Here’s some calculator online I used which shows stuff like this with colour coded graphs and such.

# Understanding reach and ETT

Posted by: gramboI am dumb and can't get my head around how steeper STA requires longer reach for descending (standing).

I think reach is just a dumb name for this measurement. When you say the word reach I tend to think of how far it is to reach the bars, which of course changes with seat height and the rider's anthropomorphic measurements. It's a number that's highly variable and constantly changing. Maybe ETT should be labeled re-labeled reach which is based on some theoretical standard human measurements and reach should just be top tube. Same thing for effective seat angle, it's a theoretical number that is highly variable.

You could have two people who are the same height but the same bike could feel wildly different if they have different leg/torso/arm measurements.

Been trying to make sense of the new geo changes with 27.5 to 29 differences and steep seat angles.

I've been mostly riding a new short travel 29er last couple of months mixed in with rides on my long travel 27.5. Both 460mm reach, 29er with a steeper seat angle.

Just pedalled around today on Sights in M 29, L 29, L 27.5. 455mm reach M and 485mm reach L's.

L Sight 29 at 485mm was far too vast, standover too tall, stack too tall, and just ungainly. Interestingly at identical reach L Sight 27.5 felt pretty close to ideal, maybe 5mm too long, but low maneuverable and confident. Also surprising was that the M Sight 29 also felt close to ideal. If purchasing a new Sight I'd probably have to think hard about what type of trails it would see to decide between a M 29 and L 27.5.

Also spent a couple of days this week riding a L Devinci Spartan 29 with 465mm reach. After a previous day on a Hightower, (brilliant bikes both), I'm convinced that 27.5 is the way to go for long travel on low speed technical trails.

Interesting findings for me, previously I thought that seated and standing reach numbers we're all I needed to translate across to a new bike.

Last edited by: Hepcat on Dec. 30, 2019, 7:03 p.m., edited 2 times in total.

You can't really understand how big a bike is just by looking at Reach. You need to evaluate Reach and Stack together. Two bikes with the same Reach values, but different Stack values will fit differently standing. Effective TT has a similar problem as you need to evaluate Stack and Effective STA at ride height to understand how he bike will fit seated.

Bit confused about this, when would stack be so extreme that it couldn't be adjusted to your norm with headset spacers or a bar swap? You mean like if you went from 29 short travel fork bike to 29 long travel fork bike?

Bikes will fit differently on the shop floor, but once you get home doesn't everyone adjust their stack to be identical across their bikes? Or even move headset spacers up or down on the shop floor if serious about purchase?

Maybe with extreme stack consistent bar height is not possible? Man, I think that would be a no-go for me. Hmm, I can see how that could be hang up point of long travel 29ers.

For my bikes it was basic: Short travel bike to long travel bike was a few mm's difference in BB height therefore few mm diff in bar height. Couple of headset spacers moved and voila identical and familiar. Mind you it's 29 short and 27.5 long travel...

I guess I took it as a given that everyone could adjust their bars to a consistent height, much like adjusting seat height to preference.

Last edited by: Hepcat on Jan. 2, 2020, 11:43 a.m., edited 1 time in total.

If you trig it out based on a 64 degree hta, you'll find that a 10mm spacer will increase stack by about 8mm and reduce reach by about 2mm give or take. So if you find a bike with the perfect reach but the stack is way too low, then you'll have to jam a bunch of spacers under the stem to get the stack up but you'll also lose a bunch of reach.

You could toss on a riser bar to increase bar height and somewhat maintain reach.

Last edited by: T-mack on Jan. 2, 2020, 11:33 a.m., edited 1 time in total.

I always think about reach and stack as follows: your body basically has to fit in between the bottom bracket and the handlebar. Reach and stack together determine how far you bar is from the BB.

For example, if you have two bikes with the same reach, but not the same stack, the bar on the bike with the lower stack is closer to the BB. If you add spacers under the stem to raise it to the same height as the other bike, you effectively reduce reach so the bar is still closer to the BB than on the other bike.

Posted by: crawIf two bikes have the same ETT but one has a steeper seat tube angle then it by definition has less reach. Is that what you mean?

Yes, so without changing Reach, as the seat tube gets steeper the ETT would be less. Or steeper STA and same ETT would increase reach

In addition, you could have 2 bikes with the same reach, but one is a slack head angle, long heat tube, long fork 29er, and the other is a steep head angle, short head tube short travel 27er, and you would have vastly different front centres (BB to front axle length) and would be like comparing a chopper to a road bike (stack would be different too).

As far as additional spacers increasing stack but reducing reach, you could just turn your stem upside down if you add enough spacers (if it is not 0 degree), or you might even need a longer stem ;-)

Last edited by: taprider on Jan. 5, 2020, 9:04 a.m., edited 2 times in total.

Posted by: HepcatBit confused about this, when would stack be so extreme that it couldn't be adjusted to your norm with headset spacers or a bar swap?

It's not that you can't [usually] adjust the bar height [a number of ways] to get them where you want them to. The problem is if you are just looking at Reach measurements in a geo chart without factoring Stack two bikes with the same Reach can turn out to be different sizes. The two FS bikes I ride are both 18.3" Reach, but the Stack values are different by over 2". When you sit on them the bike with the higher Stack is a noticeably bigger frame.

All this to say you can't shop for bikes by looking at Reach without also looking at Stack since the Reach value depends on the Stack value. If you ignore Stack you may end up buying a frame that is too big or too small.

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