cooleric1234

What would you consider the effective top tube measurement to be? In most of my reading it seems like it starts at the top of the head tube, half-way between the front and back when viewed from the side. And from there it's a horizontal line through the middle of a seat post that continues the angle of the seat tube. Is that correct?

The reason I ask is because it appears from the graph that Fuji bikes uses the same point at the head tube but goes horizontally until the vertical line that intersects the middle of the top of the seat tube. I know mere mention of bikesdirect is polarizing but this is important as it appears bikesdirect uses the same mesurement for their geometry. Very good to know if you're ordering a bike online.



You may think the difference is trivial, but it isn't necessarily. I was looking at getting a bikesdirect Titanium bike which has a "semi-compact" geometry. Not fully compact but not a perfectly horizontal top tube either. The largest frame lists the "effective top tube" length as 585 mm. However, when I emailed BD and asked what the measurement is if you continue the horizontal line until it intersects the middle of the seat post they said it would be 605 mm. That's a difference of 20 mm, which is quite significant. Are you finding your BD bikes to be larger than you would have expected?

Stickney

I have no measurements and have only ridden my BD bike about 5 miles (I just got it last week) but I actually feel like it is smaller than I anticipated. I have another bike with nearly the same geometry and it feels longer (I feel more stretched out). The stems are the same length, but I am fiddling with saddle position. The seat tube angle is 0.5 degree different.

Just my very ignorant 2 cents. And this is on a TI cx bike which has a more standard geometry and thereby could account for the difference.

cooleric1234

Thanks. I guess I should clarify my question. There are a lot of variables. I suppose the only way the bikes would feel more stretched out is if you bought specifically with the effective top tube length in mind based on previous experience. Not so much if you just bought for a given size bike.

waterrockets 

I have a 2010 Fuji SST 2.0 and it measures correctly to the middle of the seat tube (well, if it was a round tube -- it's forward of the center of the mast, right at the fattest portion of the mast).

StanSeven

If I understand what you're saying correctly, the difference is Fuji measures it to where the tope of the seat post is rather than where a horizontal top tube would meet. 2 cm difference seems reasonable if that's it. I have a Fuji track bike and its measured on the seat tube from C-Top of the seat post. That's a full 2 cm difference is the size.

So assuming they take the horizontal measurement for the top tube from there to the head tube, what they told you makes sense.

cooleric1234

I think that's right. Imagine you put a plumb bob to where the tip touches the top of the seat post in the middle. I think Fuji measures a horizontal line from the middle of the top of the head tube to where that horizontal line intersects the plumb bob.

StanSeven

I just edited my prior post. Then I think that bike will have what other companies consider a 58.5 cm effective top tube.

cooleric1234

Crude illustration attachedtt..png

dstrong 

Hmmmm...I never really thought about it...but now that I am, here's what I always believed...

"Classic" top tube length on a standard geometry bike, with a horizontal top tube is measured from the c-to-c head tube/seat tube, along the side of the top tube.

But with compact geometry, the end of the top tube is lower on the seat tube, effectively shortening the top tube.

I would contend that the effective top tube length should be measured starting at the center of the head tub/top tube intersection and go horizontally back to intersect the center of the seat tube.

Cruder illustration attached...

effective top tube..JPG

The further above the top tube you measure, the more difference in length there will be between the vertical line and the line of the seat tube.

But that's just one man's opinion who spent way too much time just now thinking about this and adding to your crude illustration.

cooleric1234

Good point. Now that you mention it I do recall it being defined as the middle of where the top tube intersects the head tube. I suppose the numbers would be very similar, the same when the seat tube angle and head tube angle are the same.

DaveSSS 

The proper way to measure the TT length has never changed. It has always been from the HT/TT intersection point, along a horizontal line, to the center of the seat tube, or the center of the seat post, if the TT slopes. That makes it possible to compare traditional frame to sloping TT with NO error. Manufacturers may call the TT length "effective" or "virtual" but it all means horizontal.

The height at which the measurement is taken will not make much difference, since the TT and HT are either parallel or close to it. A difference of 1-2 degrees over the short distance from the HT/TT intersection point to the of the head tube would make a trivial difference.

What's also being overlooked is the effect of differing STAs. If a STA is one degree steeper on a frame, it increases the reach by about 1cm.