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When I read about Giant, they talked about how in the past road bikes have to be in like 10 incremental sizes, but Giant had an idea from a mountain bike (which usually only have S M L sizes) to make road bikes with compact geometry so that it can be made with bigger jump in sizes.

How does this work? why a traditional frame can not be made in the same jump in sizes?

No matter how I compare the frames, I can't wrap my head around how a compact geometry can cover wider possibility of sizes that the traditional frame does not. The head tube angle is the same, the seat tube angle is the same, so raising or lowering stem and or seat post will be no different regardless if it is a traditional or compact. It is not like compact geometry can have adjustable stack and reach.

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    A lot of changes that we've seen over the past ~20 years in the way bikes are made are not to make bikes better, but to make manufacturing less expensive. These changes are then justified in terms of making the bike lighter or whatever. Making fewer sizes is a lot less expensive. I think that's what's going on here. – Adam Rice May 20 at 13:39
  • Frame size is fairly important for a traditional diamond frame, especially as a road bike. But newer style frames, especially those with slant top tubes, can accommodate a wider range of riders. Plus, of course, a manufacturer doesn't want to make eight sizes when three will capture the majority of the market. – Daniel R Hicks May 20 at 14:09
  • I think it is mainly about the standover height. When it comes to the distance forward you can play with the stem, move your saddle forward and backward but you could not change the top tube height with the traditional frame. But it is a question I wonder about myself. – Vladimir F May 20 at 14:15
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The claim is pure marketing and does not make any sense at all, unless you only care about standover height. It is true that Giant has smaller selection of sizes than competing manufacturers, and Giant might use the claim to explain why, but even their own models are not consistent with the explanation:

  • High end Giant models have 6, not 3 frame sizes. This is not that many less than competitors who have 9 or 10.
  • Giant has a series of non-compact "aero" road frames. These also come in 6, not 9 or 10 sizes.

On the other hand, smaller selection of sizes reduces tooling and inventory costs. If a sponsored athlete requires a bike outside standard selection, it is easier to provide them with custom frame than mass produce the wider selection of sizes.

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The head tube angle is the same, the seat tube angle is the same, so raising or lowering stem and or seat post will be no different regardless if it is a traditional or compact.

The difference is the amount of adjustability. A compact frame means a shorter seat tube and head tube and a steerer tube which is relatively long compared to the head tube. This gives more room for adjusting the cockpit size by varying the seat post length and position of the stem on the steerer tube.

It is not like compact geometry can have adjustable stack and reach.

If you are talking about the distance from the bottom bracket axis to the top of the head tube that's true, but what we care about is effective stack and reach - the distance from the bottom bracket axis to the nominal position of the rider's hands on the bars. That can be adjusted by varying the position of the stem on the steerer tube and the stem length.

Also, because the seat tube is angled backwards, dropping the saddle towards the bottom bracket to account for a shorter rider's leg length also has the effect of moving the rider's shoulders forward, reducing the effective reach to the bars.

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  • To accept the explanation, you need to believe that stem length does not affect other things than reach – ojs May 20 at 17:18
  • @ojs yes there is interplay between the various adjustments one can make, changing stem length affects steering for example, so there are limits with what can reasonably be done, I think my answer deals with why there is more scope for rider fit on a modern compact frame design than a traditional high horizontal top tube frame. – Argenti Apparatus May 20 at 17:51

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