#1
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Geo Question: Does top tube length matter anymore?
I realize that TT length has largely become the cycling equivalent of a MLB pitcher’s “win” totals: a metric that’s no longer really relevant. “Reach” and “stack” are now the coins of the realm. I don’t really have a firm grasp on those concepts.
But, in theory, if I ride a bike with a 580 TT using a 110 stem, then I should be able to ride a bike with a 570 TT using a 120 stem. Is that accurate? |
#2
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It's still one of the metrics I look at on geometry charts. To me, means more than Reach. Add seat angle and I get a good sense of how long a frame is.
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#3
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Stack and reach are more useful for me when comparing bikes with different geometries. Given the variations in things like bottom bracket drop, head angle, etc.
The missing ingredient with stack and reach is saddle setback, which can vary a lot with different seat tube angles. Most geometry charts don't provide it given variance in saddles and seatpost setback - occasionally you will see a proxy measurement. Worst case, you have to eyeball it from the seat tube angle. This is all assuming a road or gravel bike, of course. Modern mountain bikes are an entirely different kettle of fish reach-wise, as the rider's seated position has been pushed further and further forward and the reach has grown to accommodate. |
#4
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Top tube length (or ETT) + stem length still works for me but I’m still just into bikes with level or slightly sloping top tubes.
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#5
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Quote:
But if the TT stays the same, then changing the seat tube angle will change the reach. A rough rule of thumb is that a 1 degree change in seat tube angle results in change in Reach of about 10mm. So a 73 degree ST angle + 580mm TT would have about the same Reach as a 74 degree ST angle + 570mm TT. This is part of why Stack and Reach have become popular - it takes seat tube angle out of the Reach measurement. (The other main reason is that with sloping top tubes, seat tube length is no longer a good reflection of handlebar/stem height.) |
#6
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But I also recall this has been a big bone of contention here before, so.... Last edited by EB; 03-01-2024 at 11:21 AM. |
#7
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1) it matters, but not in isolation, as other parameters can affect things. But there are awesome so many ways to measure a bike, and there's no right way or wrong way, as long as you end up at a point where you're comfortable.
2) different bikes are designed differently. A lot of gravel bikes are designed to be run with a short stem, so they are going to have a long reach and a long effective top tube compared to a road bike that's designed to be run with a long stem. 3) with all that said, if I had to pick out a bike and was only given one single measurement to choose from, I would go with effective top tube length.
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#8
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Stack and reach are better metrics. You don't want to fit a stem that's too long or too short (assuming we're talking road bikes).
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#9
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Yes, if you care about what your seat post looks like
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please don't take anything I say personally, I am an idiot. |
#10
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But the other thing to consider relative to the question quoted above is that while moving the head tube back by shortening the TT but lengthening the stem will keep the handlebars in the same place, it will also shift the weight distribution a little more on the front tire and consequently a little less on the rear tire. Will that be something that's perceptible? Maybe, maybe not. I would guess that if you don't have an A/B comparison between two otherwise identical bikes, probably not. |
#11
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Also, another factor to consider is that reach really needs to be considered in combination with stack. At standard road bike head tube angles of ~73 degrees, adding 3mm of stack decreases the reach by about 1mm.
If you're old-school enough to have think about quill stems, consider that as you raise the quill, the bars move both up and back. The interaction between stack and reach is similar. So if you're comparing two different bikes and only have the reach numbers, you're not getting the whole picture. I've been burned by this before when I didn't pay attention to that when trying to decide between a 63cm and 60cm CAAD10. (To be fair, effective top tube length really needs to be considered in combination with seat tube angle, so it's not like reach is worse than ETT.) |
#12
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Or, to put it another way, if you look at the Pegoretti geometry chart, seat angle is a better indicator of size than reach.
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#13
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Thus, I suspect that as long as you know your sizing in either format, you should be able to get decent results. Of course, MTBs and tri bikes have much steeper angles than road bikes. I’m talking about comparing between drop bar bikes. |
#14
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"But if the TT stays the same, then changing the seat tube angle will change the reach. A rough rule of thumb is that a 1 degree change in seat tube angle results in change in Reach of about 10mm." And this is why using Reach (from the BB) is often better than using TT length for sizing a frame - if trying to determine the saddle-to-handlebar distance using TT length, you have to also compensate for the seat tube angle. |
#15
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