A bike weight difference of a few kg makes little difference to climbing ability.

 

Try telling that to Lance Armstrong!

A few kg extra would still only equate to a few percent less climb speed, even for Lance Armstrong.

 

Other people are more likely to overlook the usually larger negative impact of excess bodyweight on climbing ability, about a 12% penalty for both of us, for example, which I imagine the likes of Lance Armstrong should tackle as a first priority.

 

P.S. On the subject of weight watching, I find body fat percentage measurement a much better guide to optimum weight than BMI, oldosc.

 

Stuart.

Edited June 8, 200817 yr by coops

I fear we may be boring others...... But, whilst most climbers over the years have been slight (Charly Gaul, Robert Miller). Others like Armstrong were of average weight (for a competitive cyclist) and some like Indurain, and Merckx before him, were actually quite heavy. They were all fit and all sought advantage by using the lightest bikes available in the mountains, weight in the wheels (and especially the drive wheel and the rims) making the greatest difference. As a fatty, I can get up a hill best on my lightest bike. My 1980s Pinarello beating the Cytronex easily (unless I turn on the motor!).

 

Chris

Interesting (to me!) Chris I suppose once we're fit & optimum weightwise a lighter bike will be one of the best gains available, whereas until then its our best hope to get up hills!

 

I was quite surprised to learn that weight reduction off the wheel counts for double that weight from the frame, but how to apply that to hub motor wheels is beyond me - one for the engineers! (and probably for another thread as you say... )

 

Stuart.

Interesting (to me!) I was quite surprised to learn that weight reduction off the wheel counts for double that weight from the frame, but how to apply that to hub motor wheels is beyond me - one for the engineers! (and probably for another thread as you say... )

 

Stuart.

 

Triuble is, you've got me going on an issue that interests me.........

 

From first principles, and a base of knowledge from non-electric bikes, I would expect the following. All other things being equal (motor drag etc), a front hub motor should be better that a rear one (it's still the rear wheel we turn with our legs) but direct drive through the chain might be expected to be better than both (motor weight isolated from rolling weight).

 

That said there are plenty of other things to confound the above....... Hub gears offer more frictional resistance that derrailleur gears (if the latter are properly maintained), they also weigh considerably more.

 

When it comes to weight in the wheel, I wouldn't assert it is necessarily twice as important than weight in the frame (though it might well be). There's been a lot of work put into assessing the efficiency of modern racing wheels (see a French Site called Roues Artinasal: http://www.rouesartisanales.com/article-2141818.html: ). The most importand determinants of performance are thought to be aerodnamics (crucial at speed) and inertia (mostly related to weight and determining the effort to get , and keep, the wheel moving). The search for aerodynamics has led to fewer, specially shaped spokes and deeper rims. Reducing inertia has led to lighter wheels and the use of carbon fibre, especially in the rims. The rims matter much more that the hubs, because the further the weight is from the point of rotation, the greater the force required to move it.

 

So....... If I was trying to improve the climbing ability of the Trek (or any other bike) I would try to save weight, but especially rotating weight. A good pair of modern racing wheels can weigh as little as 1Kg. The Trek's weigh twice this, even without the motor. I would try lighter rims and lighter, livlier tyres thereby losing puncture resistance!) Elsewhere, a good pair of cyclo cross carbon forks, perhaps carbon bars and a lighter chainset. I guess all that could get 1.5Kg + off the weight, but it could cost £500, even building the wheels yourself!

 

Train journey nearly over, so I guess I had better leave it here.

 

Chris

So....... If I was trying to improve the climbing ability of the Trek (or any other bike) I would try to save weight, but especially rotating weight.

 

Weight at the wheel rim counts twice as much as weight on the frame, but only for acceleration, not hill climbing.

 

This is because the wheel rim travels at twice the speed of the frame and so must be given twice the acceleration to achieve this. That isn't an issue when traveling uphill where all weight counts the same (including the weight of the rider!)

Weight at the wheel rim counts twice as much as weight on the frame, but only for acceleration, not hill climbing.

 

This is because the wheel rim travels at twice the speed of the frame and so must be given twice the acceleration to achieve this. That isn't an issue when traveling uphill where all weight counts the same (including the weight of the rider!)

 

Hang about John, why do you assert that the rim travels twice as fast as the bike? The speed of the outer circumference of the tyre (which is in contact with the road) is, by definition, the same as the road speed on the bike. Indeed, this is how bike computers calculate your speed (and the reason why you have to enter your tyre size into the computer). As to the effect of climbing and wheel weight, I can only suggest you compare a few wheels in the same bike frame. I have, and I can assure you that my son's Bontrager XXX's (weight about 1200 gms) take a damn site less effort to get up a hill than his winter training wheels at twice the weight.

 

Chris

I had always assumed one of the main affects of the bike weight was in terms of the sprung/unsprung weight.

Hang about John, why do you assert that the rim travels twice as fast as the bike? The speed of the outer circumference of the tyre (which is in contact with the road) is, by definition, the same as the road speed on the bike. Indeed, this is how bike computers calculate your speed (and the reason why you have to enter your tyre size into the computer).

Chris

 

The computer measures the speed of the tyre relative to the frame, but that doesn't help us much anyway as it is just counting rotations.

 

Yes, the tyre is rotating at the speed of the road, but the whole wheel is also moving forward at the speed of the road too.

 

Let my try to explain like this...

 

Imagine you are on your bike traveling at 5 mph on ice with the brakes on, locking the wheels (entirely hypothetical). Clearly everything including the wheels is traveling at 5 mph. Now let go of the brakes and spin the wheels to roll with the road (lets say pedal for the back wheel and motor for the front). They are now rotating at 5 mph at the rim. It took 2 lots of 0-5 mph acceleration to get there.

Elsewhere, a good pair of cyclo cross carbon forks

Chris

 

You would need to be careful putting the motor into these forks, at least adding a substatial torque arm which might cancel out the weight saving.

The computer measures the speed of the tyre relative to the frame, but that doesn't help us much anyway as it is just counting rotations.

 

Yes, the tyre is rotating at the speed of the road, but the whole wheel is also moving forward at the speed of the road too.

 

Let my try to explain like this...

 

Imagine you are on your bike traveling at 5 mph on ice with the brakes on, locking the wheels (entirely hypothetical). Clearly everything including the wheels is traveling at 5 mph. Now let go of the brakes and spin the wheels to roll with the road (lets say pedal for the back wheel and motor for the front). They are now rotating at 5 mph at the rim. It took 2 lots of 0-5 mph acceleration to get there.

 

I understand where you are coming from John, but your figure of "twice" is artbitrary, you cannot assume that the force required to accelerate the wheels (in mid-air or on ice) is the same as that required to accellerate the bike (it depends on the mass of the bike + rider). Anyway, I think we both agree that wheel mass has a significant effect on acceleration. My real point of disagreement was with your assertion that wheel mass was not related to ease of climbing. As I said, just get on some light wheels and try it - it is not hard to detect the difference.

 

Ps. I quite agree on the carbon forks issue. I would be concerned about the bonding of the (alloy) dropouts into the resin - although cyclo cross forks are designed to take some punishment.

 

Chris

Edited June 9, 200817 yr by Chris_Bike

but your figure of "twice" is artbitrary, you cannot assume that the force required to accelerate the wheels (in mid-air or on ice) is the same as that required to accellerate the bike (it depends on the mass of the bike + rider).

Chris

 

The force required to accelerate the wheels (in mid-air or on ice) is the same as that required to accelerate the wheels (in a straight line without spinning) not the bike. Twice is the exact figure (assuming that all the weight is at the rim, a little less in reality), it is not arbitrary.

 

I don't deny anyone's personal experience which or course is very valuable, I just like to get the theory right too.

 

Anyway, since climbing a hill doesn't (usually) involve accelerating, the weight effect must be the same wherever you put it.

Agreed in princple John, however a further complication is if you climb a steep hill using pedal power you will accelerate on the downstrokes and slow down in between times, so you are actually doing quite a bit of accelerating!

 

Big picture though, a bit of weight can make a few seconds difference, which matters a lot to someone in a race, but on my commute to work, a few minutes either way is not significant!

Agreed in princple John, however a further complication is if you climb a steep hill using pedal power you will accelerate on the downstrokes and slow down in between times, so you are actually doing quite a bit of accelerating!

 

Yes but with a heavier wheel, the deceleration will be less, meaning less acceleration needed, the effect cancelling out exactly (at least in energy terms - the feel may be different).

 

Big picture though, a bit of weight can make a few seconds difference, which matters a lot to someone in a race, but on my commute to work, a few minutes either way is not significant!

 

Certainly agree with that, thankfully, a few seconds late for work doesn't lose me my job

I admit to no longer being a physicist, but it seems to me that when you climb you are also accelerating to defeat gravity (9.8m/s/s downwards).

 

Chris

I admit to no longer being a physicist, but it seems to me that when you climb you are also accelerating to defeat gravity (9.8m/s/s downwards).

 

Chris

 

It takes force to move the wheel vertically in order to overcome gravity but it takes no more force to keep the wheel spinning (at constant speed) when you are moving vertically (or a slope) than when you are moving horizontally.

 

BTW, I don't deny that you are noticing something when you use lighter wheels going up hill, I just can't say what the physics is.

 

Big picture though, a bit of weight can make a few seconds difference, which matters a lot to someone in a race, but on my commute to work, a few minutes either way is not significant!

 

On an e-bike the success/failure hinge point becomes important in a way it isn't so defined on ordinary bikes. Example on my Q bike (Quando) climbing my home 1 in 7 on a one third discharged battery:

 

With my weight and bike's climbs without pedalling.

 

With 5 kilos of shopping added it still just makes it.

 

With 7 kilos it has no chance, reaching stall point.

 

So there's a make or break single kilo in there. The percentage importance of that depends on the percentage share of rider effort when pedalling, but it's a factor that isn't anything like as identifiable to a precise point on unpowered bikes.

.

Weight at the wheel rim counts twice as much as weight on the frame, but only for acceleration, not hill climbing.

 

Apologies for the confusion (& now total derailment of the thread Chris!) arising from my incomplete statement of this 'rule': what John has said is what I meant, Chris, and I was thinking about acceleration (though without saying it explicitly) rather than hill-climbing :o. Apparently it takes twice the energy to accelerate wheel weight as it does the same weight on the frame, 'as a rule'. Whether the weight distribution also is a factor I don't know but I'd guess that weight on the rim makes more difference like you say Chris, so maybe those high spec wheels help you accelerate into the hill & boost your initial climb speed & confidence with it .

 

This is because the wheel rim travels at twice the speed of the frame and so must be given twice the acceleration to achieve this. That isn't an issue when traveling uphill where all weight counts the same (including the weight of the rider!)

 

Another way to say it (if I remember right...) is that that the rotational forces necessarily produced within the wheel when its rotational speed increases (i.e. during acceleration of the bike) are, coincidentally, equal to the force that is required to accelerate just the mass of the wheel (without rotation) to the speed of the bike. Therefore to accelerate and rotate the wheel takes twice the force it would if it wasn't rotating, and since the force required is proportional to the mass and the acceleration, weight on the wheel counts for twice the same, non-rotating, weight elsewhere on the bike when acelerating.

 

That the wheel is rotating is the reason the wheel speed (at the top of the wheel) is twice the frame speed (since the rotation speed of a point on the

tyre edge must equal the road speed, unless the tyre is slipping), so what I said is equivalent to John's explanation (but just a bit longer!).

 

As for the implications for hub motors, cytronex (mmmm... lemons!) etc....

 

Stuart.

In a vain attempt to get this thread back on track

 

So I've decided to stop worrying why light wheels are better on hills and just enjoy the bike. It's a fabulous evening and I've taken a circuitous route to the pub. Holland are 2 nil up and I'm having a small pint before enjoying a leisurely ride back. Fantastic!

 

Chris

Chris,

I have also made a decision, I will ignore posts from non Cytronex owners, that only leaves you and the lady with a pannier on each hip who has yet to comment, I will just form my own opinions when I get the bike.

 

What exactly is a small pint? Are you feeling under the weather?

 

J:) hn

A small pint

 

Well, it's actually a pint (as opposed to 2, which you later pretend was only one although nobody believes you). 18.3 miles at any average of 12.5 with power on low pretty much anytime I wasn't actually going downhill. I like this bike more and more.

 

Hope your's comes soon John.

 

Chris

Edited June 9, 200817 yr by Chris_Bike

Well, it's actually a pint (as opposed to 2, which you later pretend was only one although nobody believes you). 18.3 miles at any average of 12.5 with power on low pretty much anytime I wasn't actually going downhill. I like this bike more and more.

 

Hope your's comes soon John.

 

Chris

 

18.3 miles per pint, that's 146.4 mpg. Not bad.

.

18.3 miles per pint, that's 146.4 mpg. Not bad.

.

 

I guess it would get me into the lowest CO2 emission band for car tax and the congestion charge!

Personal impressions give a useful idea of the 'feel' of a bike, aswell as its performance with a particular rider, but it is very difficult for others to extrapolate from such personal experiences what performance they may get from the same bike (such as whether it will cope with their terrain or give sufficient range): calculating the power output from just the bike's battery & motor, then combining the rider's pedalpower with that, can give a roughly accurate and useful guide to performance limits for anyone, which can if necessary be fine-tuned with experience rather than innaccurately derived from it in the first place.

 

Stuart.

Edited June 10, 200817 yr by coops

Calling John (Aldby)

 

This is another attempt to get back on track. I'm travelling back to B'ham on Virgin Trains on a lovely sunny evening. I intend to get out on the Trek later. Incedently the lady with the panniers nailed to her hips is very happy with hers too (she just thinks I'm mad talking to myself on this forum!).

 

Chris