Can some one recommend a good article on 29 geometry pls?

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  • Can some one recommend a good article on 29 geometry pls?
  • JCL
    Member

    Manufacturers are trying to get the wheelbase of 29″ as short as possible, mostly by making the chainstays as short as possible and sometimes compromising head angles. In the process they’re ruining bikes but that’s a another story. If you think about the impact angle of terrain on the front wheel the 29″ will contact further ahead of the front axle in comparison to a 26″/650b which has a similar effect (but not the same) as having a slacker head angle. Basically without waffling on for hours a 29″ with a 70 degree head angle will be a lot more capable all-round than a 26″ (same travel) without any real negatives. It’ll feel far more stable, maybe more boring but that’s subjective, and if the suspension designs are the same it should climb better.

    Handling is a tricky thing to rate. One persons stable is another’s dull and one persons lively is another’s twitchy. Test ride.

    Premier Icon Wookster
    Subscriber

    Thanks fella, I agree I wouldn’t buy a bike with out a test ride but it difficult to know what’s the geo and what’s the wheel size??

    It’ll be hard to find a good article because most are biased one way or another. MTBR.com has had some good ones in the past.

    I’m a big 29er fan and The best 29er’s I’ve owned/rode have been frames built to be a 29er not ones built to ride like a 26″.

    Premier Icon Wookster
    Subscriber

    So Ive found Im fastest and happiest on a 100mm 70 degree long stemmed F/S bike, no idea why, maybe it’s because I spend a lot of the winter on a CX bike so im used to the handling. Im faster than on a 140mm f/s bike on natural trails lakes Wales at home on twisty wooded tracks and on trail centres Afan Keilder (it’s in the classifieds if you fancy!) so it’s all about me me me!!

    Im intrested as to how a 29er has been designed to get the handling on par with a 26er but include the perceved benifits?

    Does that make any sence?? Just intrested google gives me some stuff.

    I must add I like the flowy swoopy bermy type riding…the last time I hi led to flat was at 3am after shorts at uni!! :mrgreen:

    JCL
    Member

    Well the wheels themselves will only effect rolling resistance, the larger the wheel the less resistance for given tire size. Obviously weight increases too which can possibly effect acceleration. I think the former easily trumps the latter but others feel different. There is also a bit of gyroscopic effect but I personally think it’s overstated.

    By far the most interesting result of running bigger wheels IMO is it allows the BB (drop) to increase, as much as 35mm, below the wheel axle centreline. As the vast majority of the riders mass is on the pedals this has a huge effect on stability. This is what opponents of 29’ers really don’t like when they ride them, not the wheel size. A bike has to have a degree of instability to turn so a 29″ does require more effort to change direction and lift the front wheel (manual). You have to way up if you prefer a nimble but ultimately less stable (at higher speeds) bike with one that feels a little dead and dull at lower speeds but should allow you to have more confidence at higher speeds and in loose terrain (similar travel bikes obviously).

    The BB drop and longer chainstays also has a positive effect on how the suspension performs on 29″ but we’ll save that for another day.

    The kind of old school geometry/set-up you like would probably be quite hard to replicate with a 29″. Something like a Camber 29″ would probably feel like a tank compared to your current bike. I’d probably try a Anthem 29″ first and if you don’t like it I’d probably stick with 26″ or 650b.

    Premier Icon bigdean
    Subscriber

    There are a good set of vids on you tube of cy from COTIC giving a lecture for Imech. Frustratingly they cut at interesting bits. But its interesting that fork rake and offset are play a bigger role tha head angle.

    b r
    Member

    and don’t get hung up on wheelbase, as we noticed at the weekend that my 26er has a longer wheel base than a (much shorter) riding buddies 29er…

    I’m convinced that a 29er will be my next bike, BUT, it won’t be anytime soon as my 26er and all the spares/wheels/tyres have years of life in them.

    Premier Icon jameso
    Subscriber

    Im intrested as to how a 29er has been designed to get the handling on par with a 26er but include the perceved benifits?

    I think 29er handling has sod all to do with 26″ handling and everything to do with making a frame that puts contact points and wheels in the right place to get the best out of the package. A good handling bike is just that, wheels / fork travel etc aside.

    When I read ‘our 29″ bikes handle like our 26″ bikes’ from brands I do wonder what they’re on about. It won’t, and can’t, but that’s nothing to worry about. It should read ‘we want to promote a 29er to those sceptical of them but we know you trust 26″ bikes, as then you’ll try it and probaly like it anyway, since it’s not that different either’

    Weight distribution, contact points, overall balance etc. Some of these can be improved by using bigger wheels, some are harder or almost impossible to optimise.

    fork rake and offset are play a bigger role tha head angle

    Perhaps it’s been misquoted or edited, but HA is equally important, that, wheel size and rake are parts of the trail effect. Being able to adjust them is the important bit and in 2008 or so you couldn’t do that with rake, now you can.

    JCL – if a 26er and a 29er have the same bb height in absolute terms, then why would BB height relative to axle height have an affect on the overall CoG / Stability?

    Or is it to do with the fact that if you are sitting “lower” in the bike, then your centre of mass and the bikes centre of mass are both nearer to each other, thus contributing to a feeling of togetherness?

    JCL
    Member

    A bike has very low mass compared to the rider (unlike motorbikes) so you can almost ignore it’s influence. The reason for the stability is a lower rotational inertia. While it isn’t exactly the same as lowering the CoG it has a similar stabalising effect.

    The polar moment going on between contact patches, BB, wheel axles etc is nuts.

    It is quite easy to overthink the effects of geometry and bb position relative to axle height. The best thing to do is to get out there and ride a few bikes and see what you like the feel of .

    Premier Icon jameso
    Subscriber

    why would BB height relative to axle height have an affect on the overall CoG / Stability?

    Harder to endo ime, that’s all. The BB to axle height thing in corners, I don’t see it making any real difference in itself since the bike leans on the contact patch. I think people feel the cornering stability of a generally longer/thinner contact patch and assume the bb drop is what adds cornering bite. There may be some stability from a heavier wheel adding to that impression but it’s not much.

    Or is it to do with the fact that if you are sitting “lower” in the bike, then your centre of mass and the bikes centre of mass are both nearer to each other, thus contributing to a feeling of togetherness?

    That’s probably part of it too.

    (sorry not a q aimed at me I know, just my take on it)

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