Geo Question: Does top tube length matter anymore?

[XXtwindad]

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?

[tv_vt]

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.

[EB]

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.

[Spaghetti Legs]

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.

[Mark McM]

Originally Posted by XXtwindad

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?

That depends on the seat tube angles. For the same seat tube angle, then yes, a 580mm TT + 110mm stem would have the same reach as a 570mm TT + 120mm stem (all else being equal).

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.)

[EB]

Originally Posted by Mark McM

That depends on the seat tube angles. For the same seat tube angle, then yes, a 580mm TT + 110mm stem would have the same reach as a 570mm TT + 120mm stem (all else being equal).

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.)

I thought reach was measured from the bottom bracket forward, thus two bikes with different seat tube angles can have the same reach (but different setback), assuming the TT is lengthened - which it often is as bikes size up. So you also have to pay attention to that TT number, to see what changed - your rule of thumb only works if the TT is static.

But I also recall this has been a big bone of contention here before, so....

[prototoast]

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.

[MikeD]

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).

[bicycletricycle]

Yes, if you care about what your seat post looks like

[mhespenheide]

Originally Posted by XXtwindad

... 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?

Others have already agreed that reach is a better measurement because of the top tube's interaction with the seat tube angle.

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.

[mhespenheide]

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.)

[prototoast]

Originally Posted by mhespenheide

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.)

No single metric can capture everything. In two-dimensional space, you have 3 contact points: feet, butt, hands. If you define everything in reference to the bottom bracket you still need 4 parameters to define butt and hands. Because seatposts have so much adjustability, we can often talk about just 3 parameters, and in the old days of lugs when the angles were largely fixed, we could often talk about just 2, but again, any reduction beyond 4 parameters depends on making some assumptions.

Or, to put it another way, if you look at the Pegoretti geometry chart, seat angle is a better indicator of size than reach.

[weiwentg]

Originally Posted by Mark McM

That depends on the seat tube angles. For the same seat tube angle, then yes, a 580mm TT + 110mm stem would have the same reach as a 570mm TT + 120mm stem (all else being equal).

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.)

Within broad categories, I suspect that STA only varies by 1.5-2 degrees. For example, for stock bikes in my size, I typically see 74 degree STAs, with some at 73.5 and some at 73. Gunnars in my size had 75, which was really an outlier. Some smaller Colnago or Pinarello frames might have had 75, but I think they’re smaller than what I would ride.

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.

[Mark McM]

Originally Posted by EB

I thought reach was measured from the bottom bracket forward, thus two bikes with different seat tube angles can have the same reach (but different setback), assuming the TT is lengthened - which it often is as bikes size up. So you also have to pay attention to that TT number, to see what changed - your rule of thumb only works if the TT is static.

Well, yes, which is why I included that in my statement:

"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.

[Mark McM]

Originally Posted by weiwentg

Within broad categories, I suspect that STA only varies by 1.5-2 degrees. For example, for stock bikes in my size, I typically see 74 degree STAs, with some at 73.5 and some at 73. Gunnars in my size had 75, which was really an outlier. Some smaller Colnago or Pinarello frames might have had 75, but I think they’re smaller than what I would ride.

Thus, I suspect that as long as you know your sizing in either format, you should be able to get decent results.

It's often good enough for a rough approximation, unless you're at extremes of Reach for a given Stack. For example, for typical road frames with appropriate stack, I find that with modern compact handlebars I often have to use a 12cm or 13cm stem. If a frame had a particularly shallow ST angle, then for the same TT I might need a 14cm or 15cm stem for the same fit, which can limit stem options. At the other extreme, if someone is using a 6cm or 7cm stem with a steep seat angle, then for the same TT length they might need a 4cm or 5cm stem - also limiting stem options. So in some cases, one needs to pay attention to both TT length and STA to determine if a frame will fit. (Which again points to why Reach can be a more direct measurement for frame fit.)