[Closed] Do small riders need so much suspension travel?

I learnt yesterday that they're 'overwheeled' on 29ers so I can see them being 'oversuspended' on anything more than 120mm of rear axle travel.

I'd have assumed that a lighter rider needs more, so that they stick lines through rock gardens more easily.

The obstacles, drops and most of the bike is going to be the same size (ignoring can of worms), so the travel needs to be the same just correctly set for weight.

What flatboy said.

Anyway how can a small person know what a large person's experience is?

They probably have shorter legs and arms, so less natural suspension.

Maybe they need more mechanical suspension?

Indeed, as flatboy and Al concur, a smaller person just has less air in the shock (or a lower weight spring), so the travel can still be used properly. The women still race the same designs of bikes as the men in downhill races and they don't seem to have any issues with having too much travel.

Apols for the word concur!!!111!!

cynic-al - Member

Anyway how can a small person know what a large person's experience is?

Stilts.

It's down to weight and ride style rather that how tall you stand. Not everything is linear and related to leg length!

To summarise: what theflatboy said!

Another +1 for theflatboy. Unless the smaller riders ride smaller trails, their requirements are the same.

I see where flatboy and his supporters are coming from but should probably justify where I was coming from to start with

Imagine hitting a drop to flat (because it's easier to analyze than anything else). The drop is the same size no matter what the rider size. Scale down suspension travel s with rider height x, and assume that mass scales with x^3 and limb cross sectional area with x^2

The potential energy from the drop is absorbed into the suspension travel, so
mgh = Fs
or dropping constant terms
m = Fs

And limb strain is force per unit area, so
strain = F/x^2

Putting it all together
s = x, m = x^3
strain = m/sx^2 = 1 i.e. constant provided we scale s with x

So yes the drop is just as big no matter how big the rider, but the lighter rider has less body mass to slow down on impact. You can drop rider style out of it - for sure people who ride different get a different ride, that's another topic altogether.

Translate the analysis to sucking up a rock on the trail into your suspension. Yes if you don't have so much travel the front of the bike will kick up more. But maybe a lighter rider would handle that kick better as their limbs feel less strain from it? Although they'll possibly lose some traction?

Interesting point about racing pros. Does everyone at the top end of dh race with the same fork travel? If so is that because something else sets the upper limit of how much travel you can get away with on a bike (CoG height versus BB clearance?) and everyone racing proper* dh just wants as much travel as they can get?

* i.e. not including courses that get won on 160mm 29ers no matter that I'd love to have one of those 😉

does this take into account tubby shorties and lanky tall people?

surely its about weight and not height?

Anyway how can a small person know what a large person's experience is?

This is a very valid point, on one level we can't (though Dave MacLeod tries to compare the two in his book from a rock climbing perspective and notes the strong/weak points of each body type).

In practical terms though if each person tries out lots of bikes to find their ideal trail bike (aaah) then if on average smaller people choose shorter travel that would tell us something.

does this take into account tubby shorties and lanky tall people?

[from original post] assuming similar body shape

That would be no 🙂

I think, in amongst doing the maths, which I haven't checked (although I suspect that if it stands up, you are calculating the wrong thing), you have forgotten that everyone falls at the same rate, so on your drop example, regardless of the weight of the rider, they are falling at the same velocity when they land from a given height of drop. Therefore in order to all experience the same level of deceleration, everyone need the same amount of distance to slow back down to zero vertical velocity and that distance is provided by the suspension travel. So if the smaller rider has less travel they will experience a higher g-loading when they land, just like you will on a shorter travel bike. The actual loads may be smaller, in whatever units you favour, but as a proportion of their mass, it will be higher.
Also, you are missing that the point of suspension in an ideal sense is to allow the wheels to move up and down sufficiently that riding over a bump doesn't cause the rider to accelerate upwards at all. The wheel travel relative to the rider should be exactly the height of the bump. this is independent of rider size and reducing the travel for the smaller rider simply limits them to smaller bumps before the suspension is overwhelmed and runs out of travel.
In short, you are wrong, or missing the point 😉

sideshow - Member

flatboy and his supporters

This is a first!

Interesting question.

I'm 5'8", my mate is 6'7". If we ride the same rock garden, he has a much bigger range of movement in his arms & legs to work the bike than me, so in effect possibly uses less suspension travel than I do.

But working on the assumption both our bikes are set up correctly in terms of sag, compression & rebound damping etc, I'd expect to see similar travel ranges for a given trail as the bikes should both get the same travel for a given obstacle.

Getting back to your original question of *need* I would say no. There is so much difference in rider style and ability that makes the minutia of whether a shorter rider *needs* any different travel, as you describe, invalid.

Set your suspension to suit your weight, adjust rebound to taste as we are getting pedantic - ride - ride more/improve. If you reach world class level then perhaps your mathematics could be examined further. 🙂

Robin your first paragraph is misguided (apart from the suggestion that I might be calculating the wrong thing, you may be right on that point). Yes discounting air resistance we all land at the same speed, but heavier people are carrying more kinetic energy when they do. Put another way it's not about the rate of deceleration but the forces experienced during an equal rate of deceleration (which will depend on how much mass you are trying to decelerate) and the area over which those forces are spread (which depends on how thick your bones and muscles are). This (and terminal velocity issues) is why a mouse can supposedly survive a fall from pretty much any height where it won't suffocate/freeze, but elephants can't jump.

In response to your second paragraph I am more inclined to agree.

If we ride on forks of different lengths, the small fork will bottom out first when hitting the same size rock. (Actually I'm not 100% sure of this - the smaller rider will be thrown upwards more which postpones the bottoming out... confused there).

If we ride on forks the same length, then if sag is set appropriately they should bottom out at the same point. But a larger rider will be experiencing more stress at the moment when the fork bottoms out, than a small rider would be.

This raises the point, how are we defining a similar ride? Traction issues may show up differently to fatigue on long descents for example. Either way it's a fair point that performance of a fork over bumps is nonlinear - most notably at the point where travel bottoms out - but before you even reach that point linearity is lost especially on an air shock.

In conclusion maybe the truth is likely to be somewhere between the two viewpoints - smaller riders can get away with smaller forks - but not completely in proportion to their height.

Damn all this theory though - still (alas) no small rider has chipped in to tell me about their preference on travel for a trail bike!

andyrm, assuming you're both riding bikes that fit you. You on a medium 26" and your mate on a large 29er, both identical abilities, I can't see how your mate would be able to work the bike more. He may be able to roll over rocks a bit better on a 29er but that's all I can see…

Thanks Andy for the first vaguely empirical answer 🙂

I'm 5'8", my mate is 6'7".

but heavier people are carrying more kinetic energy when they do...

So are you talking about smaller people or ligher people?

woodsman has a good point on rider style anyway, overtaking full sussers downhill on a rigid bike puts it all into perspective...

gonfishin - both. I'm working with an x, x^2, x^3 scaling law. Any variation on that (small and fat for example) you'd have to work out separately. Also it would depend on where you carry your weight - arm/leg muscles or elsewhere.

There's a project for a sports science student - make an online calculator for impact stress based on individual rider stats. We could all post up our scores and see if they correlate to riding ability. Though you'd have a hard time measuring the latter.

sideshow - you mean the first answer that vaguely agrees with you more like 😉

I do think it's down to weight really, and as most bike riders are likely to be not obese, the likely hood is that shorter riders may select less travel, as they are likely to be lighter in general. Which kind of agrees with you in part but not for the reasons you suggest/caculate.

I think your second post is too oversimplified. I can see what you're trying to do in trying to find which terms are proportional to each other, but I think the first equation mgh=Fs is far too simplistic a view. In reality, a suspension shock will slow the descent of a rider upon landing a drop, but it will also absorb a certain amount of energy in the fluid and reduce the impulse on the rider.

A smaller rider will have a weaker spring/less air, but could also have different "tune" on the suspension in other ways affecting how the energy is absorbed.

I agree that the larger rider will have accumulated more momentum when he reaches the bottom of the drop, but the set up of his shock will be such that it absorbs more energy, so the impulse on the two riders is reduced by similar amounts.

The shock will also use up its travel more slowly for the heavier rider, assuming the shocks are set up to use the same amount of travel for the same drop, so the momentum change will be spread over a longer period, resulting in similar forces being applied to the two riders.

+1 I hadn't considered that. The peak force may not be proportional to the average force during shock compression.

Seems to me that the biggest flaw in all this is assuming that weight and height are completely and directly corellated.

The points you're raising are perhaps valid insofar as applying to a heavier rider. Their size is a red herring innit?

I’ll grant that the smaller rider will likely have a lower stress in their bones at the same acceleration since bone cross section will increase roughly on the square of height while weight is more likely to be on the cube, but I’d contend that looking simply at that is reducing it to a consideration of survivability, when what we are implicitly trying to consider is to do with tolerance, i.e, assuming the riders are similarly fit, and therefore have a similar strength to weight ratio then the issue is to do with stress in their muscles (in the true engineering sense of force per unit cross-sectional area). It seems reasonable to assume that at the maximum tolerable limit of load bearing, any size of rider will have approximately the same stress in their muscles, since everyone has pretty much the same kind of muscle fibres, the larger rider simply has more. Providing the riders have same strength to weight in order to have the same level of fitness, they will therefore be able to tolerate the same level of deceleration, which brings you back to needing the same length of travel to slow them down.

Now, you might, probably correctly, assume that in general the smaller, lighter riders will in fact have a higher strength to weight ratio, and indeed at maximum fitness this tends to be true (as evidenced by the fact that top rock climbers tend not to be all that tall, nor hugely muscled and therefore heavy). That would imply that the smaller rider can actually stand more impact and might therefore need less suspension but from watching different sized riders at DH races, it seems to be an advantage to be tall and reasonably heavy, at least in terms of enabling you to smash straight through rock gardens. Watching Steve Peat at Fort William a few years ago compared to smaller riders, he was clearly able to use his longer limbs and greater weight to allow the bike to kick around under him more without himself being in danger of being bucked off, so perhaps the answer is that smaller riders can land bigger drops and larger riders can straight line the rock gardens better and on balance everyone needs about the same amount of suspension but sets it up and uses it differently.

The mouse vs elephant thing is just a red-herring in this context, the size differences we are talking about are far smaller and terminal velocity definitely doesn't come into it here.

Agreed terminal velocity has nothing to do with this, just the mouse (and not the elephant). +1 for another empirical observation. So many variables affect the theory that I think stats speak louder then algebra. (A bit like the what-makes-a-bicycle-stable debate).

I think it's more to do with stiffness than travel. I would think lighter riders would want a lower spring weight which would in itself lead to longer travel. That's how it's worked for me anyway. I'm 60kg and much happier now I have the lightest spring available in my forks.