Bowie69 Posted November 17, 2010 Share Posted November 17, 2010 Some of that could be due to the difference in grip between tyres with unequal pressures. More likely difference in rolling resistance, or the above change in rolling circumference, if proven true, I'm keeping out of that one at the moment . Quote Link to comment Share on other sites More sharing options...
slomofo Posted November 17, 2010 Share Posted November 17, 2010 Whatever the values are, a deflated tyre will have a lower rolling radius and so will cover less distance per revolution. errr....no. The tread/contact patch of the tyre will retain its linear properties, so unless its 'shuffling'(Tread Blocks bunching together) or 'hopping'(Grip Slip Grip) the contact patch will continue to cover exactly the same distance in every revolution. A difference in axle rotation speed vs wheel speed on a completely flat tyre can occur through slippage on the bead seat on the rim; as a direct result of torsional shear through the bead/sidewall/tread relationship (fixed wheel wants to rotate faster then level of grip will sustain) Spent a few years playing with tyres at MIRA a long time ago, don't think they have changed much in terms of physics.....!! Quote Link to comment Share on other sites More sharing options...
rtbarton Posted November 17, 2010 Share Posted November 17, 2010 Whatever the values are, a deflated tyre will have a lower rolling radius and so will cover less distance per revolution. errr....no. It must do, the radius is smaller the distance is less. Yes tread bunching or whatever must occur because the rubber approaching the contact area has to go somewhere as it slows down. Somebody PLEASE do an experiment! Quote Link to comment Share on other sites More sharing options...
Range Rover Blues Posted November 17, 2010 Share Posted November 17, 2010 But can you prove it? As I said before it has been tried and the conclusion was it couldn't be measured. Quote Link to comment Share on other sites More sharing options...
rtbarton Posted November 17, 2010 Share Posted November 17, 2010 I reckon if you get a sack cart, put a bag of sand on it, let one tyre down, mark the top of each tyre with a bit of tape and push it in a straight line you should see one wheel executing more turns than the other. Quote Link to comment Share on other sites More sharing options...
landroversforever Posted November 17, 2010 Share Posted November 17, 2010 Little ammounts of pressure difference - it probably wont be measurable difference. BUT the maths says that it has to be more turns when the wheel is flat. The radius is changed at the bottom, the tread must bunch in some way, surely thats where the head build up is? Quote Link to comment Share on other sites More sharing options...
LR90 Posted November 17, 2010 Share Posted November 17, 2010 I'm with the 'circumference remains unchanged brigade'. The specialist sand tyres in my garage are designed to be able to run near flat with the radial banded tread acting just like a track when deflated. The length of that track doesn't change because the tyre is deflated and it doesn't slip and bunch or magically travel over the sand any faster as this would break the surface and defeat the whole purpose of their design. What does change, dramatically at low pressure, is the change in distance (radius) from the hub centre to tread as the wheel rotates causing all that flexing in the side walls on every revolution. This is what can generate heat and can damage the carcass if the speed exceeds the manufactures design at that operating pressure. Running at lower pressures does increase rolling resistance though which is why the vehicle will tend to pull to one side if the tyres on one that side are running a lower pressure than t'other. Quote Link to comment Share on other sites More sharing options...
Night Train Posted November 18, 2010 Share Posted November 18, 2010 My 2p It depends on the tyre. That is all. Some tyres will hold the same rolling circumference at a wide range of pressures. Other tyres will grow a little extra circumference at higher pressures. Some tyres will, at very low pressure, maintain tread contact, others will bunch up. Also it will depend on how extreme the pressure diffence is. At maximum pressure the the rolling circumference calculations holds true, and allowing for any change in circumference for the specific type of tyre. With no pressure the wheel rim will roll on the side walls and the tread of the tyre will bunch up in the foot print area. Then the rolling radius calculations will hold true. At various pressures in between there will be a transition between the rolling radius and rolling circumference calculation being true. Where that happens, in the pressure range, will depend on the specific tyre. Other variables will be the sidewall rigidity of the tyre, the tread thickness (and hence stiffness) on the tyre and the effective weight of the vehicle over that tyre. A light car on lorry tyres will probably show little or no difference in rolling radius even when the tyres have no air in them at all! Quote Link to comment Share on other sites More sharing options...
rtbarton Posted November 18, 2010 Share Posted November 18, 2010 What does change, dramatically at low pressure, is the change in distance (radius) from the hub centre to tread as the wheel rotates causing all that flexing in the side walls on every revolution. This is what can generate heat and can damage the carcass if the speed exceeds the manufactures design at that operating pressure. True, the reduction in radius means a proportional reduction in linear speed at the road. The heat caused by side wall flexing is excessive heat which damages the tyre as Night Train says, but IMO heat is also generated by the tread bunching, this heat is allowed for in the tyre design and doesn't cause a problem. Quote Link to comment Share on other sites More sharing options...
slomofo Posted November 18, 2010 Share Posted November 18, 2010 So if Euclids Theorems don't answer the question, let's try schoolboy logic: If an axle completes one revolution of 360degrees and it has a tyre fixed to it that has a circumference of say 30", irrespective of tyre pressure the constant will be a 30" distance travelled for every complete revolution of the axle - simply because the axle cannot complete a revolution independently of the piece of rubber wrapped around it. My wheelbarrow does it most weekends....! Quote Link to comment Share on other sites More sharing options...
rtbarton Posted November 18, 2010 Share Posted November 18, 2010 So if Euclids Theorems don't answer the question, let's try schoolboy logic: If an axle completes one revolution of 360degrees and it has a tyre fixed to it that has a circumference of say 30", irrespective of tyre pressure the constant will be a 30" distance travelled for every complete revolution of the axle - simply because the axle cannot complete a revolution independently of the piece of rubber wrapped around it. My wheelbarrow does it most weekends....! Here's looking at Euclid Yes but the velocity at any point on the circumference is proportional to the radius, reduce the radius, the velocity reduces. As mentioned several times the rubber travels quickly into the footprint where it compresses or spreads out or whatever, slows down and accelerates again as it leaves the footprint. Quote Link to comment Share on other sites More sharing options...
slomofo Posted November 18, 2010 Share Posted November 18, 2010 By jingo I think we have an accord! The OP question was does a flat travel a shorter distance than an inflated tyre? The answer is........ Wheelbarrow Arf! Quote Link to comment Share on other sites More sharing options...
rtbarton Posted November 18, 2010 Share Posted November 18, 2010 By jingo I think we have an accord! The OP question was does a flat travel a shorter distance than an inflated tyre? The answer is........ Wheelbarrow Arf! Does that mean it's less work to carry concrete in a wheelbarrrow with a flat tyre because you don't have to push it as far? Quote Link to comment Share on other sites More sharing options...
Night Train Posted November 18, 2010 Share Posted November 18, 2010 Here's looking at Euclid Here's a Euclid with both flat and inflated tyres. Just chalk a line on each tyre and then drive it to see if they make the same number of turns. Quote Link to comment Share on other sites More sharing options...
rtbarton Posted November 18, 2010 Share Posted November 18, 2010 Here's a Euclid with both flat and inflated tyres. Just chalk a line on each tyre and then drive it to see if they make the same number of turns. .... Sod the chalk, I'd just like to drive it! Quote Link to comment Share on other sites More sharing options...
berti1554 Posted November 18, 2010 Author Share Posted November 18, 2010 The answer is........Wheelbarrow If it's that easy I would have never asked that question, believe me. We had already everything from wheelbarrows (the dia of its tire is changing with pressure as it isn't steel reinforced) to track type tractors and so on. No real answer. I think rtbarton is very close, I'd just like to see some maths confirming that Thanks guys! Quote Link to comment Share on other sites More sharing options...
berti1554 Posted November 19, 2010 Author Share Posted November 19, 2010 I think rtbarton is very close Buddy, you are not only close, you got it!! What counts is the dynamic radius of the tire to calculate the dynamic circumference. The dynamic radius is somewhere between the static radius (distance between the center of the axle and the ground when the car is not moved) and the radius of a fully round tire. The higher the tire is inflated, the less the weight is on it, and the faster the car is going the less differnence between the circumference of a fully round tire and the dynamic circumference. Depending on the type of tire and the above mentioned variables this can be from nearly nothing to really much. To answer the question in my first post: Yes, a lower inflated tire turns more often over the same distance than a higher inflated tire. Thanks for all your help guys!! Robert Quote Link to comment Share on other sites More sharing options...
slomofo Posted November 19, 2010 Share Posted November 19, 2010 "The higher the tire is inflated, the less the weight is on it..." I'll try helium in my tyres and see if it saves money at the weighbridge! Quote Link to comment Share on other sites More sharing options...
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