Geometry and sizing

[Montana St Alum]

I'm new. Well, I was new 70 years ago, but I'm new to ebiking.
I'm on the border between small and large frame sizes generally, at just over 5'8" tall (call it 174cm). Bike frame sizes have increased considerably in reach over the last few years. Do people tend to go down in size?
That would be my inclination based on R.A.D. sizing.

 

[Nicho]

I'm new. Well, I was new 70 years ago, but I'm new to ebiking.
I'm on the border between small and large frame sizes generally, at just over 5'8" tall (call it 174cm). Bike frame sizes have increased considerably in reach over the last few years. Do people tend to go down in size?
That would be my inclination based on R.A.D. sizing.

When I got my Giant Trance I should have been on a'small' according to Giant's size chart, but when I stood over the crossbar my legs were almost against the handlebar and it felt ridiculously cramped, so I went for the medium.
You don't really know if the size is correct until you have sat on it.

 

[Montana St Alum]

When I got my Giant Trance I should have been on a'small' according to Giant's size chart, but when I stood over the crossbar my legs were almost against the handlebar and it felt ridiculously cramped, so I went for the medium.
You don't really know if the size is correct until you have sat on it.

For the seat to handlebar length, I agree. For the reach it's a bit more complicated. Medium was great for me on the trance initially, but I have since shortened it out and it rides better. My current reach on a medium Trance is 30mm shorter than the medium SL and right on for the small. It's hard to appreciate reach differences while seated. And the ETT on my Trance is closer to the small, than the medium SL. Given the changes in design, sitting on a small or a medium would both feel fine, but I'm wondering if that really translates to how they work on the trail. The video makes a compelling case for the smaller bike, generally, if you're equally comfortable on both sizes.

This is interesting as well. In this case, it seems to be model dependent:

 

[steve_sordy]

I'm sure that there must be an "average" or "typical" rider around which bikes are designed, but I'm not average at all. My legs are too short for my body despite me being 6'1". I look normal, but feed my measurements into a bike selection gizmo and it might very well come out with "we don't have a bike to fit you, but we think a Large might suit you best". I have always chosen a Large, but I have come to realise that all size "Large" bikes are not the same. They vary in just about everything except wheel size. (Well they did until 29ers and then 27.5ers entered the market!) Then of course bike geometry changes over the years, bars get wider, stems get shorter, reach grows astronomically (forward geometry anyone?)

But the thing is that once you know what suits you, you can pretty much buy any bike unridden as long as you can check out the geometry chart (I find Reach and Stack particularly important). Any slight differences can be adjusted once you get the bike. You can move the saddle back & forth and up & down, the stem length can be altered, even crank lengths! Bar furniture can be moved and rotated, even the bars can be rotated, lifted, dropped, or even replaced with something wider or with different geometry.

This from the Bike Radar website may help:

What size mountain bike do I need? Plus tips on bike fit

Use our guide to help you determine your perfect size of mountain bike

www.bikeradar.com

 

[Montana St Alum]

True, Steve.
I have my current bike dialed in pretty well.
Since stack and reach work together, I have taken the diagonal of that right triangle to help narrow it down. Thank you, Pythagoras.
My current bike, with adjustments by way of spacers, bar sweep and stem length is about 31" on the actual R.A.D.

The calculated diagonal on my "dialed" bike is about 30". So I can just look at the diagonal from published stack/reach and come up with a value that I can use to judge how stem, bar and spacers might work.

The Shuttle SL has shorter stack, greater reach and the Fuel EXe is the opposite. Bottom line on the diagonal is, they're about the same, and they feel like that.
I have a "sit bones" to head tube length (with the seat centered) that I like for seated feel, so I can check that at the shop on various bikes. It just narrows the selection process a bit.

But the bottom line is that I could go Sm or Md or both bikes. The closest is the small EXe followed by the small Shuttle SL. That really surprised me!
I used to climb, ages ago and I remember a story that when Yvon Chouinard (a little guy) started Patagonia, he insisted all clothes be sized so that he could wear a medium. Kind of how I'm feeling - I ought to be on a medium!

 

[steve_sordy]

True, Steve.
I have my current bike dialed in pretty well.
Since stack and reach work together, I have taken the diagonal of that right triangle to help narrow it down. Thank you, Pythagoras.
My current bike, with adjustments by way of spacers, bar sweep and stem length is about 31" on the actual R.A.D.

The calculated diagonal on my "dialed" bike is about 30". So I can just look at the diagonal from published stack/reach and come up with a value that I can use to judge how stem, bar and spacers might work. ...........

I can see why you have worked out a diagonal. But TBH I believe that approach is flawed. To take an extreme case I could give you half a dozen combinations of Reach and Stack all with the same diagonal, but with radically different values of R&S. In the real world of course, the values of R&S do not change in a radical way (until something like Forward Geometry comes along). However, when it comes to R&S, even small changes in the dimension make a noticeable difference. Buying a bike on the basis of the diagonal may work for you, but I prefer to know what the values of R&S actually are. I really want to know that the Reach is longer and the Stack is shorter, or indeed the other way around. The bike would fit me differently, despite having the same diagonal.

 

[Montana St Alum]

I can see why you have worked out a diagonal. But TBH I believe that approach is flawed. To take an extreme case I could give you half a dozen combinations of Reach and Stack all with the same diagonal, but with radically different values of R&S. In the real world of course, the values of R&S do not change in a radical way (until something like Forward Geometry comes along). However, when it comes to R&S, even small changes in the dimension make a noticeable difference. Buying a bike on the basis of the diagonal may work for you, but I prefer to know what the values of R&S actually are. I really want to know that the Reach is longer and the Stack is shorter, or indeed the other way around. The bike would fit me differently, despite having the same diagonal.

I agree, you do need to know reach. R.A.D. is based on diagonal. I like to consider reach and diagonal, but they tell you different things about fit.