joejack951

As a slightly-below average size male, most of this is academic for me but since I have found myself fitting others to bikes I am curious if others have some insight. I recently checked out the geometry charts for a bike being promoted on Kickstarter and was surprised to see that many of the sizes had nearly the exact same reach particularly at the small end, with only stack changing significantly. I assumed it was an anomaly until today when I checked out the geometry for a Specialized Allez being discussed in another thread: https://www.specialized.com/us/en/me...5-elite/115475

The three smallest sizes 49, 51, and 54cm all have exactly the same reach! The only real difference from one frame to the next is the standover height due to headtube length differences. 'Fit' is being adjusted by using shorter and shorter stems. I recall reading some marketing piece from Cervelo years ago about using stack and reach to define frames and in it they highlighted that some frames had this issue with smaller sizes. I assumed it was uncommon and definitely thought that anyone still building frames that way would have changed things up by now but apparently not.

So what's the deal with smaller cyclists getting screwed on bike fit, presumably because the industry doesn't want to build a 'road' bike with anything smaller than 700c wheels? Why should smaller cyclists have to ride bikes that handle differently than 'big people' bikes and/or have them improperly balanced on the saddle (shifted forward relative to the BB to compensate for the long reach)? Discuss.

redfooj

if you shrink the bike horizontally for stack (i.e. shorten the wheelbase)... you'd face pedal strike. i imagine theres also clearance issue b/t front tire and downtube. or you'd have to verticalize the tubes or change the rake and etc, otherwise transforming the handling characteristics. so one cant simply downscale all the dimensions in a linear manner.

joejack951

Unless they deviate from 700c wheel size...

rpenmanparker 

You are ignoring that the seat tube and post tip backwards. A taller saddle position is further from the bars than a shorter one. There is frame reach and there is actual reach of the finished bike as fit to the rider. Also larger sizes can have longer stems. The 54 doesn't need to have a longer frame reach in order to have a longer bike reach than the 50.

Carbonfiberboy 

Most builders at least used to decrease seat tube rake in the smaller sizes in order to decrease reach while avoiding foot strike. This isn't optimal either because it changes the rider's balance and causes more weight on hands. There's not a perfect way to do it. 650 isn't the solution either because there's so little availability both overall and during a ride. Woe to the 650 rider who goes through both tubes or ruins a tire. Personally, I'd take better balance and a shorter stem.

valygrl

my bet is that that's a mistake on the web site. that doesn't make any sense.

Sy Reene

Methinks a lot of builders also cheap out and don't really adjust fork rake to maintain common trail between frame sizes. If you ride a 54-56cm frame consider yourself lucky, as you're probably safely within the makers' design target.

Drew Eckhardt 

It doesn't do anything because most people adjust for some desired relationship between saddle and pedals like KOPs or saddle aft enough to allow balancing in the drops position with hands off the bars. A steeper seat tube just means the saddle will be placed back farther on its rails which is more likely to call for an offset seat post.

That position will probably be farther forwards for a smaller rider with shorter femurs and less lengthy torso to counter balance, so a steeper seat tube will be more likely to keep the saddle close to the middle of its rails.

Carbonfiberboy 

On my '99 52cm Trek, with a 25mm offset post and the saddle all the way back, there's still a little too much weight on my hands. None of my saddles, all the way back on this offset post, are more than 4cm behind the BB. Doesn't even pass UCI rules. I could get a 35mm setback VO post but I haven't bothered.

joejack951

Here's the Cervelo article I referenced earlier: Bike Geometry, Sizing and Fit - Cervélo

I guess my question is: who's full of it? Cervelo says stack and reach are what really matters. Specialized's frame geometries suggest that reach can be virtually identical for someone 5'0" up to 6'+ (there's a 5mm reach difference between their 49cm and 56cm frames).

They can't both be right, right?

San Pedro

I wonder how many bikes are like that. Giants at least have different stack and reach numbers across all their sizes (at least the TCRs).

It does seem like seat post and stem length can vary the fit quite a bit.

Have you tried a longer stem? That helped me quite a bit when I got my new bike.

joejack951

Here's the other example I mentioned: https://www.kickstarter.com/projects...built-in-power

Sizes S, M, and L have a reach within 2mm of each other.

Carbonfiberboy 

This 52 has a 52.5cm TT and I put 100m or 110mm stem on it, I don't remember which. My position is fine because as others have pointed out, it's easily adjustable with stem length. However balance is not as adjustable. Be that as it may, the bike is fine for me. I've done many long endurance rides on it in the course of 50,000 miles or so. I'm just commenting on the issue of reach as it's defined when considering bike sizing.

black_box 

Look at the "B" measurements, which appear to be the effective top-tube: 518mm, 537, and 548mm. These measurements take the difference in seat tube angle into account (75.5, 74, 73.5). Steer tube angles are basically the same, so you can adjust the overall reach between the ETT values with a stem swap.

Fiery

There's a few reasons behind this.

1. Stack and reach as defined are simply not very good measurements of how a bike will fit, because they are not independent. Take two bikes with same reach, but one has a 3 cm more stack than the other. By the time you've added 3 cm of spacers to the other bike, its actual reach will have decreased by 1 cm. So when a smaller size frame has less stack and same reach, it is actually shorter than the bigger size.

2. The actual sitting position on the saddle is typically behind the centerline of the seat tube. This means that the lower the saddle goes, the steeper the seat tube needs to be to keep the actual angle between the sitting position and the bottom bracket the same.

3. Where the wheels are relative to the bottom bracket is important, but not because of toe strike (I've always had some toe strike on my bikes, and I've always had sizes 56-58). Wheelbase and chainstay/front center measurements have major effect on how a bike will handle, and to get a road bike that handles like road bikes do, they need to be within a certain range. If you try do simply linearly downsize a frame while keeping all the angles the same, the wheelbase quickly becomes too short to handle well. If you extend the wheelbase by making the head tube angle slacker and front center longer, but keep the seat tube angle the same, you end up with too much weight on the rear wheel and not enough on the front. If you lengthen the chainstays and shorten the front center to improve the weight distribution, you decrease the responsiveness of the bike to steering "from the hips". So you leave the chainstays shorter and the front center longer, and you make the seat tube steeper to move the center of mass forward, and thus extend the measured reach - and you get a bike that balances and handles the way a road bike does.

4. Finally, some manufacturers simply have some strange ideas about bike geometry and how it should scale through the size range (or, worse, they don't really put that much thought into it).

redfooj

some manufacturers treat geometry scaling differently

example here

rpenmanparker 

As several other posts in this thread have suggested, even FRAME stack and reach doesn't perfectly tell the tale of bike size and fit. In order to know how a bike will fit, you need to diagram the frame with the seat tube extended up as high as the saddle will be mounted and the head tube extended up as high as the bars will be mounted. Then the horizontal distance from the top of the seat tube with post to the top of the head tube with spacers and stem is the reach measurement that really matters. As far as stack is concerned, what you need really depends upon your tolerance for spacers and planned saddle to bar drop. There is no substitute for experience.

chas58

I’m trying to figure out the same thing.
Post #14 seems to have the only accurate answer here. Top tube length changes, and seat tube angle changes, thus the length of the cockpit changes, even if the “frame reach” does not change.
(well, #17 by rpenmanparker hits the nail on the head, but no one publishes that info – it would vary based on your inseam. Some manufacturers give a height range for a bike - which comes close).

I’m trying to understand why reach has become popular. I’m trying to configure a mailorder only bike (Canyon), and the stack and reach are not helping me much. You identified the problem with reach.

For me, cockpit length is of prime importance. This is (effective) top tube length + stem. Frame reach really is not relevant unless the seat angle never changes.
Stack is a little better, but really it is a range based on steerer tube length and available spacers. (and the height of the headset).

Fiery

@chas58, when setting up a new bike, how do you set the saddle setback? Do you go relative to the bottom bracket (say, nose of saddle X cm behind center of BB), or relative to the seat tube and seat post (say, center of saddle rails X mm behind centerline of seatpost)?

Dean V

Everything is set from the BB. That is why stack and reach measurements are better than previous systems as it is based on the BB as a datum.
Only real gremlin is it the reach measurement being effected by the stack due to the steering head angle. That can be worked around with some simple maths.

Racing Dan

Mistake maybe? The top tube length, that I would argue is as important, if not more, does in fact change as expected. Smaller bikes do tend to have different geometry than their bigger brethren. Usually steeper seat tubes, slacker head tubes and more trail (unless a fork with more rake is used on the smaller model). I agree that many smaller bikes get screwed in the geometry department. I believe its mostly because of fear of foot-front wheel strike and because many manufactures spec the same fork rake for all the models even if they have slack head angles. Imo, Outside of reach this particular bike does not look too bad. Many small bikes have much slacker head angels and much more trail. Trek tend to spec different forks for the smaller models.

Kontact 

It isn't a mistake. As several people have pointed out, a horizontal measure that intersects a 73° head tube is going to vary as the head tube gets larger or smaller. Reach is just a poor tool for comparing frame sizes unless stack is already very close.

So the "same" reach on two bikes with different stacks are actually different reaches. Add spacers to the lower stack and watch the reach get shorter.


ETT is a better tool, but you have to normalize it for differences in seat tube angle. 1° = 1cm of TT length.

Racing Dan

I haven't done the math, hence the ?. I believe you didn't do it either.

EDIT: I have read a number of your posts and honestly, you often appear to only answer the the first line in what ever you reply to.

Sy Reene

How many bike makers do this? The Jamis Renegade Escapade model (Renegade® Escapade )offers this, but other Jamis models don't go to 650. That said, Jamis makes a big marketing plug about their practice of Size-specific frame designs, and though I don't own a Jamis, I've been attracted by their approach, ie. from Jamis Expat webpage:

"As you start to review the geo chart you will noticed we have 3 different fork offsets, BB drops and rear center measurements. Add size specific tubing and 2 different sized rear triangles and you have what we feel is the perfect endurance geo for all sized riders. The reason for this is that we wanted the 48 and 61 to fit the rider correctly and make the geo changes needed to provide the perfect ride.

SIZE SPECIFIC FRAME DESIGN
Size Specific Design (SSD) is the Jamis design philosophy and technique used to create the best possible riding bike for each size rider. Rather than limiting frame size variations to just different length top, seat and head tubes lengths, we take an all-inclusive look at each frame’s total configuration. Every SSD frame will also feature size-specific BB drop, rear center, fork offset/trail and SST technology.

rpenmanparker 

Are you asking what someone actually does, or what one should do? Relative to the BB is the only correct method.