top tube. Then put in the top tube with different degree of slopes. It will be apparent to you what else needs to be adjusted along with the change in top tube angle. (length of seat tube and where the seat stay mounts on the seat tube will be affected).

With a sloping top tube you get a lower stand over height (measured center of the top tube) that is really the only difference that is worth mentioning that isn't mearly a visual preference regarding the change in top tube slope.

[Ralph]

When looking at new bike frames, and most have sloping top tubes, they give the TT length as if it was horizontal. So you can compare to older way.

The exact formula would probably be 8-9th grade math......trig probably. If you know say two angles...head tube and seat tube, and know two sides....seat tube and down tube, you could solve for top tube length. But hey....it's been a long time since I did any of that.

[wc1934]

From a user's perspective a sloping TT affects you in only two ways:

1) Reduces stand-over height
2) Aesthetics

There are a few other things like seatpost length, but IMO those are the two biggies.

Edit: Be sure to compare apple to apples when it comes to TT length:
"virtual" for a sloping TT frame = "normal" on a horizontal TT frame

Thanks Louis - that is the kind of info I am seeking.
So, if I usually ride say a 52 in a horizontal tt frame what size should I order in a sloping tt frame?

[choke]

So, if I usually ride say a 52 in a horizontal tt frame what size should I order in a sloping tt frame?The manufacturer should list the 'virtual top tube' in the specs....that number is the equivalent of a horizontal top tube.

[Ralph]

You could do it backwards. Pick a frame that gives a good chart of it's dimensions.....like Cannondale CAAD 10 or similar....goggle it's geometry and see what the actual TT length w/b on a frame with a 52 or so sloping TT. You will get close.

[wc1934]

The manufacturer should list the 'virtual top tube' in the specs....that number is the equivalent of a horizontal top tube.

AHHHHH - now I get it. And thus the reach to the bars would be the same.

[Louis]

And one other thing - for a sloping frame ST length and frame size will likely no longer match, since the ST-TT intersection is "too low".

Assuming nothing else (e.g. ST angle, HT length or angle) is changing, all the sloping TT does is move the TT-ST intersection to a lower point on the ST.

http://georgenaterry.com/wp-content/uploads/2012/05/sloping_tube.jpg

[Louis]

Remember, fit-wise the critical items are where your sit bones are relative to the BB and where the bars are relative to that. What the frame tubes do between those points is meaningless (for fit).

(Ignoring some other secondary stuff like stand-over and toe-clip overlap.)

[Curve_in]

Remember, fit-wise the critical items are where your sit bones are relative to the BB and where the bars are relative to that. What the frame tubes do between those points is meaningless (for fit).

(Ignoring some other secondary stuff like stand-over and toe-clip overlap.)

+1 which means a steeper seat tube angle frame needs to have a shorter top tube to fit the same way.

[johnmdesigner]

Also consider the seatpost area. Is it conventional round or proprietary?
More post will be exposed on the sloping frame. Sometimes much more. Be sure you have enough to get your saddle in the proper position.

[bicycletricycle]

The angle of the top tube is relatively meaningless as far as fit goes. Stand over is the biggest/only "fit" issue a top tube slope effects.

More stand over can also be gained with an extended head tube.

Also, level top tubes are nicer to sit on when your waiting to start a ride or taking a break. As long as the brake cable isn't routed externally on the top of it.

A more compact frame can technically add some vertical compliance because of the relative change in orientation of the seat stays as well as the increased cantilever of the seat post (especially if it's a flexy seatpost) I think the seat post is the bigger factor in ride quality change.

[Peter P.]

For example, is it possible to replicate/duplicate the same reach and fit as a bike with a non sloping tt - If so, is there a formula?

Yes; it's easily possible. You're interested in duplicating the "virtual" top tube length, which is how most companies specify their top tube lengths.

You will also want to duplicate your head tube length or, at least be able to duplicate your favorite handlebar height by adjusting the number of stem spacers as well as the stem's angle.

How would I transfer those numbers to a sloping tt bike?

You are primarily interested in duplicating your STACK and REACH and seeing how closely a prospective bike's stack and reach numbers copy your current bike's numbers.

HERE'S (http://velonews.competitor.com/2012/05/bikes-and-tech/technical-faq/technical-faq-setting-your-bikes-up-identically_216035) a very easy method to measure the stack and reach of your current bicycle.
Currently, not all companies are including stack and reach measurements in their geometry charts because the concept of stack and reach is relatively new.

What are the pros/cons of a sloping tt?

A sloping top tube enables a builder more flexibility in designing a frame that can get the bars high or low enough for a rider. It eliminates the constraints of a horizontal top tube bike where a rider might need or want the bars in a higher or lower position but is constricted by the necessity of having a seat tube long enough to get proper seat height. Since you can make the top tube slope virtually any angle you want, you can determine seat tube length independently from head tube length/handlebar height.

There are really no drawbacks to sloping top tube frames.

[shovelhd]

Good point about the head tube length. Some bikes like the Giant OCR get the slope from a longer head tube. This puts you in a more upright position.

[Catdr]

good point about the head tube length. Some bikes like the giant ocr get the slope from a longer head tube. This puts you in a more upright position.
+1