Assuming no change in strength/fitness/endurance between the two weights and no difference in travel of suspension (and in general assume similar quality of components) is there any reason for thinking there'd be a difference between say a 150lb person riding a 20lb bike and a 140lb person riding a 30lb bike in terms of the speed at which he/she can ascend and descend, and how long he/she can ride for?

I can see how it might be easier to manipulate the lighter bike (e.g. bunny hopping, or shifting the back around or lifting the front wheel) so that would help in some places on ascents and descents but setting that aside any differences?

Too many variables. Is the heavier person carrying 10lb of extra lard or 10lb of extra muscle??

you know what – I was about to ask the same question

ie lose 5lbs weight or 5 lbs off bike – does it make any difference? I think losing the weight off the bike makes more of a difference (something to do with unsprung weight) but would like to know the science

**pulls up chair**

Well, take it a stage further and ask yourself whether a 140lb rider on a 40lb bike would be riding at the same speed uphill as a 160lb ride on a 20lb bike… Tells you everything you

Well, a rucsack feels lighter after you've eaten the food in it. So yes.

a rider is used to moving under their own weight so the smaller the additional weight (the bike) the easier its going to be.

The 'roadie' view as far as I can see is that if you can maintain the same (or close to the same) power output and lose weight then this will give you a bigger gain than shaving a bit of weight off your bike…and it will probably be cheaper too.

Losing weight off the rider saves money.

All things being equal then the lighter bike will be faster than the heavier one due to less unsprung mass.

@rkk01, explicitly ruled out such variables in the statement of the problem – 'assuming no change in strength/fitness/endurance …'.

@Badly, actually that doesn't really help me, my intuitions are silent about which would be the fastest assuming the bikes are of the same quality.

@thepodge, I kind of see that, but again if we're assuming same strength etc doesn't this rule out this kind of effect?

@ctznsmith, would be interested to know what the rationale for the roadie view is, no doubt it's based on something (experience perhaps) but kind of interested why it should be the case that weight loss on person is better than on the bike.

@Ian, will google 'unsprung mass'.

Depends on where the weight is in the bike, rolling weight would be much more difficult to shift than static weight (I think).

@Ian, suitably googled, don't see why it makes it difference.

@daniel, OK thanks, so suppose both bikes had the same wheels/tires, I see there's some benefit to be had in reducing the weight of wheels/tires.

150lbs rider – Their bike is 13.3% of their own mass
140lbs rider – Their bike is 21.4% of their own mass

150lbs will be stronger than 140lbs rider, and is only bringing 13% extra mass. So would be quicker.

@ooOOoo, no explictly ruling out difference in strength between two riders.

I think the 'roadie' rationale is that it's easier to remove weight from the person (i.e. 10 of lbs) than the bike 1 or 2lbs.

In your question/example if you were on a road logically the person on the lighter bike should climb better (same strength and therefore power but less weight to shift up the hill against gravity) and the person on the heavier bike descend faster (same power but more potential energy).

On a loose or rough surface then all sorts of variables come into play surely. Does the heavier person put more weight down onto the back wheel which equals better grip on a loose surface for example?

explictly ruling out difference in strength between two riders.

Don't see how you can. If they are the same strength, then their strength to weight ratios will be different. If the ratios are the same then their power outputs will be different

OK
PE= mgh

(150+20)gh=(140+30)gh

Erithacus rubecula jv

All things equal for the same power output(from the rider) then both would climb at the same speed as total weight(bike plus rider is the same)
However it would mean that the lighter ride had a higher power to weight ratio to achieve this

what the bobbins has unsprung mass got to do with anything?

if two different people have exactly the same strength then (despite adding in more variables than can be really taken into account) the lighter bike still wins

@rkk01 didn't say anything about their strenght to weight ratios being the same, clearly they'd be different if they had the same strength and different weight.

@ctznsmith, don't see why the person on the lighter bike has less weight to shift up the hill, and I'm with ooOOoo re the potential energies being the same. But yeah, lots of variables on rough track which I guess are hard to remove, so probably the roadie answer is of most interest, though if there's any big reason why there'd be a difference off-road keeping everything as similar as possible that'd be interesting too.

oooo heavier rider will not always be stronger the only fact in that statement is that they are heavier!
bwd not strictly true 140lds of muscle and venom will get a 40lb bike up a hill faster than 160ld of lard will get any bike up anywhere – never had your arse kicked by a lighter guy on a heavier bike I have
g

@thepodge, I don't see why the lighter bike still wins, in both cases the weight of bike+rider was the same, why is it not this overall weight that's most important? But do agree that I cannot see a reason for unsprung mass making a difference, now I know what it is.

I guess if there is an extra 10lbs on the wheels the rolling weight will be a lot higher rolling resistance, so it will be harder to push the wheels round. Also you muscles are used to carrying around your body weight around all day so having a heavy bike is a shock to the system.

I’m 15 stone and have a 22lbs fulsusser and a 30lbs hardtail and it is a lot harder riding the hardtail (it is setup like a trail bike and not a race bike)

I should say the light bike is better 🙂

@charliemort – unsprung mass is the mass (or weight) that is not supported (or above) the suspension, so your suspension fork lower legs, disc brake caliper, wheels etc are un-sprung, the frame and the rider are sprung. Reducing the un-sprung mass means that the suspenion works more effectively as it less mass must be moved during responses to terrain and rider input.

@ctznsmith – that view can only be true if other factors are taken into account, i.e. it is much easier/cheaper for the average rider to lose 10lb from their own weight than the bike, and in some cases (wheels/tyres) the mass lost has a greater effect on speed than losing body mass.

As far as rider / bike weight goes, the bike is easier to move in all aspects (uphill or in the air) if the mass is lower, but so is the rider – if you compare the 150lb rider on a 20lb bike to the 140lb rider on the 30lb bike, I'd say that the two would be as quick IF their power outputs remained identical (not considering different rotational mass etc).

On a decent bike, after a decent set of wheels there is little point in spending big money on weight reduction unless the rider has a decent power output, strength and realistically low body amounts of body fat.

kingtrout is on the money – overall weight is far more important (assuming similar power output).

@Swalsey, this is more or less what I thought, and what prompted the question.

@Nezbo, out of interest how much of a difference does the weight make do you think, I guess they're different types of bikes so makes it a little unfair to compare, but roughly how much slower is the heavier one assuming say you do a route that they can both cope with?

The only difference really could be through rotating mass.

If the wheels and drivetrain were identical on both bikes, and only the frame weight varied, then in theory they would climb identically.

I'm an idiot the PE would be the same – d'oh.

@kingtrout – aye, I think my answer should simply be 'no', but with all the other crap added on to justify myself 🙂

kilgoretrout – I recon the new Full susser has knocked about 30 to 45 mins off an 100k race.

my full susser is a racing MTB ie lighter than light, race geometry, etc… but the hard tail is built for comfort, wide bars, short stem, etc… so they are totally different bikes. But i think it is all to-do with the weight.

i have used the same tyres on both bikes (i have changed my tyres on the hardtail to bigger gripper ones and confort) and i think it is still the weight difference.

@Nezbo, thanks for that, what sort of percentage of overall time does that saving represent? I guess still pretty significant even at a slowish pace for 100k, that's pretty interesting.

@thepodge, I don't see why the lighter bike still wins, in both cases the weight of bike+rider was the same, why is it not this overall weight that's most important? But do agree that I cannot see a reason for unsprung mass making a difference, now I know what it is.

Even relatively smooth ground is made of lots of little bumps. When a mass is raised by going over a bump it converts some of the horizontal kinetic energy(speed) into potential energy by gaining height. However on the downside of the bump, you won't reclaim all this energy back as horizontal speed, so you'll be a fraction slower. If you've got 10 pounds of weight on the bike, you'll be constantly raising and lowering it a bit more than if it's on your body, where your body tissue acts as a suspension system. This holds true whether the bike is a full suss or a hard tail, but I guess is more profound on a HT
But if you want a simple example compare the difference in effort between riding a full-suss vs. a hard tail through a good horse rutted then solid set bridleway. A smother trail has exactly the same effect, just to a lesser degree.

Unsprung mass from my understanding is a way of describing mass on a bike which is not suspended ie, wheels, callipers, fork lowers, swing arm, any component which is unsuspended, considering the question made no reference to suspension I’m not sure what it has to do with the OP…

I think the power to weight ratio answer was pretty much the most useful the heavier rider is shifting less mass relative to their own weight and thus is doing less work relative to the lighter rider…

I’m not sure what the point of this exercise is meant to be, as it has almost no bearing on the real world, as you’ve removed far more variables than you ever could in reality…

kilgoretrout – I used to do 100k'ers in around 6 hours, but now I am down to around 5 hours. (Some of that time difference is me getting fitter and faster though) I know not fast but I am training hard for next season, but fast enough for a 15 stone fatty 🙂 I mean I am not Nick Craig (maybe next year hehe)

But I am crap at maths (and grammar/spelling), just give me a crayon and I might just be able to colour in within the lines, so I an not to sure what percentage it is 😉

Unsprung mass from my understanding is a way of describing mass on a bike which is not suspended ie, wheels, callipers, fork lowers, swing arm, any component which is unsuspended, considering the question made no reference to suspension I’m not sure what it has to do with the OP

Unsprung means (or can mean) with reference to whatever spring you're looking at in the complete system. The different springs having different magnitudes of effect. If you take the rider as the spring, then it's the bike that's unsprung mass, if you take the bike's suspension systems then it's the parts you list, if you take the wheels, then it's the tyre carcass etc. etc.
The important point being that any mass that ends up having to moved up and down wastes energy, and the 10lbs on the bike will be moving up and down more than 10lbs on a person.
Assuming you don't hit the resonatant frequency of moobs 🙂

@Nezbo cheers for that, pretty big difference between the bikes then.

@Ian, still not seeing quite why the bike mass gets move upwards more than the rider's. When you go over the bump rider and bike both rise don't they? It sounded like you were saying that the rider doesn't rise as much because there's natural suspension built into the body, but wouldn't that mean that when they get to the other side of the bump that they're now lower than they were before they arrived at the bump, and that if they want to raise themselves back to the appropriate riding position then they do have to raise their weight as well as the bike?

in the last few months ive lost 24 pound in weight and got a new bike thats 6 pounds heavier than my old one and im much faster on it than i was the old one

that said i wouldnt mind seeing what i was like now on my old race dale ht and my anthem lol