“100g lost from the wheel is equivalent to 300g lost from the frame”

I overheard someone in a cafe quote this once.

Do we all agree (when talking about the effort required to accelerate a bike) that reducing rotational mass has more of an effective than reducing non-rotational mass?

Could someone clued up on physics give a good explanation of the reason behind this?

Then following on from that I would love to know by what factor weight lost at the wheel is more effective than weight lost at the frame.

I suspect the above quote cannot be correct as it’s going to depend on whereabouts on the wheel the weight is lost. So factors relevant to current wheel sizes for weight lost at the rim/tyre would certainly be interesting to know.

Just some Easter Sunday rubbish weather musing.

Sprung or unsprung?
Also if it means your wheel snaps it’s not worth it.

It is true to a degree, as you have to accelerate the mass of the wheel rotationally as well as forwards. Although I wouldn’t like to put a figure on it, The 3 to 1 ratio above seems too high. Also where you lose the weight from the wheel makes a difference, as the parts have more inertia the further you get from the centre, therefore losing 300g off a tyre will be more significant than off a hub.

In the real measurable world, it’s pish, according to tests.

It is easier to feel though, which is what most folk go on (but is as meaningless as unsprung weight etc)

Do we all agree

I very much doubt it

bicycles are fundamentally low speed devices, mainly because their power source is pretty weedy! As such the total inertial energy transfer is also pretty small, even if their speed is dynamically changing a lot, compared to the energy lost to drag (frictional and aero).

In fact, look at your speed profile on a bike, and you’ll see you probably spend most time between say 8 and 15mph. So unless you are riding trails where you arrive at Vmax (say 25mph) have to brake to say 1mph, then must accelerate to 25mph again, and have to repeat this continuously, the inertial mass (reciprocal and linear) of a bike is really pretty irrelevant (unless you’re riding a 29er with concrete filled “plus” tyres….. 😉

You’ll only notice a difference after you’ve spent your money.

So unless you are riding trails where you arrive at Vmax (say 25mph) have to brake to say 1mph, then must accelerate to 25mph again, and have to repeat this continuously

More like mountain biking than Road biking then…

There is also an upside to higher rotational mass which is the flywheel effect … handy for keeping you moving over rough stuff.

Heavier wheels feel slower because you perceive the increased rotational inertia through each pedal stroke. However, that increased rotational inertia all provides increased rotational momentum, so although a heavier wheel is harder to accelerate it’s also better at carrying speed.

Heavier tyres normally feel slower because of increased hysteresis loss in the carcass, not the because of the change in rotational inertia.

Weight is the only bicycle characteristic which is easy to measure so it inevitably gets obsessed over.

Heavier wheels can be better when you’re pointed downhill, especially somewhere rocky, they roll over stuff better.

If your mountainbiking is all about blasting around twisty trails in the woods… then the lighter the better.

The worth 3x as much thing is for road bikes really, no?

It is true yes, not sure where the 300g figure came from but in principle yes.

Also worth noting (I’m talking physics here not noticeable difference) that the rim is where you want to save the weight, can’t remember the exact formula off hand but the weights are multiplied by the radius of gyration (distance of weight from axis) to give effort required to accelerate.

26″ makes alot of sense on paper….

The above is accurate enough for this consideration, I may come back with formulas, I’m currently cooking easter lunch and have almost finished the first bottle of red so memory is fuzzy!

So how significant is the effect of 2-5kg of rotating mass in the context of 80kg odd of overall mass?

At say 20mph, rotating 8 times per second?

The word “flywheel” is Shirley stretching it?

This is why 650b wheels kill more kittens

cynic-al – Member
…The word “flywheel” is Shirley stretching it?

Try stopping it by sticking a finger in the spokes and you’ll be convinced it’s a flywheel. 🙂

26″ makes alot of sense on paper….

But they have to rotate faster for a given speed so the radius bit cancels out, we’re just back to weight again.

I’ve never ridden a really light wheel/tyre combo that feels better to ride than something with cushier tyres, wider+heavier rims, etc. I’m with cynic-al on this one, low weight wheels are more about preferences in feel rather than actual benefits. No point having dead weight on a bike but also there’s places I’m happy to add weight to get the right spec for the job, wheels included.

I have 2sets of road wheels,more or less identical weights,but one with a part carbon rim. This wheelset does seem to accelerate better and i assume its due to the lighter rim. It also has cup and cone bearings compared to cartridge in the other and this seems to make a difference as well.Nothing scientific,its just how they feel.

exactly jameso lots of elements in play here..

cycnic-al I may actually calculate that for you once sobered up

For wheels, you need to provide kinetic energy to move them forward and rotational energy to make them go round. For the frame it is just kinetic energy.

KE = 0.5 m*v*v

Where m is mass and v is velocity.

For wheels, there is rotational energy

RE = 0.5 I*w*w

Where I is the moment of inertia and w the angular velocity.

I ALL the mass is at the rim (it isnt) the I = m*r*r and since v=r*w you get the same energy for rotation and movement.

In fact this ignores hub weight so the ratio is less than a half.

Not only that, at constant velocity, the proportio n of total energy in the wheels (compared with the 70kg lump above them) is negligible.

There are small benefits in accelerations uphill. But they are just that, small.

There are small benefits in accelerations uphill. But they are just that, small.

So still benefits then.

Or to put it another way, heavier rims/wheels make life harder – maybe that is worth a thread title “Are heavier rims/wheels harder to pedal?” 🙂

The “benefits” come down to the numbers.

So 100gm lighter rim weight per wheel will make you 0.2% faster uphill for a 100kg bike/rider.

Aero is generally more important on road, and maybe even mtb!

Exactly. The adage is (almost) true (it’s 200g or less). But the net effect is very small. If you are racing for GC, then 0.2% might matter at the end of 180 km and 3500m of climbing. For most of us it wont!

I climbed 2500m on some fairly heavy wheels and a stupid light bike on Friday. Lighter wheels would not have got me up Dartmeet 20% any faster!

Tired has it, moment of inertia. Given by mass x radius squared

I think tyre pressure, compound and tread have more effect.

In the simple case of a road bike heading uphill, weight lost from the wheel is equivalent to weight lost from the frame. If you go from a standing start, then there is a tiny penalty to getting the wheel spinning, but it is tiny – imagine a bike sat on a turbo with no resistance. You could easily spin the wheel up to climbing speed with a single pedal stroke, and the actual penalty is less than that as if the weight were on the frame you’d still have to accelerate that.

For a mountain bike with suspension there may be more of an advantage to reducing unsprung weight.

I’m no number crunching boffin, but surely, beyond just the rim rotation, there are also some gyroscopic forces to consider…

What if it’s raining? 😀

I do believe there are considerable benefits in a lighter wheelset.

I’ve tried the 2 extremes. I have a 20″ wheeled Dahon Hammerhead equipped with incredibly light Rolf wheels, and I also have ridden my fatbike with 4″ slicks. There’s nothing nicer than a light wheel. 🙂

Hills and gravity are the issues here, and this argument may appear to be more philosophical than physical.
Imagine a point on your wheel and draw a line of that point’s movement going uphill. You’ll notice this point stops and starts but never goes down hill, and as you’re fighting gravity constantly, lighter wheels are better, at least going uphill. So, yes, you do need new wheels. What was the question?