Bike "resizing"

Education
1

Hi,
what dimensions do you think should not be changed / somehow preserved when designing various sizes of the same bike - to have similar ride quality? Maybe there is some reading you could recommend? Thanks.

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2

What style of bike are you discussing? Road? Gravel? Mountain hardtail? DS?

3

Well, I’m into road/gravel but I’m looking for some general thoughts.

4

Are you looking for thoughts centered around “best ride”, “best compromise” or “best efficiency?”

The PoV from a Product Line Manager is very different than from a custom frame builder or from a small/niche brand.

5

I think something between best compromise and best efficiency from a small builders’ view.

6

I try to keep everything constant across all sizes. But you end up with certain trends across the size spectrum due to:

  • wheel and component availability
  • rider expectations
  • marketing/consumer expectations

Things I try to keep constant:

  • the bar drop to saddle height ratio
    – This determines the stack height
    – gravel example: Saddle to bar drop ~8% of saddle height
  • Trail (handling characteristics)
  • Front-rear weight ratio
    – this determines the chainstay length
    – I try to keep this constant on mountain bikes, but it ends up varying due to component availability and consumer expectations
  • aestetics

Things I scale proportionally:

  • “hypotenuse” (sqrt(stack^2 + reach^2)
  • bar width
  • crank length
  • wheelsize
  • tube spec

For example, this is the massive spreadsheet that I use to design sizing ranges. I look at things like bar drop, hypotenuse (stack and reach), and front-rear weight ratio.

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Nothing is ever constant (horizontal line) or perfectly linear as the size range changes. There is definitely some fine-tuning and massaging of the numbers to get bikes to look right and meet people’s expectations.
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Additional reading:

I would take the RAD discussion with a grain of salt. I think their methodology is good, but their conclusions are based on too narrow of a view.

Here are some related slides:

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7

I think to best understand sizing adjustments, you need to have a really good handle on what makes a bike feel like “this bike” and not “that bike”. If you can take a set of parameters (trail, reach/stack, CS length, tube choice for example, or however you would normally define your design for a given size) and say “I expect this bike to function with X, Y and Z characteristics” then it should pretty easy to extrapolate to bigger and smaller sizes. I guess the hard parts are a) knowing how to do that in the first place, and b) knowing what proportions to aim for with a given rider body geometry.

Find a bunch of mass-production bikes that are similar to what you’re working on and look at their geometry charts - you probably shouldn’t copy directly but you can get a good idea of where to start. Also look at ratios between given parameters, eg trail:wheelbase (hat tip: @BS_Industries post in the 14" kids bike thread).

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8

@Daniel_Y I truly appreciate the work and knowledge you put in your posts here. Thanks. It’ll take me now probably a year to understand all of this :slight_smile: What you wrote is somehow is similar to what my intuition tells me, but puting it in numbers is a challenge.

@JoeNation I think (hah) that I have “the feeling”, but would like to put in numbers at least a bit. I’ve already looked through some mass production stuff, but - big producers usually have marketing and accountants messing around the process, so optimal geometry be a bit “obfuscated”.

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