Why doesn't reach change much?

londoncommuter

I've been pondering bike fit of late and am maybe missing something obvious but I'm confused as to why reach often doesn't change much between sizes. Effective top tubes get longer but reach often doesn't. The growth in ETT is effectively behind the bottom bracket as it's the seat tube angle that gets slacker. As your "correct" saddle position is always fixed compared to the bottom bracket this doesn't seem to make a difference as you'd just have to shuffle your saddle forward or switch to an inline seatpost to compensate for the slacker angle.

Assuming then that the reach is about right for you, can the choice of one size or the next really just be all about stack and how many spacers you want to run? What else might you be changing?

I guess the amount of exposed seat tube will reduce as the size goes up so is it really just a question of whether you want spacers at the front and more exposed seat tube or run the front slammed and have less seat tube?

What am I missing? Here's a few frames to illustrate this (excuse the formatting):

Reach Stack ETT Seat tube angle

Supersix Evo - 52cm 384 526 535 74.0
Supersix Evo - 54cm 384 544 545 73.5

Scott Foil - 52cm 384 527 535 74.0
Scott Foil - 54cm 385 549 550 73.3

S-Works SL4 - 52cm 386 526 537 74.0
S-Works SL4 - 54cm 387 543 548 73.5

Look 695 - S 380 535 528 74.5
Look 695 - M 384 560 544 74.0

Dogma 65.1 - 530 386 542 545 73.7
Dogma 65.1 - 540 386 550 550 73.4

ai_1

Just from a quick look at those figures, you're looking at adjacent sizes, not opposite ends of the range but still the reach figures are pretty similar as you suggest. I suspect in most or all of these cases the larger bike will ship with a 10mm longer stem which would make roughly the right increase in reach in each case.

londoncommuter

Yes, I just wondered why you get a steady progression in stack with frames but not reach. I'm not sure that frame designers would expect people to adjust for length with a longer stem if as they get larger they want more reach.

Partly answering my own question about choosing between two sizes with the same reach, I guess running the same height bars on a lower frame would need more spacers which would tend to reduce reach as the head tube is at an angle.

Interesting bit from Cervelo here, also wondering about this:

http://www.cervelo.com/en/engineering/t ... d-fit.html

JayKosta

Increasing 'reach' would also increase the overall wheelbase, and probably decrease the lateral stiffness of the front portion of the bike. It would also make the bike appear to be more 'stretched out'.
With the current preference for short wheelbases - typically well under 100cm, the frame needs to be designed so the reach doesn't result in a long wheelbase.

With current stems being very easy and quick to change, getting an acceptable 'fit' on the bike is fairly simple. Adjust seat height, and position the bars with suitable spacers and stem.
If the amount of spacers and stem length is reasonable, then fine. If not use another frame size to find an acceptable blend of dimensions.

Jay Kosta
Endwell NY USA

Zerotails99

Yeah small frames with long stem is the trend these days

Grill

As you can see the seat tube angle changes at about the same rate as the top length, and I'd also expect a change in head tube angle. When you consider that when specced as full builds from the manufacturer a 52 will typically have a 100mm stem and a 54 will have a 110mm stem, it makes perfect sense.

londoncommuter

Thanks guys, that's really interesting. I'd always assumed that changing stem size was to correct for minor fit issues and that the frame should grow with the rider. Looking again at the geometry chart for the Supersix Evo (just as an example), the reach changes by 3.1cm and the stack by 10.7cm across the range so it does seem you'd need to rely on longer stems.

Still confused though why Cervelo move their reach out in "nice" increments across the sizes (roughly 9mm increase with each step up on the R5) where other manufacturers seem to have random changes with many having tiny changes or even none at all around the 52/54 models (e.g. Supersix Evo is 38.4cm on both).

MichaelW

Crank length remains fairly constant over the size range , so larger riders move rear-wards to maintain "KOPS" or however they prefer. This is one reason why larger frames have a slacker ST angle. If cranks were proportionate, angles would be more similar.
I wonder if the riding experience of XL and XS riders is the same as average M/L riders, in terms of balance, weight distribution, agility and stiffness, even on the same model of bike.

on-yer-bike

londoncommuter wrote:

Thanks guys, that's really interesting. I'd always assumed that changing stem size was to correct for minor fit issues and that the frame should grow with the rider. Looking again at the geometry chart for the Supersix Evo (just as an example), the reach changes by 3.1cm and the stack by 10.7cm across the range so it does seem you'd need to rely on longer stems.

Still confused though why Cervelo move their reach out in "nice" increments across the sizes (roughly 9mm increase with each step up on the R5) where other manufacturers seem to have random changes with many having tiny changes or even none at all around the 52/54 models (e.g. Supersix Evo is 38.4cm on both).

Cervelo always have 73 degree head and seat tubes on every size, or used to.

on-yer-bike

If you look at Colnago c59 the reach is nearly the same on every size. Probably the same with the C60.

freefall_junkie

There is a very interesting article on this on the Cervelo website http://www.cervelo.com/en/engineering/t ... d-fit.html , the general gist being that many manufacturer don't size their frames correctly, especially in smaller sizes.

londoncommuter

OK, so my summary (and sorry if I've been slow on this) is that as riders get larger frames accommodate this mainly by slackening off the seat tube angle. Reach hardly changes and stems can be used to fine tune things.

For any given rider though, as reach doesn't change much between sizes for a lot of brands, their choice is driven solely to get the stack right so as to minimise the number of spacers needed. They do perhaps need half an eye on making sure the lack of exposed seat tube (standover in extreme cases) doesn't get too silly as the frame gets higher.

If that's the case, you'd effectively be choosing frame size based on flexibility i.e. a flexible person would go for a 52cm as they can handle a larger saddle to bar drop whereas the exact same sized person but less flexible person would go for a larger 54cm?

Feel free to point out where I'm going wrong there....

Manc33

The reach on my 48 bike is the same top tube length as my 57 bike and even on that 57 that particular frame has a reputation for having a long reach.

People used to say the China carbon frames always had a short top tube. :roll:

Don't ask me, I use flat bars on a road bike. :P My reach is a good 13cm back due to that alone. I'm not racing, I just want to be comfy and the bike to be as light as possible.