Roadies_Rok

I grew up and started riding and racing in the days of the so-called "Double Diamond" frames that had horizontal top tubes and the bikes were for the most part sized by the seat tube. Then along came the "Compact" road bike frames and it threw me into a huge amount of confusion that to this day I have never really sorted out. When the bike makers switched over to compact frames I continued to ride large frames (X-Large and XX-Large) and continually became more and more stretched out with the 60-ish cm effective top tubes and wound up selling several bikes for half to a third of what I paid for them because I was tired of being in pain. Some of the shops tried to put me on a Large or a Medium frame to get the top tube under control but with me being 6'2" tall with a short torso and most of my height in my legs I was never comfortable having 18" of seat post sticking out. The thought of being ***** by a jagged piece of aluminum when the seat post snapped was something I could never get past.

Two years ago I got lucky and found a used good condition Fuji bike with the traditional frame and traditional geometry. My bike has a 59cm seat tube c-t with a 56cm top tube c-c. Even though my preferred size is 60cm or 61cm I bought this bike because from the first time I rode it the bike felt like it belonged under me and I have not regretted it to this day. I still have a fair amount of seat post sticking out but so far it hasn't snapped off and maimed me.

I have a couple of questions. Should the day ever come where I go looking for another bike I will probably have to content myself with a bike with compact geometry and risk being impaled by an 18" seat post snapping off but I guess that's the price for "progress" :-( I am somewhat familiar with Stack and Reach. My Fuji has a Stack of 60cm and a Reach of 39.5cm. If I look for a Compact Geometry bike with this Stack and Reach (or numbers that are close) will I be OK riding it and not in a lot of pain from being too stretched out?

Second question: I have never understood why the bikes changed from the Double Diamond configuration to the Compact Geometry. Just looking at the questions here in this sub-group there are a lot of people who are unsure as to bike fit and are struggling with understanding this Compact Geometry. Why did the bike makers change over to Compact Geometry? I was always taught "if it ain't broke don't fix it". Any thoughts??

Polaris OBark

I had the exact opposite problem, with short legs and a long torso. I eventually had a custom frame built (which is my recommendation for you), but it came out having geometry nearly identical to a Trek Domane 54cm.

The sizing you find now, with effective TT and effective seat tube, attempts to extrapolate to a conventional double-diamond frame with a horizontal top tube.

Why are the frames shaped like this now? Probably because for most people they are more comfortable, and carbon in particular permits these sorts of frame modifications without compromising the strength of the frame.

There are modern road frames with nearly horizontal top tubes. Canyon and Allied come to mind.

Iride01 

Those long seat post and tubes don't snap off and impale the cyclist. They do tend to make the ride a little more comfortable as they flex to a certain extent to absorb some of the jolts that you encounter along the ride.

If you think the your fuji gives you the position you like and want to keep, then frame stack and reach will help you narrow that down somewhat. But you have to realize that if you look at bikes with significantly different seat tube angle, BB drop, fork trail and wheel base, those and other things will all impact whether that bike feels right to you. And just frame stack and reach will not necessarily give you the same reach to the bars if the stem length and bar reach are different.

The old double diamond shape with horizontal top tube had some limits. I don't really know the many reasons, but the large frames I rode with the double diamond had a ridiculously long head tube and were the only bikes I so far have ever road rode and gotten that high speed wobble in the front wheel that is known as death wobble. Some attribute that to the long head tube, but there is nothing really conclusive that I've seen. And supposedly some get death wobble on bikes that aren't the double diamond. But I never have so far.

My current bike, a 2020 Specialized Tarmac is about the most fun bike I've ever road rode . And as comfortable as any. If you like the fairly aero position it's made to give you. You shouldn't fear the demise of the double diamond bikes. Though I might agree that the skinny tubed double diamond bikes were some of the prettiest bikes ever made.

rando_couche

Way off base, IMO. "Compact" sizing/geometry makes life easier (ie, cheaper) for manufacturers since each size fits a wider range of body sizes. ie, fewer sizes fit all. Sort of. Just like most "advancements" in cycling hardware in recent years: external bottom brackets, 1x drivetrains, threadless headsets, etc, etc. Any benefits to the consumer are incidental; it's all about cutting costs.

Polaris OBark

So, looking at my example (Trek Domane), the sizes offered are: 47, 50, 52, 54, 56, 58, 60, 62.

I'm calling B.S. on your explanation; it has absolutely no explanatory power.

(I do agree with the BB, but press-fit was more of a liability for the manufacturers, so it bit them in the arse eventually).

What drives sizing is what the competition offers. Giant and specialized have some nearly identical bikes, so if trek only offered a subset of those sizes, they would lose customers.

zandoval 

Fantastic...

I would sagest that you get someone to help you take DETAILED measurements of you on that comfortable bike.

Type of saddle -
Saddle position on its post -
Top of saddle to floor -
Top of saddle to the lowest pedal -
Position of your butt on the saddle -
Distance from the center of the saddle to the Stem -
Distance from the center of the saddle to your bars -
Position of your bars -
Width of your bars -
Thickness of your bars -
Distance from you bars to the crank -
As you are in your favorite position on your bike the distance and possibly the angle from your eyes to the front hub -
Your favorite hand position on your bars and brakes -

Help me out guys. I am sure I am missing something.

In the old days I could just about jump on any bike and ride off. At my age and de-conditioning I can no longer adapt. Few years ago I took these detailed measurements on my most comfortable bike (UNIVEGA) and have now applied them to my other bikes successfully. Truly they are not more then a few milimeters different from each other in set up but all maintain their own character. Some times I see new riders going to extreme and expensive lengths to fit a bike. You are so far ahead of others just trying to find what they can ride. Oh... Keep that FUJI!

tomato coupe

Does that happen a lot?

Kontact 

There is nothing mysterious about compacts. It is the standard frame geometry, but with the seat tube shortened and the top tube angled down to meet it. The stack and reach didn't change, nor did the rest of the geometry. The entire top tube is below a traditional top tube. The seat post is about 5cm more exposed, not 10 inches.

Most modern bikes aren't compacts. They have sloping top tubes, but they slop from a point below the traditional seat tube length to a point above the traditional head tube length. So they're top tubes cross the traditional or virtual top tube about mid span.

Compacts are great for short legged riders because they offer more standover clearance.

Modern sloped tube geometry is good for long legged riders because you get the stack of a larger size bike (taller headtube) with the virtual top tube of a smaller one. In other words, a size 56 will still reach like a 56 but have the saddle to bar drop of a 58.

Some modern bikes have very little slope to the TTs and are only a little different than a traditional frame in stack or standover.

Trakhak

A persistent myth. Most road bikes sold in the '70s and into the '80s came in either three or four sizes. And, believe it or not, some entry-level road bikes used the same top tube length across all sizes back then. A few---very few, in terms of numbers sold---upper-end bikes came in more than four sizes.

One big motivator for keeping the size range limited---most bike dealers hated the idea of the costs associated with the extra inventory they'd need to stock. And in multiple colors, too.

Plus, comparatively few bike dealers across the U.S. knew or cared much about bike racing, so they didn't see the need for all those sizes. (And a few of the bike racers who showed up in their stores were annoying prima donnas, expecting special treatment. One local bike dealer used to donate prizes for bike races---anonymously.)

A couple of manufacturers briefly tried restricting their earliest road bikes designed with sloping top tubes to three sizes but quickly returned to sizing them in the same increments as the equivalent horizontal-top-tube bikes.

unterhausen

I see no significant advantages to compact frames. The way I look at it, it's just styling. I have built one frame with a sloped top tube. I use effective seat tube and top tube length. To me, it seems like going with stack and reach is lacking a number, the seat tube angle. Although stack and reach is a good start if a company doesnt' know how to do effective seat tube length sizing. Kona comes to mind, they seem to use actual seat tube length on highly sloping top tubed bikes. So any other company's 64cm bike is listed as much smaller by Kona


The funniest example of "all top tubes the same" was the better Austro-diamler bikes from the early '80s. If you were short, the top tube was the same size as someone that was 6'4" We only ever sold the ones that came with Campagnolo, I think it was the Vent noir. Loved that smoked chrome finish.

Trakhak

No real disadvantage to a compact frame, either, though, except style. And you see threads here from time to time where someone complains about a bike with too little standover height and is told to keep a foot on the pedal and lean the bike to one side when stopped. Followed by posts chortling about the possible consequences of emergency stops.

The Vent Noir was the most memorable A-D model name, but I remember, dimly, that the top model might have been the Ultima. Or maybe that was the Puch. Was Puch A-D's line for entry level? We carried both. I know we bought them from the same importer.

Kontact 

What is the important function of seat tube angle? (Aside from making small bikes look like they have shorter top tubes.)

I like the companies that use 73 for the entire size range

Alan K

I thought it was supposed the be saddle and not seat!

Raising the seat tube beyond a certain point, may change the relative position of bottom bracket in relation to a bicyclist - not sure if one can compensate completely by moving the seat forward.

Kontact 

Yes, if you have a crazy angle. But 73 works for nearly everyone, and the common 74 to 72 range seems to work fine with modern saddle rails. Small frames with 76 STAs mostly create headaches because short people don't have a different angular relationship to the BB than tall people. Set back is automatically reduced by moving the saddle down.

So unless the bike is designed for unusual anatomy or a different fit paradigm (triathlon), the STA is more of something you work around. Your setback is not driven by it.

unterhausen

All the frames I build are 73/73. Someday I'll make myself an mtb frame and break from that though. I'll admit the main issue is I am set in my ways and have no feeling at all for stack and reach. Tell me an effective seat tube length and I have a pretty good idea about the size of the bike, tell me stack and reach and I don't. And it seems that manufacturers aren't really sure how to measure stack and reach given some of the weird inconsistencies I have seen.

The final problem is a lot of bikes that are sized by stack and reach also have an unpredictable location for the seat. I don't know if any sector of the bike industry is immune to this issue.

masi61

To me, sloping top tube frames are more nimble climbers. When out of the saddle the top tube can sway lower down the legs (maybe even below the knees? I’d have to check this). Certainly more to it than greedy manufacturers maximizing their profit margins.
I think also that a competent frame builder has more options for handling, stiffness & comfort when designing a “compact”.

Kontact 

I don't think anyone really sizes by stack and reach, since the two don't have a very much connecting them. People like to look at them so they don't have to figure out head tube lengths and TT adjustments by STA differences.

Personally, I would replace stack and reach with a stack number based on 73 degrees like the seat tube, and a corrected top tube based on a virtual 73 degree STA. That would make numbers more comparable and reference existing measuring methods.

elcruxio

Personally I feel stack and reach are the perfect end all measurements. Just by glancing at them I can tell whether a bike will fit. In road bikes I can't ride anything with lower than 630mm stack. For reach I can make 400mm work, but that'll mean a short stem. 370mm would be optimal as then I could use a 120mm stem, but such dimensions would mean a custom frame.

If the seat tube angle is in the 72-74 degree region I can makr that work so it's pretty much irrelevant. Head tube angle is considered for handling purposes. BB drop is for deciding whether the bike can be converted to 650b.

Top tube length, standover, headtube length etc. I have no use for. Stack accounts for head tube and fork length AND headset height. At least it should. Reach tells me exactly how long the bike is since it isn't measured from an seat tube angle and head tube dependent point in space but from the BB, where my feet, ie. most important contact points are located.

oldbobcat

Cervelo.

Generally, a slacker seat tubes allows a longer top tube without extending the wheelbase. They also facilitate this by making the head tube steeper. And in my experience a slacker seat tube makes the ride a little more compliant. Once the rider starts fiddling with saddle setback and stem lengths, though, all bets are off. That said, most of my bikes have been 72.5-73.0 in back and 73-73.5 up front, and I've been happy with that.

Kontact 

Not exactly. Head tube angles are defined from the top down, while the seat tube angles are defined from the BB up. Slacking the seat tube makes the TT longer, but it isn't important since the TT length itself isn't how you're fit. Whatever the angle is, you'll choose set back ignoring it. And then you set your reach, ignoring the TT and choosing whatever stem length necessary.

Depending on the material, a slacker seat tube could change the ride, but if you used the same saddle set back, that's now going to be mounted to the seat post different and that will also have an effect on ride.

Ideally, seat tube angle should serve the type of posture the bike style is built for - like how a rando or touring bike favor a posture with more set back and higher/closer handlebars. So it might make sense to have a 72 STA for your touring bike and a 73 for you racing bike because you are choosing to sit back and up on the tourer.

Head tube angle is just there to set front center, just like chain stays set the rear length. HTA doesn't change TT length. Wheelbase/front center is about cornering and toe overlap. Stability and handling characteristics come much more from trail.

Cannondale also went to all 73 STAs in the late '80s.

oldbobcat

This does not explain why so many builders slacken the head tube for small frames and make it steeper for the big ones. And the conscientious builders will adjust fork offset to ensure some uniformity in the trail figures.

When I started racing in the early '70s, 72-72.5 parallel was pretty much the norm for stock medium-priced bikes like a Raleigh Pro or Peugeot PX-10. Italian boutique builders were moving toward 73, but if you looked at the brands that favored long top tubes, like LeMond, and the custom builds for big guys like Tom Boonen, you'd see slacker seat and steeper head angles. And towards the end of the steel bike era, special builds with slacker angles were common for the cobbled classics. The differences between my made-for-export Gios and the ones that Roger DeVlaeminck rode, even for stage races, were obvious.

Of course, riding positions for the racers have changed over the last 10-12 years. Today's racers look a lot different from the guys Cyrille Guimard coached in the 1980s, and the guys who were influenced by them. Just the same, if I were shopping today I'd be hard pressed to find a 56 or 57 cm racing frame with 73-degree parallel angles.

Kontact 

Well, they steepen on small frames (74) and slacken on large frames (72.5). And that has been happening for decades - at least since the early '80s, if not long before. Factor into that when builders started referencing population anatomical numbers and when builders started embracing set back as an independent metric - like KOPS.

So I don't know why it started, but it doesn't seem to have any real basis in fit, unless you believe that shorter riders have a different relationship with set back than taller ones - and there is no evidence of that. If anything, steep seat tubes just prevent short people from getting their saddle far enough back with a standard seat post.

There also was a period when "crit bikes" had steep head AND seat angles, which may have just been referencing the way track riders move forward under power as well. But that position is usually not considered sustainable for long rides because of saddle nose pressure and weight on hands.


What we do know is the manufacturers continue to steepen STAs on very small bike sizes to make top tubes appear shorter. And all of that is what innovative companies like Cannondale and Cervelo rebelled against by setting 73 degrees across the product line. It is total BS to sell a short person a frame that says it has a 51cm TT, but that's because it has a 76 STA, which makes that TT identical to a 74 STA with a 53cm TT - which is actually long for even a 51cm bike.

Especially considering that modern bikes aren't made of pre-butted steel tubes, using STA to garner ride qualities should be unnecessary and pointless.


On the flip side, as long as you have enough adjustability on your saddle rails, the STA doesn't matter much. Which is why people have bikes with a range of STAs and are able to duplicate their fit across all of them.

elcruxio

Slackening of the head angle in small frames is a reaction to toe overlap. You put 700c wheels on a 50cm or smaller frame and that starts to be a real issue.

Ideally small frames would have smaller wheels as well, but that'd require good tires in two or three diameters. That seems unlikely.

elcruxio

this is where I feel reach again reigns supreme. It tells you exactly how long the frame front end is, which is really all that matters when determining frame length. Instead of faffing with eff top tubes and seat angles you have one number and the maximum tolerance for it (eg. 370mm) and that's it. Then you have the stack and with those two numbers you can check whether a bike has potential or if it's a total non starter.

Of course this is for road bikes as in the MTB world the field just blows up and nothing has any meaning any more. Especially when a bike has tolerance for 90mm of change in the fork length. That's a big change in geometry when you go from 483mm to 551mm axle to crown

Kontact 

But reach doesn't tell you how long the frame is - it tells you how long the frame is at a relatively arbitrary point that intersects the steerer tube - and not a point where you can mount the stem. The reality is that we have always treated the "reach" to the bars as a function of the distance between two roughly parallel lines - seat tube and head tube, and that worked because of how we raise and lower stem and seat. But suddenly we are measuring the distance between a vertical line and a 73 degree line - and then documents where the headset bearing goes. It makes little sense.

As long as you correct for variations in seat tube angle, ETT always works - if you know that you need around a 55 TT off a 73 STA, you can compare multiple geometries simply and easily, and not need to adjust for stack. All you need to know is to add or subtract 1cm of TT for each degree of STA difference.