Sick of lack of geometry information! (**rant alert**)

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Comments

  • neeb
    neeb Posts: 4,467
    rich164h wrote:
    I can also see it from a slightly diffent angle though, that for the seat post setback, I rather think of that as a replaceable part (along with stem, saddle and bars) that I may or may not have to replace almost immediately to get the fit right. The fact that it's not published doesn't therefore really change my decision about whether or not to buy the bike overall. Maybe they also think about things in this way? They can only supply components that fit an "average" person. How many people have exactly "average" dimensions for each and every aspect of their body? Very few I'd guess. I don't know anything about seat masts though and whether these are much harder to find replacement for though, so perhaps if there's only one size and no after market alternatives that fits the frame this is a major issue.
    I agree it isn't an issue with traditional frames where the seat post is not an integral part of the frame - even if one is supplied with the frame, you can just replace it with one having the correct setback if necessary. The problem is that with modern carbon frames with unique seat tube cross sections, you can usually only use the manufacturer's own seat mast, and usually the only one that exists is the one supplied with the frame....
    rich164h wrote:
    With respect to the fork angle, I'm not sure that having that angle known is that important either, unless it is at an extreme angle. I agree that a change in the angle can have a dramatic affect in handling etc but then so can weight distribution, stem length, bar width, wheel weight, tyre choice etc etc and so you're only really going to understand what the handling is like from riding the bike yourself, or relying on reviews from people that have. I'm not sure I could infer anything about the difference in handling between brand A bike with an angle of X vs brand B with an angle of Y. There are too many variables, unless of course, as I said above, the differences were significant/extreme.
    What is important to know is the trail - this does significantly alter how a bike steers and feels, and a single degree of head tube angle can alter the trail very significantly if the fork rake is the same. E.g. with a 43mm rake fork, a 73 degree head tube angle gives a trail of around 58mm, while a 72 degree head tube gives about 65mm. I know from previously riding a bike with this amount of trail that I don't like it... To get the trail down to under 60mm with a 72 degree head tube angle you would need a fork with quite a lot more rake.
  • neeb
    neeb Posts: 4,467
    simple infomration that could be measured by anyone if they had access to a frame
    P.S. It's actually fiendishly difficult to measure frame angles from the frame itself unless you have special equipment - I know because I've tried.. :wink: You simply can't line things up accurately enough over these distances to measure angles to within 1 degree or less...
  • rich164h
    rich164h Posts: 433
    edited January 2013
    Yes I understand that, but, as I see it, you say you don't like a trail of 65mm, but that was with a stem of a certain length, a bar of a certain width, a frame geometry and put a certain amount of weight over the front wheel, a certain weight of wheel/tyre, a certain material/layup of the fork etc. By changing any of those parameters would change the way the bike handled and with a different combination of those things you might find that a bike with a trail of 65mm might handle in a way that you do like (or at least not hate). I have no idea if 65mm is an extreme trail by the way, maybe it is.

    Ok, so some of those parameters you would fix based on other more pressing needed (i.e. bar width linked to body size etc) but all of the other would change the handling of the bike, so it's difficult to know for sure what that number is going to mean the is going to feel like and you may end up excluding frames from your buying process and missing out on what may have proved to be a cracking bike.
  • rich164h
    rich164h Posts: 433
    neeb wrote:
    simple infomration that could be measured by anyone if they had access to a frame
    P.S. It's actually fiendishly difficult to measure frame angles from the frame itself unless you have special equipment - I know because I've tried.. :wink: You simply can't line things up accurately enough over these distances to measure angles to within 1 degree or less...
    Really? I'm trying to think why it would be so difficult. It's just a series of triangles so with a tape measure and some basic trigonometry you should be able to calculate everything. I guess it would be harder if you were trying to directly measure the angles though using protractors etc.
  • Pross
    Pross Posts: 41,149
    Fair point on the BB drop. I should also have used the centre to top or effective seat tube length I think. You're right, the chart is s**t and it does seem ridiculous that they don't provide the information :lol:

    However, I think if I were buying a bike like that I would ignore geometry charts and be getting a bike fit and decent trial ride.
  • neeb
    neeb Posts: 4,467
    rich164h wrote:
    Yes I understand that, but, as I see it, you say you don't like a trail of 65mm, but that was with a stem of a certain length, a bar of a certain width, a frame geometry and put a certain amount of weight over the front wheel, a certain weight of wheel/tyre, a certain material/layup of the fork etc. By changing any of those parameters would change the way the bike handled and with a different combination of those things you might find that a bike with a trail of 65mm might handle in a way that you do like (or at least not hate). I have no idea if 65mm is an extreme trail by the way, maybe it is.
    These other things will affect handling too, but in different ways. Changing the stem length isn't going to compensate for an extreme trail, they have different and separate (if slightly overlapping) affects on handling.
  • neeb
    neeb Posts: 4,467
    edited January 2013
    rich164h wrote:
    neeb wrote:
    simple infomration that could be measured by anyone if they had access to a frame
    P.S. It's actually fiendishly difficult to measure frame angles from the frame itself unless you have special equipment - I know because I've tried.. :wink: You simply can't line things up accurately enough over these distances to measure angles to within 1 degree or less...
    Really? I'm trying to think why it would be so difficult. It's just a series of triangles so with a tape measure and some basic trigonometry you should be able to calculate everything. I guess it would be harder if you were trying to directly measure the angles though using protractors etc.
    One problem is that you need very precisely anchored points from which to measure the frame angles, and the centre of a convex head tube or moulded seat tube / top tube junction doesn't easily provide this. Also, any non rigid measuring device or line such as a tape measure or piece of string will have enough wobbliness in it to make it impossible to measure angles to within one degree or less. Try it if you don't believe me! :wink:
  • neeb
    neeb Posts: 4,467
    Pross wrote:
    Fair point on the BB drop. I should also have used the centre to top or effective seat tube length I think. You're right, the chart is s**t and it does seem ridiculous that they don't provide the information :lol:

    However, I think if I were buying a bike like that I would ignore geometry charts and be getting a bike fit and decent trial ride.
    In my case I've already had a couple of bike fits and know my measurements, so it's just a case of narrowing down the selection to bikes that can be made to conform to these measurements. And to do that I need geometry.. :wink:

    Ironically, I sometimes wonder when people choose bikes entirely based on trial rides whether perhaps as often as not they end up going away with the bike that is set up best for them just by chance... When you do a test ride, unless it's at a very good shop that knows a lot about setup and you have already been fitted, each bike you try is going to be set up slightly differently. That's probably going to make as much difference to how the bike feels to ride as the qualities of the frame itself do...
  • rich164h
    rich164h Posts: 433
    What I'm saying is, don't measure the angles at all. Measure the lengths (rigid tape - it's only straight lines we're after) and calculate the angles.
  • neeb
    neeb Posts: 4,467
    rich164h wrote:
    What I'm saying is, don't measure the angles at all. Measure the lengths (rigid tape - it's only straight lines we're after) and calculate the angles.
    I think the problem with that is that the main triangle isn't actually a triangle, but rather a quadrangle (four sides and four angles - top tube, seat tube, down tube and head tube), and so the angles aren't determined by the lengths of all of the sides as they are in a triangle. You can squash and stretch a quadrangle, changing the angles, while keeping all of the sides straight and of the same length.

    Hey, we're learning some serious geometry on this thread.. :)
  • rich164h
    rich164h Posts: 433
    Yes, but you can also measure the diagonals, to effectively create two triangles from your quadrangle.
  • neeb
    neeb Posts: 4,467
    rich164h wrote:
    Yes, but you can also measure the diagonals, to effectively create two triangles from your quadrangle.
    True... I still bet you couldn't do it accurately enough on a frame where you didn't know the angles to come up with the correct figures. Might be wrong though!
  • My attitude is much like the old joke about the difference between scientists and engineers (which I won't repeat here)

    You sound like a scientist - must have all the data before being able to make any decision.

    Engineers have a 'close enough' attitude if it pretty much works and are likely to get on a bike without wondering if this angle is right, or optimal etc.

    I think most people aren't quite so anal about the geometry to care enough and the manufacturers cater to the majority.
  • neeb
    neeb Posts: 4,467
    My attitude is much like the old joke about the difference between scientists and engineers (which I won't repeat here)

    You sound like a scientist - must have all the data before being able to make any decision.

    Engineers have a 'close enough' attitude if it pretty much works and are likely to get on a bike without wondering if this angle is right, or optimal etc.

    I think most people aren't quite so anal about the geometry to care enough and the manufacturers cater to the majority.
    I am a scientist of sorts actually (nothing related to physics or applied sciences though) - but I'd say that I'm right and "most people" are wrong of course.. :wink:

    I don't think it's anal to want to be sure you can duplicate the saddle setback you are most comfortable with on a 3 grand bike frame. I pretty much agree with the "close enough" philosophy (which is why I think custom frames are unnecessary and a waste of money for most people), but if I can't set the saddle up so I can comfortably sit on the back of it (which for me is having the tip of a Specialized Toupe about 5.5cm behind the BB), that's not "close enough".