Why the Slopping Top Tube?

John has noticed that top tubes or cross bars are now running down from the head tube to the seat tube, unlike the more traditional frame designs where the top tube is horizontal. He wants to know why this is? These are the reasons

Question:
why does the cross bar on bikes are not straight like the originals. but at a slope or off lower down the down tube?

Hi John,

The slopping down tube or cross bar was a design that originally appeared on Giant bikes quite a few years ago.

There are a few different reasons for this design:

First it makes the main triangle and the rear triangle smaller so that it will be lighter and stiffer than the more traditional frame design.

Secondly it cuts down the amount of different frame sizes that the manufacturer has to produce as now they need only small, medium and large and then possibly an extra small and an extra large. This is great for carbon frame production as a mould has to be made for each size and they are very expensive.

The handling of the frame is very similar to the original designs and some frames feel more “lively” with the compact design. Is this what you needed to know, John?

3 Responses to “Why the Slopping Top Tube?”

Ken Wilkes August 17, 2009

The editors pretty much hit the high points on frames with slopping top tubes. It also allows a taller head tube, without the frame looking funny. This allows the rider a more upright riding position and more comfort.

Justin August 17, 2009

I had a cheaper Avanti with the sloped bar and I personaly think it was a bad idea as the seat post ended up so frigging long that that it flexed a great deal kind of defeating the purpose of the stiffer frame. I agree it makes frames cheaper to build which is why they do it, not that it is actually a better design. I think it also gave them a destinction between old and new bikes for a period so that people could look at a new bike and say “oooh thats a new bike”.My new bike is a standard frame (granted all C7 carbon) and is a lot stiffer and doesn’t have the flex in the seat post, it is also a lot more comfortable to sit on the frame at the lights when the bar is higher and horizontal.

Mark Gaines August 17, 2009

I agree with the answer as far as performance bikes are concerned, but I have found that comfort and touring bikes are much more comfortable if they have the old style large triangles with level top tube. Thank you.