Wheels - Weight

[jethro10]

Ok, I'm back with my probing questions.

To me, a leisure cyclist, weight don't matter 'too' much, but I've read many times, wheels are a first thing to upgrade re weight.

I'm lost as to why.
Is it because the weight is rotational, ie centrifugal foce makes it seem heavier? Basically 500gramms rotating is worth more that 500grams weight in the frame?

I'm just kinda lost and have this insatiable thirst for knowlege.

thanks folks
Jeff

 

[BrumJim]

Rotating inertia plus linear inertia, plus gain in potential energy (if you are going up-hill).

i.e. you have to spin the wheel faster: take front wheel off, and it doesn't rotate itself, you need to put effort in to spin it,
and
you have the inertia of speeding the thing up along the road (imagine pushing it along ice, wheels not rotating, but no friction between tyre and ground),
and, if going uphill, you need to create some potential energy that you change into kinetic energy when you fly down the hill going "WEEEEEEE" (doesn't work as efficiently without the sound effects).

With frame weight, you just get the last two.

 

[Ant]

That and the fact that, apart from the frame itself, the wheels are the heaviest thing on the bike.

If you're going to reduce weight you may as well start with the heaviest component that's easily replaced.

 

[accountantpete]

Although weight has a part in it I don't think it is everything.The top end wheels are able to transmit power a lot better due to the design and materials used -there's less flex and the power is transmitted to the rim that bit quicker.

 

[Tollers]

Jethro, you're right and BrumJim is correct too...this post is just me re-wording....

When you accellerate, most of the work is done in getting you and the bike accellerated in whatever direction you want to go. Basically, any weight at all makes this require more energy to get up to speed. Whether its wheels, saddle...you...anything.

However, you have to consider that to actually get motion there are lots of parts that need to move to stationary to rotating motion which is totally independent of the direction you go(wheels, chainset, pedals, casette, chain). The energy you put into accellerating these (estimated 3kg or so) into their rotational state is where the extra weight savings for rotating peices of the drivetrain become valuable.

Another way to think of it....imagine yourself on your back with bike upside down on top of you. Even though there is no resistance from the ground you still need to put in energy to getting the wheels moving. THIS is the critical energy that can only be saved by lightening the rotating drivetrain.

Make sense?

Accountantpete also makes a valid point about transmitting power. Basically "efficiency". If there is a lot of friction of flex in the drivetrain. The energy in your muscles will have percentage losses at each point at the drivetrain until it reaches the ground. These inefficiencies can be signicicant. Generally between the pedals and the hub you could expect to lose 1-6% of your power. There will be an additional losses between the hub and rims via the spokes (although very small), and another loss at the interface between the tyre and the ground (choose tyres carefully and check pressure).

Tollers

M.Eng Mechanical Engineering & Manufacturing Systems

p.s What you refered to as centrifugal force is actually "moment of inertia" (measure of an object's resistance to changes of it rate of rotation).

 

[Steve Austin]

also on nearly every bike you can buy, the wheels are cheaper than they should be. All bikes under 1k could do with an instant wheel upgrade

 

[jethro10]

Thanks folks, well I was kinda there, although my physics 'O' level was 32 years ago!

one point I never thought about was stiffness of wheels, especially at the bottom end of the price bracket. I gather it's the law of diminishing returns, as ever with any subject.

It's common to buy your first bike as a £300 ish product by looking at newbies on here. These will no doubt have what, £30-40 worth of wheels.

Obviously your £1500 "dream machine" (to me anyhow) will have what, £200-300 worth of wheels.

Now weight is easy to check, ie your getting a good improvement, but where would you start to decide if a pair of £300 wheels at a given weight are stiffer than a £100 pair at the same weigh - I've just checked on chain reaction, there are many wheels at £100-£300 that overlap in weight.

I'm guessing that £300 wheels, that are not the lightest, but still light, are infact a better choice, as the slight addition of weight would add stiffness? Ultimatley I guess you need to see the shape of the section it's made off and wall thickness etc??? I'm kinda getting lost again here.

Perhaps on bike web sites, items should have a 'stiffness factor' displayed as well as the all favourite weight.

Well this started easy, but has ended up way heavier (no pun intended) as a subject.

Thanks

Jeff

PS : I'm finding this really interesting

 

[accountantpete]

Now weight is easy to check, ie your getting a good improvement, but where would you start to decide if a pair of £300 wheels at a given weight are stiffer than a £100 pair at the same weigh - I've just checked on chain reaction, there are many wheels at £100-£300 that overlap in weight.

I'm guessing that £300 wheels, that are not the lightest, but still light, are infact a better choice, as the slight addition of weight would add stiffness? Ultimatley I guess you need to see the shape of the section it's made off and wall thickness etc??? I'm kinda getting lost again here.

Perhaps on bike web sites, items should have a 'stiffness factor' displayed as well as the all favourite weight.

There's also the quality of the hubs to be taken into account as they are also very important in the sceme of things. A lot of hubs seem very smooth when you turn them in your hand but it's how they,and indeed the rest of the wheel, perform under load that is crucial.

Personally I go by price and stick to the established wheel manufacturers where £350 will get an excellent wheel.

 

[andyhunter]

first thing to start of in weight reduction is frame but you could have a really light frame but it is not aerodynamic so it would not help you that much and but if you start of and work from the frame first as it is the most important aspect of a bike as everything is built around it, dont c the point in wasting time other products part from frame that will not benefit you that much as then you can work on other things such as wheels or pedals or handle bars or cranks or brakes etc etc over time when you are happy with your frame weight. thats what i do start from the top and work down by prioritising everything. thus happy with my bike apart from changing my pedals at the moment when i get a buyer from my time rxs ulteam ti carbon to time iclic(titan) titanium carbon pedals as there lighter and word on the block is there the best on the market at present due to the amount of power that can be transfered using them.

 

[jimboalee]

There's also the quality of the hubs to be taken into account as they are also very important in the sceme of things. A lot of hubs seem very smooth when you turn them in your hand but it's how they,and indeed the rest of the wheel, perform under load that is crucial.

Personally I go by price and stick to the established wheel manufacturers where £350 will get an excellent wheel.

Hubs are only as good as the bearings that they hold.

In my skateboard days, the team and I washed the grease out of the SKF 608s we used.
Without grease, they span like spinning tops and we rode the vert easier.

Bike hub bearings, if you have precision race 'cartridge' bearings, can be washed out for a record attempt, but I would not suggest running 'dry' races in an 'everyday' situation.
The hubs you buy with precision race bearings will have the races packed with grease causing a braking effect from the outset.

Cup and cone bearings can be adjusted and the grease/oil used can be changed quite easily.
For one-day races, a looseish ( not loseish ) adjustment and the meagrest drop of oil will allow them to spin easier than 'correctly' adjusted cones packed with thick grease.

'Looseish' means just on the verge of detectable lateral movement.

 

[Globalti]

Blimey - if you're going to go that far you might as well neck a good laxative a few hours before the race as well as the weight loss will make more difference.

 

[gavintc]

I made the mistake when I built my first bike of buying lightweight wheels without considering their strength and lateral stability. Yes, they were very light for the price, but were not that effective for a 12st rider. I replaced them after the rear wheel rim broke with a slightly heavier, but stronger wheel. The increased performance was remarkable. It felt like a new bike, sprightly and with new found accelaration - quite incredible.

 

[Dan B]

Hubs are only as good as the bearings that they hold.

In my skateboard days, the team and I washed the grease out of the SKF 608s we used.
Without grease, they span like spinning tops and we rode the vert easier.

Two things come to mine. One, the moment of inertia of a typical skateboard wheel is practically negligible compared to a cycle wheel. Two, the rotating speed of a diddy little skateboard wheel is appreciably higher than a 26" cycle wheel, even allowing for the cyclist probably going faster

Oh, and three: don't neglect the psychological effect. I speed skate (on inlines) and I know an awful lot of people who think they can tell the difference between one bearing lube or type or brand and another. My estimate is that about 2 of them (maybe 3) would actually notice anything in a blind test, but the effect of having paid ninety quid for "full ceramic" bearings makes the others think they're faster and so put more effort in.

 

[jimboalee]

Two things come to mine. One, the moment of inertia of a typical skateboard wheel is practically negligible compared to a cycle wheel. Two, the rotating speed of a diddy little skateboard wheel is appreciably higher than a 26" cycle wheel, even allowing for the cyclist probably going faster

Oh, and three: don't neglect the psychological effect. I speed skate (on inlines) and I know an awful lot of people who think they can tell the difference between one bearing lube or type or brand and another. My estimate is that about 2 of them (maybe 3) would actually notice anything in a blind test, but the effect of having paid ninety quid for "full ceramic" bearings makes the others think they're faster and so put more effort in.

The 'proof of the pud' was a simple test on the half pipe in the factory's back workshop.
The same skateboard with OE bearings was let free from the 'blue tile' and a marker was chalked where it stopped on the opposite 'transition'.
The bearings were changed for dry bearings and the test repeated.

Lo-and-behold, the dry bearing 'stick' rolled further up the halfpipe.

No change to trucks, hangers or paintwork on the skateboard's underside. Not even an additional G&S sticker.

 

[jimboalee]

By the look of your Avatar Coruskate, you probably weren't around when Tony Alva was God.
No disrespect intended but these guys ( professionals ) kept their secrets to themselves.
We wondered how the guys at Santa Cruz got so far out of the pool.
Then we learned about 'dry' bearings.
What a revellation. I was handplanting the coping and getting 3 foot airbournes the very next week.