Linkage Suspension Fork Mock-Up

I don’t think there’s much to gained from trying to compare the unstable characteristics of aerobatic planes to bikes TBH.

If I might be so bold, OP has some odd dimensions, I can imagine that trying to find the bike he wants that performs well on his specific terrain would be hard to impossible to get via an industry that is set up to shift large numbers of bikes to a formula, building a custom bike seems like a great idea, I imagine it’d be a great way to experiment. Good luck to him, I hope it works.

Interesting blog – I shall read more of it!

Regarding BB height, this is something I’ve experimented with by adjusting the geometry on my current full-sus and hardtail (with movable dropouts, different fork lengths, anglesets and sag). Although it’s just anecdotal evidence based on my messing about I can say with a fair degree of certainty that as soon as a bike slides laterally a lower BB height makes it easier to keep the bike balanced. It almost feels like cheating!

I don’t think there’s much to gained from trying to compare the unstable characteristics of aerobatic planes to bikes

why?
❓
lets say, small cruising airplane: designed to be stable. Makes hands off flying possible.
I would compare this airplane to: road race bike for example

aerobatic airplane: designed to be not stable. Airplane follows every control input right away.
This is very much what I want from an mountain bike…
💡
How reacts the machine to the control input? That’s an important question for the design of a machine. No matter if airplane, seakayak or bike.

Touches this question right away:

I’m not sure anyone who rides a mountain bike for fun would actually want a ‘really’ stable bike.

My answer (but might be wrong): no. This wouldn’t be fun.

Linkage Suspension Fork Mock-Up

No question a great, great project. Love it.
😉
But as mountain biker: don’t fully agree with the “stability ideas” presented in the http://www.daveypushbikes.com blog.

lets say, small cruising airplane: designed to be stable. Makes hands off flying possible.
I would compare this airplane to: road race bike for example
aerobatic airplane: designed to be not stable. Airplane follows every control input right away.
This is very much what I want from an mountain bike…

It’s what you might want from a trials bike or a BMX, but I can’t see it being much use for the majority of mountain biking. I agree there might be a place for less extreme variations, though, but still limited.

Airplane follows every control input right away.
This is very much what I want from an mountain bike…

That only works if you’re very strong and very skilled with excellent balance etc. A mountain bike being ridden on gnarly trails is constantly subjected to near-random destabilising forces, which not only cause the bike to deflect from its path but also unbalance the rider causing them to apply unwanted inputs to the bike.

That only works if you’re very strong and very skilled with excellent balance etc. A mountain bike being ridden on gnarly trails is constantly subjected to near-random destabilising forces, which not only cause the bike to deflect from its path but also unbalance the rider causing them to apply unwanted inputs to the bike.

Good point.
Indirectly also hint how complex mountain biking is!

Don’t have the know how or expertise.
But very possible as well:
Above is different for each of the 6 degrees of freedom (6dof).
In certain degrees of freedom the mountin bike should be “more” stable in others not?

In my opinion funny as well, history:
Oliver and wilbur wright had both bike background. Knowing about the 6dof controls issue from BIKES.

They used this know how to design the first powered airplane. 1904 or so?
And their goal was, for this aiplane: to be very stable in all 6 dof.

At this time in history: this was extremely smart.
But indicates as well: airplane controls and bike controls might have something in common.

Above question:

I’m not sure anyone who rides a mountain bike for fun would actually want a ‘really’ stable bike.

Maybe helpful to answer this – for mountain bikes – for each of the 6 dofs?

No idea how to do this.
Just an idea.
💡

Hi andreasrhoen – I think that we are possibly both arguing for the same thing? In my first response to nick_c I made the distinction between a stable bike and a stable rider. While these things are rarely if ever binary, they are often discussed in such ways. I believe that the current trend in mountain biking (longer, lower, slacker) is creating more stable bikes at the expense of rider stability. I also believe that, within reason, rider stability (i.e. being able to control a bike from a position of stability) is central to having fun on a bike.

First and foremost, what I would like to create is a frame that places the rider in a more stable position than I have been able to achieve with previous bikes. The stability of the bike is almost a separate (but very much interrelated) issue.

Hi nick_c – you’re right, I do have some pretty odd dimensions – I have very long legs for my height. But while this is almost certainly the main reason why I have played around so much with handlebar position and, ultimately, decided to build my own frame, I still believe that many riders might benefit from the thinking that I have already described above and discussed in my blog. This is particularly true as a riders height increases (regardless of physical proportions) as many manufacturers fail to increase a bikes stack in proportion to its reach. For example, Trek maintain exactly the same stack height across the first four frame sizes of its 2017 Fuel EX trail bike (something that I’ve never seen mentioned in a review of this bike). Essentially, the taller you are, the more you are expected to reach down to the ‘bars (placing the rider in a less stable position). I’ve plotted some similar numbers for a range of bikes here http://www.daveypushbikes.com/blog/balancing-act-part-2 and its a similar story for all of them (Rolo did something similar a while back for road bikes). Therefore, I suspect I am not alone in experiencing this issue (even if in my case it is more extreme than most).

Essentially, the taller you are, the more you are expected to reach down to the ‘bars (placing the rider in a less stable position).

Is that really the case or are the shorter riders expected to use flat bars and the taller riders riser bars with more spacers under the stem?

@oliverdavey80

I believe that the current trend in mountain biking (longer, lower, slacker) is creating more stable bikes at the expense of rider stability. I also believe that, within reason, rider stability (i.e. being able to control a bike from a position of stability) is central to having fun on a bike.

First and foremost, what I would like to create is a frame that places the rider in a more stable position than I have been able to achieve with previous bikes. The stability of the bike is almost a separate (but very much interrelated) issue.
Thanks.
Good explanation of your goal. Start getting your point… 😉

Interesting approach. You seperate rider and bike stability.
Mmmhhh
Have to admit: I was wrong.

possibly both arguing for the same thing?

Funny enough, answer is: yes
😉

like this thread! Makes me thinking!
Neat.

Hi chiefgrooveguru – using spacers and riser bars is certainly one way to get around this issue, but it’s hardly an optimal solution. I would suggest that this sort of thing should be kept for fine tuning rather than something as fundamental as rider height. From what I can workout, the four different Trek frame sizes that I mentioned are supposed to cover everyone from 5’1” to 6’4”. That’s quite some range.

I don’t think that its unreasonable to expect that each frame size should be optimised for the height and proportions of Mr or Mrs average who sits in the middle of each size range. But really this issue is just one part of the bigger challenge of achieving a more stable riding position.

I do agree that head tubes should get a little longer as bikes get bigger but not a lot. I think there are multiple aspects which affect bike fit:

1. Height and proportions (leg length, back length, arm length).
2. Weight distribution (ie build).
3. Flexibility
4. Strenth
5. Natural riding style (I suspect this is usually a product of the previous four aspects).

I don’t think it’s unreasonable to suggest that a rider with a longer torso and shorter limbs, plus more weight higher up, will benefit from a longer reach bike (both for fit and stability reasons) but could need a relatively short head tube to get the handlebars in the right place.

Conversely a rider with long legs and short torso will want less reach, especially if more of their weight is in their legs (lower CoG) and if they have short arms.

Another thing to bear in mind that is a bicycle (engined or not) is most stable if the weight is concentrated low down and forwards. So even if the rider is in a less natural attack position, that could be outweighed by the increased stability of the rider+bike system.

All very interesting stuff (in a massively geeky way!)

I had a set of Quasar Link forks back in the day. Looked trick but were utterly awful in use. All the damping was friction based; tighten up the bolts holding the spindles in place for more. Naturally the damping worked equally badly / well on either compression or extension – and with a sodding great elastomer shock in place, it usually never worked. The whole enterprise was a disaster – worse even than the original “stop working at all when it rains” RC36’s. An interesting historic diversion, and it would take a lot to convince me to ever bother again.

Not a fork as such, but I’ve just seen this bike mentioned on a Bike Radar vid
http://structure.bike/

Main reason it won’t take off is because it’s ugly as sin. But it’s all about the linkage!

Went for a ride with a bloke from Middleburn back in the day who had an AMP – those forks were even more flexy than our Pace RC36s. He kept crashing 😆

I believe that the current trend in mountain biking (longer, lower, slacker) is creating more stable bikes at the expense of rider stability.

That’s the bit that I disagree with, I don’t think they are generally I can see why these bikes might not be optimal for you specifically, but for me (Mr average at 5’10” with 32″ inseams), I am much more stable on them than older style geometry secondly I sadly think rider awareness, confidence, ability, and understanding is never going to match the capability of bikes, and in some ways that’s fair comment from an industry aware that folks want to go fast, and are largely providing the tool for most folks with average capability to achieve that. That might not be what you (or I) think of as fun, but that’s not for us to judge, is it? and thirdly dropper posts, which most people use these days which has a massive effect on CofG that you haven’t mentioned in your blog, and I’d be keen to hear what effect they have on your ideas for you bike

I love your experimentation though, I think more cyclists should question the accepted view as you’ve done.

andreasrhoen, I think the aeroplane analogy works up to a point, after all I think you’re right that everyone would prefer something perhaps “flicky” and “fun” and so on. but the difference is of course the thing that mostly effects the airplane is input from the pilot, that’s NOT true of mountain bikes, where loads of the input is out of the riders control.

I’m interested to understand the biomechanic differences/similarities between foot “input” forces being applied via
1. Feet on solid ground (Tennis Athletic stance)
2. Feet on fixed foot pegs (Motocross Athletic stance)
3. Feet on a pivot point located in line with point of contact. (Push Bike)

Hi chiefgrooveguru – don’t get me wrong, I completely agree that if your proportions sit someway outside of the middle of the bell-curve (like mine do) then custom geometry may be the best bet.

Characteristics like strength and flexibility are an interesting one. For a discipline like time trialling where there are competing and conflicting demands on body position (in this case, for example, to generate power and to be aerodynamic at the same time) then they definitely come into play. As for a mountain bike designed for having fun (rather than for racing) I’m not so sure where something like flexibility or even strength comes into the mix? I must admit, I haven’t spent a great deal of time thinking about this (because I’m only designing for myself with a fixed/declining (!) level of strength and flexibility), but it’s an interesting question. If greater strength means higher speeds then there is an argument for geometry that is more stable. But when it comes to the riders position on the bike, I would still argue that it should still be governed primarily by stability, which will be very similar, if not the same, regardless of rider strength (all else being equal).

As for CoG, low is more stable when braking or accelerating, but (unlike four wheeled vehicles) this doesn’t hold true when turning. A bike can be thought of as an inverted pendulum (the analogy of balancing a mop handle vertically in the palm of your hand is often used). Rather counter intuitively, the higher the centre of gravity, the more stable a two-wheeled vehicle (or mop!) will be when turning or riding over rough terrain, because more force is required to knock it off course by a fixed amount. Equally, more energy is then required to correct the bike by the same amount.

andreasrhoen, I think the aeroplane analogy works up to a point, after all I think you’re right that everyone would prefer something perhaps “flicky” and “fun” and so on. but the difference is of course the thing that mostly effects the airplane is input from the pilot, that’s NOT true of mountain bikes, where loads of the input is out of the riders control.

@nickc-bloke:
agree.
The airplane thing might have been not the best example…
But following situations might be similar – when talking about airplanes: flying in turbulence

And: airplanes with too much stability tend to be very bad in turbulence!
Certain airplanes are just great in calm weather conditions and behave nasty when it get’s “wild”. 😯

But overall: the airplane thing might guide us onto the wrong path…
😥
Thanks for thinking about it so!
😉

As for a mountain bike designed for having fun (rather than for racing) I’m not so sure where something like flexibility or even strength comes into the mix?

Flexibility and mobility are hugely important for riding a MTB well. When my hips are moving well I can flick the bike about better, balance it on the limit of grip and flow with more style and fun. And when I’m feeling strong I have more spring when jumping and can hit technical or rough sections with more speed and confidence.

Often the biggest difference you’ll notice between two riders of differing descending ability is the better rider is moving their hips more.

Rather counter intuitively, the higher the centre of gravity, the more stable a two-wheeled vehicle (or mop!) will be when turning or riding over rough terrain, because more force is required to knock it off course by a fixed amount.

I read this on Geoff Apps’ site some years ago and I didn’t believe it was true then and I still don’t now. I don’t know if it’s the case of applying an incorrect analogy or incorrectly applying an analogy – I wonder if it could be that however hard you try a MTB will be destabilised by external forces and the bike with the higher CoG will be harder to restabilise?

If you don’t believe me, borrow a bike with adjustable geometry (like my Banshee Spitfire). The increased cornering stability (and increased reluctance to flick slaloms turns) when in the lowest BB setting is very obvious. It takes about 5 minutes to change the geometry so it’s easy to AB test.

Hi chiefgrooveguru – sorry, I’ve just re-read my last post and I should have been more explicit / reiterated that the point I was trying to make was specifically in regard to rider position. I completely agree with you that flexibility and strength are important to riding a bike well. I would certainly love to be more flexible and stronger on the bike! My point is that I’m not certain how flexibility or strength would significantly affect the placement of a bikes contact points (and therefore a riders body position) if, like me, achieving rider stability is the main goal? Whether a rider is strong or weak, flexible or inflexible, all else being equal I suspect that a stable body position will be the same.

As for a bikes centre of gravity, this is my best attempt at trying to explain what’s going on http://www.daveypushbikes.com/blog/bottom-bracket-height-the-most-misunderstood-dimension

Whether a rider is strong or weak, flexible or inflexible, all else being equal I suspect that a stable body position will be the same.

But it isn’t because the stable body position is part of a dynamic series of positions and the greater the rider’s strength and flexibility the better the bike geometry can be optimised for bike handling rather than rider stability. It would be different if we rode one hour plus descents but we don’t – a few minutes is normal and twenty minutes is pretty much the max – so we don’t need our riding position to be the easiest to hold stably, we can compromise to get better handling.

The easier you find it to hip hinge, the lower the bars can be for your height.

As for a bikes centre of gravity, this is my best attempt at trying to explain what’s going on http://www.daveypushbikes.com/blog/bottom-bracket-height-the-most-misunderstood-dimension

That was the first bit of your blog I read and the only bit I really disagreed with. The cited Wikipedia article doesn’t help either. And Geoff made similar claims with his Cleland which again don’t stack up.

I wouldn’t bother dropping the BB on my Spitfire for uplift days if it didn’t make a difference to cornering stability – it isn’t like I need to slacken the head angle, it’s already at 64 deg, I do it to make the bike lower. I’ve yet to see any convincing maths to counteract my own experience.

I’m sure the BB height does relate to both the axle heights and the height from the ground, and to the crank lengths and riding style.

Also, before my Spitfire and Zero AM I had a Soul with 140mm forks. The BB height on the Soul was 40mm higher than on the Zero. Corners in mud when both wheels are drifting are so much easier on the latter – you could say that’s because of the longer wheelbase which is why my original examples were smaller adjustments (up to 15mm) on the same frame but it was a BIG difference between those bikes.

I am really unsure of the relevance of bb height above/below axle height as a separate issue (separate from the other issues of wheel size and bb height, with which it is obviously intimately related).

Really, there is no substitute for mechanical analysis of this with maths and stuff, everything else is just words.

Is stability the same thing as handling, or is it something different?
http://cycleseven.org/bicycle-stability-and-centre-of-gravity-or-mass

This is all very interesting, but any chance we can get back to the original subject? 🙂

You’ve got some time to do stuff before the framebuild course – I want to see the CAD development of a tubular steel spaceframe version of the fork. This list has all the cheap and easily obtained Columbus cro-mo plain gauge tubes (8-10-12.7-14-16-19-22mm etc).

http://www.framebuilding.com/Spare%20Tubes.htm

Have you also seen the Nukeproof Reactor frame / fork from the 90’s?

Cool project, be interested to see how it pans out.

I just realised why the balancing the broomstick analogy does not hold up. Once the limit of grip has been exceeded we are not moving the contact patches of the bike to balance it, we’re moving our own mass.

So a better illustration would be to get a set of stilts and see how balancing on them is easier or harder as you vary the height of the footrests. And then repeat the experiment on a low friction surface.

Hi paton – we’re drifting into semantics here, which is always dangerous territory! I will simply say that the definition of rider stability that I subscribe to is the ability for a rider to remain in a steady state (i.e. maintain balance) when subject to external forces. This is not the same as handling.

Hi mick-r – You’re right, and apologies for veering off on such a tangent.

I’ll see what I can do about the CAD development! I have produced some simple 2D drawings, which are included in my blog, but my next steps are likely to be some form of structural analysis in parallel with the pivot designs. Unfortunately, right now I’m not quite sure when I’m going to get the time to do this, but I’d love to see it become a reality.

Back on Linkage forks, I’ve seen most of the designs everyone else will have and one thing struck me on the point about axle paths;

Pretty much all of the designs I can remember seeing use either a parallel or trapezoidal linkage (USE are an exception in using a sort of oleo link as pointed out in the lefty thread the other day) and therefore they all result in some degree of curved axle path…
But has any one ever attempted to use a scissor type link to produce a linear axle path? You’d inevitably need more linkage members, but you could reproduce that linear axle path that all of us Tele’ riding MTBers are now used to.

I do like a linkage fork, and sort of remember my old girvin with a retro fondness

Love the new wraith bikes mantis stuff, totally bonkers

But has any one ever attempted to use a scissor type link to produce a linear axle path? You’d inevitably need more linkage members, but you could reproduce that linear axle path that all of us Tele’ riding MTBers are now used to.

There’d be a boatload of extra moving parts, to replace the one moving part in a tele fork. That’d cost a fortune and be heavy.

Wonder what that mantis rides like.

That mantis is glorious in a “two fingers up to the establishment” sort of way 😆

The axle path on a telescopic fork is pretty close to idea from a bump absorption perspective, being heavily rearwards. The downside is the steepening of head angle and shortening of front centre and wheelbase.

Really you want the front centre to stay constant rather than shortening, the trail to remain constant and the axle path to be rearward (especially between about 20 and 80% travel). Something that resists brake drive (without losing grip under braking would be nice too. But they’re mutually exclusive. So which to prioritise?

Full Mantis madness, gearbox, 5″ travel fs, variable offset linkage fork, bonkers…totally. bonkers…. and apparently rides really well.

That looks a lot of fun!

They’re missing a trick not making the fork single leg 🙂

Edit: Oops, accidental thread resurrection