Geometry explained

[froggy]

Looking for a trials geometry explained guide. Have tried the search function but couldn't find anything. Does such a thing exist? I'm keen to understand what things like BB rise, chainstay length, wheelbase, stem length, stem rise, bar rise, bar pitch and other factors have on the feel of a bike. 

I was also wondering if there's any sources of collated information for frames which include detailed geometry measurements.

TIA

[La Bourde]

• BB height: the higher the bb is:
  • the easier it is to stay on the rear wheel or to move on the rear wheel, the easier it is to spin,
  • the harder is it to stay balance on two wheels or once you are in manual (less stability)
• Chainstay length: the shorter the chainstay,
  • the easier you can go in manual or lift the front wheel, the easier you can move the bike on the rear wheel
  • the less time you have to lift the rear wheel, e.g. when climbing a step, you have slightly less time to execute your extension
• wheelbase: the longer
  • the more stable the bike feel on two wheels
  • the more time you have to lift the rear wheel (see chainstay)
  • the harder it is to put weight on the front
  • the harder it is to move on the rear wheel or spin
  • the most place you have to execute an extension
  • it can ease tap or rolling onto high obstacle
     

Regarding the other factors, it is difficult to explain clearly.
I think it is important to understand that:

• every change in the geometry is a compromise
• mostly a single change impacts the whole system (and thus feeling)
• it is all about weight distribution and lever effect
• stabilty or maniabily: you cannot have both at the same time
• amplitude and reaction time shall also be considered
• stiffness impacts also the overall bike feel

 

Consider weight distribution and lever effect and you will better understand how each factor affects the feeling of the bike.

Let's take some examples:

• if you rise the bottom bracket, your center of gravity is now slightly higher compared to the rotation axle that are the wheel axles. It means, your body has an higher lever effect when you move it backwards for example. So a front wheel lift is easier ... sort of because there is another important point to consider: by raising the bottom bracket, you basically load more the front with your mass (you have more weight on your arms).  This makes the front wheel lifting for the first centimers/inches more difficult ... To compensate that, you can raise your bar. It will be easier to lift the front wheel, but more difficult to load it, i.e. front moves will become more difficult. And because you increase the distance between bottom of the front wheel and your arms, you have less place to low down you front wheel before jumping, which is also a drawback ...
• if you put your bar further away on an horizontal axle (i.e. same height) : you have now more place to execute an extension, so the amount of work you deliver is higher. But also you need to move more to load you front wheel, which is maybe more exhausting. You can put more weight on the front wheel if you are able too. On the otherside, it makes raising the front wheel more difficult, cause your center of gravity is slightly more forwards and your arms are more stretched
• if you use a wider bar: the bike is more stable, it lower your chest, you load more the front wheel. But amplitude of movement of you upper body decreases (the arm being futher apart) and because your body if further from the axle of rotation, it is harder to spin.

Hope it helps!

 

[froggy]

Thats absolutely fantastic and plenty for me to get my head around. Thank you for the detailed response 🙂

 

[The Dark Knight]

Good topic for me as I've been contemplating changing the bar/stem on my comp bike as been advised to fit a more modern combo. But I've no idea what I like to be honest, it's just a bar/stem to me at the moment. It's an Echo 24 I'm riding for comps, picked it up as really enjoyed my first ever couple of comps last year.

[stirlingpowers]

Try a new short wheelbase, flat stem & high BB bike at some comp. It's day and night for backwheel moves. The pogo stick meme is real. You will have to say goodbye to rolling bunnyhops though. 

For geo there's only the shop and frame brand websites. And these miss many important values. No effective stack or reach info, for example. Street seems to be 73 to 75 cm above bottom bracket.

From experience, at 181-182 cm body height, 70 cm stack is way worse compared to 74 for bunnyhops. 70 or 71 degrees steering angle is hard for front wheel spins compared to the 73-74 degrees on street bikes.

[La Bourde]

I agree, it would be nice to have reach and stack values for the bikes.
Somehow wheelbase length seems to be more important in trials... One can interpolate how much is the reach knowning the head angle, bb height, chainstay, headtube and fork length, but it is not really precise.

 

Bar height relative to the bottom bracket is an important figure to understand how the bike is easy to bunny up.
Effective reach (with the backsweep the bar considered) but also bottom bracket height compare to the wheel axle and width of the bar (if the difference is more than a few inches/cm) play also a role.

As I wrote earlier, always consider the weight distribution and the amplitude of movement you get.