Science Questions

Why do trolley wheels rattle?

Tue, 7th Jan 2014

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Peter asked:

To keep fit, I deliver weekend newspapers. So I've got a trolley for doing that. The trolley in question has got no suspension, but it does have fairly broad wheels to take a load. Now, I've noticed that when the trolley is empty, if I'm putting it along the path, it's actually extremely noisy. It rattles a lot. But if I pull it along the verge adjacent to the path, which is not particularly smooth, but it is at least grassy, the noise drops by probably 20 to 30 decibels. Any idea why, considering the surface really should be considered to be a bit rougher than the surface so that's supposedly smoother than the path itself?


Dave -  I guess, this is down to sort of how much energy absorption there is because if you're driving over a very, very hard surface which is a bit rough then if your wheel drives into even quite a small bump, then the only thing that can happen is that the wheel has to get after the way of that bump so the trolley is going to move up.  Imagine that they're really quite fast because itís got to get over that bump quite quickly.  And so, it will get thrown up and then the trolley move, fall back down again and go thump especially if itís very light.  If there's a lot of weight on there then the tires might deflect instead.  But when itís very, very light, the trolley is just going to move up and it will bounce and rattle.  Whereas if you're on a soft material like grass, then there's two options.  Either the trolley can go up or the ground can go down.  If you're lucky then the ground will go down and so, it shouldnít throw things up and down as fast, and should be a lot quieter.

Dominic -   I think you're right.  Itís about the absorption because the grass is softer, so itís a more absorbent material.  So, that means it can absorb the energy heating up the grass whereas concrete is very hard.  So, that's not going to absorb the energy by deforming.  Itís going to have to dissipate that energy by sound.

Chris -   One thing that's quite interesting in my experience, I don't know if you've ever driven on dirt road.  Either of you have driven on a dirt road?  And you get corrugations forming on the road.  Have you seen this?  Have you seen this, Peter?

Peter -   I can't say I've driven much on dirt road, but what you're basically saying is the direction of travel.  Itís like going across furrows, the corrugation.

Chris -   Yes, that's right.  You see these corrugations, you think, where do they come from and actually, this is down to the oscillations of the shock absorbers on the vehicles, because as the vehicle goes up over a little bump, the shock absorber has a damping effect.  And so it takes time for the car to stop moving up and down.  That oscillation is pretty standard from one vehicle to the next.  So, it ends up pushing lots of dust into little ridges that slowly build-up to make these corrugations in the road.  I thought - when I was driving on these in the middle of nowhere in Australia - because if you drive slowly on them, it rattles the car to pieces - I thought "they must be made of rock or stone or something."  In the end, I got out and went over to one of these things and kicked it, in frustration, because it was slowing me down so much - and itís just dust.  So I thought "well, if I drive really fast actually, then the period of oscillation of my shock absorbers will be out of phase with the people whoíve gone along here slowly.  And, therefore, I will miss these bumps and therefore it should be a gentle journey because I will just be like driving over piles of sand.  So, I tried it and, instead of going 50 km an hour I went 70 for a bit, and it solved the problem.  So, you're driving your trolley too slowly, Peter! That's what it is!  Go faster!

Peter -   [laughter].  Okay, I should mention that the trolley doesnít have tyres.  Weíre talking solid nylon wheels here.  So, your explanation would actually fit very well.  But it does mean that the supposedly smooth paved surface actually is far from smooth.  It wasnít actually concrete.  It was apparently smooth tarmac.  So, even that must have sufficient grain to cause quite a lot of vibration.

Chris -   Indeed.  Well thank you very much, Peter.  Itís very kind of you to phone in.  Good to have you on the programme.

Peter -   Very interesting answer, thank you.


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