aguycalledsteve
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The bigger wheel will have more mass and inertia once rotating than a smaller one.
- Thread starter cardiffrob
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The bigger wheel will have more mass and inertia once rotating than a smaller one.
rtbcomp
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I'll go with the leverage argument.
The work done to stop the vehicle from a given speed is going to be the same irrespective of the relative tyre & disc sizes (ignoring the fact the the bigger tyres will be heavier).
But because the leverage on the discs is greater with the bigger tyres it will take longer to stop, hence the stopping power will be less (Power = Work / Time)
"20kw of stopping force" is a misnomer, Kw is power, not force.
The work done to stop the vehicle from a given speed is going to be the same irrespective of the relative tyre & disc sizes (ignoring the fact the the bigger tyres will be heavier).
But because the leverage on the discs is greater with the bigger tyres it will take longer to stop, hence the stopping power will be less (Power = Work / Time)
"20kw of stopping force" is a misnomer, Kw is power, not force.
PhillipM
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A bigger wheel has more mass and intertia for a given size than a smaller wheel and a taller tyre does.
And no, tyre flex doesn't impede the ability to grip or brake - the entire reason a tyre provides grip is flex and slip.
Tyre grip is directly related to flex and microslip - without it there would be no grip.
Your power is only less if you can no longer lock the wheels and tyres with your brakes - which on anything from the last 30-40 years would likely mean the brakes are shagged, not that you need smaller tyres.
PhillipM
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No, because they were not beyond the recommended limits, OEM's work very closely with a tyre manufacturer and the usual case for most cars is the tyre is built specifically for them after a lot of modelling with the chassis team at the OEM - this is the case even for mundane stuff like insignia's, mondeos, etc.
That means the standard tyres that come on the car usually are not the same as the 'same' model you can buy aftermarket, for example.
And even in the case that they are, the tyre has been validated and approved for that setup, geometery, pressure and force levels, so it's not outside the recommended limits because the tyre manufacturer will have approved it for those specific circumstances.
8ob
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Have a bit of science on the subject, its quite a damming report on the effects of gurt tyres on yank trucks with a loss of braking efficiency in the region of 25% in some cases. That 25% may come handy one day if its going to stop you killing somebody, its a good read but if its just the effect of tyres/brakes read pages 8-10 https://www.aamva.org/uploadedFiles...izedTiresOnAlteredHeightVehicles_Findings.pdf
Bob
Bob
keithski122
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These are the tyres I had on my beetle, small ones on the front which are slightly bigger than standard.Big ones on the back, ten inch wide whereas a standard was four and a half inch.
I'm still confused how/why the larger tyres make a difference
I don't think the extra centrifugal force of the taller/heavier tyres will make that much difference... not 25% on a 2.5ton truck anyway
the report you linked didn't really go into reasons why / details
actually... I dismissed the leverage idea earlier thinking the breaking power stays the same so taller tyres = lower rpm and it'll even out... but... I guess the mass of the car is acting over that leverage....
The larger tyre reduces the the leverage factor/point of brakes, it's the opposite of fitting bigger disks.
It's an interesting subject that will get people who think learning physics was a waste of time talking.
If you spin a wheel it is easier to stop it on the outer circumference than grabbing hold of the hub cap.
I can totally see how going to 40" tyres would reduce the breaking ratio.
It's an interesting subject that will get people who think learning physics was a waste of time talking.
If you spin a wheel it is easier to stop it on the outer circumference than grabbing hold of the hub cap.
I can totally see how going to 40" tyres would reduce the breaking ratio.
cardiffrob
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Direct from the DVSA this morning...
Hi Rob.
We have had the following response from our Vehicle Testing and Roadworthiness section: There is no legislation that dictates the maximum or minimum tyre width that can be fitted to a particular wheel rim. The Construction and Use Regulations only require that the tyre is suitable with regard to the use to which the vehicle is being put and to the types of tyres fitted to its other wheels. The tyre industry recognises the ETRTO Standards Manual and their tyre/rim fitment recommendations, but these are simply that – recommendations rather than law. As the original 5.20-14 cross-ply tyres are no longer readily available, a 145x14 radial is generally considered by the industry to be a direct replacement, whilst a 155/80x14 is also considered to be suitable alternative.
Hope this helps
Thanks
Leann
Might be worth keeping a copy of this!
Hi Rob.
We have had the following response from our Vehicle Testing and Roadworthiness section: There is no legislation that dictates the maximum or minimum tyre width that can be fitted to a particular wheel rim. The Construction and Use Regulations only require that the tyre is suitable with regard to the use to which the vehicle is being put and to the types of tyres fitted to its other wheels. The tyre industry recognises the ETRTO Standards Manual and their tyre/rim fitment recommendations, but these are simply that – recommendations rather than law. As the original 5.20-14 cross-ply tyres are no longer readily available, a 145x14 radial is generally considered by the industry to be a direct replacement, whilst a 155/80x14 is also considered to be suitable alternative.
Hope this helps
Thanks
Leann
Might be worth keeping a copy of this!
cardiffrob
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...and yet the numpty on the Owners club pages still maintains that he's right.
8ob
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The devil in the detail
The Construction and Use Regulations only require that the tyre is suitable with regard to the use to which the vehicle is being put . I can understand going one profile size to replace an obsolete tyre will have little effect but that very sentence with the term "suitable"is their **** covering bit. "Suitable" clearly puts the onus on the person fitting the tyre to ensure its not to the dertrement of the cars handling or braking efficiency.Bob
PhillipM
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It's not very good science though is it, it doesn't run controls with the braking efficiency high enough to lock the tyres, controls on whether the tyre compounds and friction levels are similar, and how much extra weight, and weight transfer, the much taller vehicle CoG has added (since it's mainly investigating lifted trucks), and it's direct influence. Not whether the tyre profile itself is the direct contribution.
In fact it pretty much avoids that as it's just investigating lift kits and tyres used to lift the car/truck further, lifting the CoG, rather than anything to do with tyre profile - fitting a 50 profile instead of a 40 on a passenger car is not likely to make the braking distances worse, all else controlled.
The highest performance tyres optimised for maximum traction without regard to lateral grip in motorsport and even aircraft, are all high profile tyres with sidewalls that are very soft radially. Not stiff, low profile tyres.
Pete.
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Put the above quote in your signature line. He's sure to melt down.
but... there's still the same power acting on the wheel... a 2ton car traveling at Xmph has the same momentum no matter it's wheel size?
I could see an argument being made for the different contact speed between a brake disk and brake pad... but I can't find any hard numbers to help.
also... as you go to a larger size tyre there's naff all change in it's rotational speed to keep the car moving at the same speed... a few percent at most? - no where near the 25% linked to earlier
so... I'm going to go back to saying I think taller tyres makes no difference to stopping power of a car
add in (as mentioned earlier by PhillipM earlier - the vast majority of modern stuff you can lock the wheels up (if you want) at pretty much any speed.... so the limiting factor in stopping distance is tyre traction on the road not the power of your brakes
This is so, Moggy Van & Pickup wheels though outwardly identical are often wider, think this may have been done later in the production run when they upgraded from 6cwt payload to 8cwt payload. Its worth checking as the proper van wheels fetch more money! Anyone who has driven one of these with its full payload or more will realise why modern cars have brake servos.
cardiffrob
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Yes, 8cwt ones have 4 inch rims and they are now being remanufactured for about £60 a pop. Not a bad price. Not sure if they have safety beads on the rims, though.
I think the thing about taller tyres was the whole USA truck thing of moving from tyres with a diameter of 26" to 40+.
Going up a small bit isn't going to make any difference and there are many other factors. Let's not forget what is being quoted is a relative reduction in braking power so in the real world the vehicle would have had way more braking power than it needed and the reduction wouldn't make that much if any difference.
The problem here is 25% is a big number and there would have to be a significant difference for that to happen.
I am totally with you that it wouldn't make that much difference but you can't argue with the laws of physics.
8ob
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I agree that the yank thing is extreme but the same physics do apply. On an IVA brake test there is a set pedal pressure and the brakes have to make 60% of the vehicles weight in stopping force. So there is in fact a "standard" to follow so that is something to bare in mind, lock up or over braking a vehicle is something car manufacturers dont do/try to avoid which is what abs is all about.
Bob
PhillipM
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Manufacturers have massively overpowered brakes these days, that's why we have huge servos to make it possible for your nan to stand 3 tonne Merc on it's nose when she's nipped out for her pension. The reason is the other way around - they can do it because we have such good abs systems. They'd rather everyone just slapped the pedal pressures well past what's needed to stop and let the ABS and stability control stop the car.
If I took the ABS fuse out on the Merc I could probably easily generate 4-5x the force actually required to lock the tyres, and that's a 2.5+ tonne car.
We also have abs and stability control on all 4 corners that can sort out the brake bias from changes in the CoG, unlike the 'merican trucks.
If I took the ABS fuse out on the Merc I could probably easily generate 4-5x the force actually required to lock the tyres, and that's a 2.5+ tonne car.
We also have abs and stability control on all 4 corners that can sort out the brake bias from changes in the CoG, unlike the 'merican trucks.
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