Scammell Explorer fixes and workarounds

[gritineye]

The important thing is the rolling radius, as radial sidewalls spread more, the measurement from ground to hub centre may remain the same with a R15 as a 1400 cross ply tyre.

[antarmike]

The important thing is the rolling radius, as radial sidewalls spread more, the measurement from ground to hub centre may remain the same with a R15 as a 1400 cross ply tyre.

Rolling radius will not affect speedo reading.

 

But if you assume that there is no slippage between tyre and road, then 15.00 has bigger circumference than 14.00 so each rev takes it further down the road, Speedo should be under recording correct speed.

[Steifbear]

Just been to have a look at the spare, Its a 1400 grrr. this is not a question of legality but in a get me off the highway situation could you put a 1400 tyre on the walking beam and drive like that?

 

Cheers

 

Pete

[antarmike]

Just been to have a look at the spare, Its a 1400 grrr. this is not a question of legality but in a get me off the highway situation could you put a 1400 tyre on the walking beam and drive like that?

 

Cheers

 

Pete

 

What have you got on the front axle?

 

If it is 15.00 allround, obvious answer is you get a flat rear tyre is to move a front wheel to the back, and put spare on the front, undriven , axle.

[gritineye]

Rolling radius will not affect speedo reading.

 

So that's why you feel you need to have a disclaimer..................................:coffee:

 

I worked this stuff out for myself using my own brain, I only Googled it and found this in order to save you the bother of replying with anything more than an apology.

 

A Speedometer error means an odometer error too.

 

It stands to reason that if you change the rolling radius of your wheels and tyres, and the speedometer no longer reads correctly, that your odometer will also gradually become inaccurate. Assume for example that you bought a car brand new and changed the wheels and tyres on day one from 195.65R14 to 205/50R15 - not an uncommon change. By the calculator above, that makes your speedometer over read by 1.7%. Consequently, the registered odometer reading will also be out by the same value. So for example, when you get to 10,000km of driving (in the real world), your odometer will actually read 10,170km. OK so that's not a huge difference but it is one of the reasons why most car dealers have a disclaimer on their secondhand vehicles telling you that they won't guarantee the displayed mileage. ("Clocking" the odometer is the other reason). Odometer errors due to mis-matched tyres and wheels will happen on regular odometers as well as the newer digital ones.

 

 

Read more: http://www.carbibles.com/tyre_bible_pg4.html#ixzz0hz9Ibi15

 

Notice the reference to squish when measuring..this certainly does affect rolling radius. http://www.dbraun99.com/technical%20index/Calculating%20TPM%20of%20your%20speedometer%20cable.pdf

 

 

 

 

 

Edited March 12, 2010 by gritineye
additional quote added for extra clarity

[N.O.S.]

The important thing is the rolling radius, as radial sidewalls spread more, the measurement from ground to hub centre may remain the same with a R15 as a 1400 cross ply tyre.

 

As Bernard says the difference may not be much at all. And even if it is, you could always let air out of the radial tyre (if you ever have to use it) until the hub (or edge of wheel rim) is exactly the same height off the road as the other one on the same walking beam (subject to driving 'carefully' of course :cool2:). Would save any undue stress on the gearing too. And black marks on the road from the crossply tyre on the same walking beam :-D

[antarmike]

The important thing is the rolling radius, as radial sidewalls spread more, the measurement from ground to hub centre may remain the same with a R15 as a 1400 cross ply tyre.

Rolling radius will not affect speedo reading.

 

But if you assume that there is no slippage between tyre and road, then 15.00 has bigger circumference than 14.00 so each rev takes it further down the road, Speedo should be under recording correct speed.

 

So that's why you feel you need to have a disclaimer..................................:coffee:

 

I worked this stuff out for myself using my own brain, I only Googled it and found this in order to save you the bother of replying with anything more than an apology.

 

A Speedometer error means an odometer error too.

 

It stands to reason that if you change the rolling radius of your wheels and tyres, and the speedometer no longer reads correctly, that your odometer will also gradually become inaccurate. Assume for example that you bought a car brand new and changed the wheels and tyres on day one from 195.65R14 to 205/50R15 - not an uncommon change. By the calculator above, that makes your speedometer over read by 1.7%. Consequently, the registered odometer reading will also be out by the same value. So for example, when you get to 10,000km of driving (in the real world), your odometer will actually read 10,170km. OK so that's not a huge difference but it is one of the reasons why most car dealers have a disclaimer on their secondhand vehicles telling you that they won't guarantee the displayed mileage. ("Clocking" the odometer is the other reason). Odometer errors due to mis-matched tyres and wheels will happen on regular odometers as well as the newer digital ones.

Read more: http://www.carbibles.com/tyre_bible_pg4.html#ixzz0hz9Ibi15

 

Notice the reference to squish when measuring..this certainly does affect rolling radius. http://www.dbraun99.com/technical%20index/Calculating%20TPM%20of%20your%20speedometer%20cable.pdf

 

 

 

 

You have missed what I am trying to say, Artificially altering the rolling radius by letting air out of the tyre will not alter the speedo reading of a 1500 tyre. If the same tyre is properly inflated or deliberately under inflated, the ride height will alter, (and the rolling radius) but for that size tyre the Speedo reads will stay the same.

 

I run radial 14.00 on my matador and cross ply 14.00 on my Douglas. both tyres are almost the same height, and axle height from the ground is the same for both. There is no evidence of greater side wall defection on the radials, The only way a radial 1500 would flex enough to be same ride height as a 14.00 is by deliberate under inflation.

 

The item you quote assumes tyres are fully inflated ( and of identical construction) and therefore of different radius or ride height, Clearly in this context Rolling radius does affect speedo reading, {tyres of different sizes have different rolling radii but they also have different circumferences} but I was specifically answering the suggestion that for a 15,00 tyre where there is conciderable side wall flex causing an artificially smaller rolling radius that matched that a normally inflated 14.00 crossply tyre ( but where the circumfernces differed), then the Speedo would continue to work accurately.

 

My point is the effect of letting air out of a 15.00 to make it ride at the same height as a 14.00 will not reduce the distance that tyre moves forward for one revolutiuon (ie for that particular tyre you cannont alter how many revolutions it will turn as it moves a given distance along the road) and even if you do not let air out.

If however there is naturally more flex in the bigger tyre sidewalls at normal pressures and both 1400 and 1500 have the same ride height (rolling Radius) the 15.00 will travel further down the road per turn than the 14.00, therefore the ride height (rolling radius) of the 1500 compared to the 1400 is of no significance to the arguement, what matters is the relative circumference. Whatever the ride height (rolling Radious) of the 1500 it will not affect the speedo reading, only its circumference has an effect on the speedo.

 

(I am assuming there is some suggestion of letting air out of a 1500 tyre so the side walls flex more and lower the rolling radius, since properly inflated the height from ground to wheel centre should be greater for a 15,00 than a 14.00, having the same axle height can only result from incorrect inflation of the bigger tyre)

 

Perhaps what I should have said is "The ride height (rolling radius) of one tyre compared to another, when they are of different construction, and probably inflated to different pressures does not have an effect the speedo reading, ie if the rolling radius is the same for a 1400 and a 1500, but the circumferences are different, then having equal rolling radii will not result in equal speed or distance readings." But I assumed that in the context of the question I was answering I thought this would be assumed.

 

Assume you have a tank with different numbers of links in the each track.

 

The tracks could be taken of the tank formed into to round wheels. Obviously the one with more links in it will travel further if you roll it along the road one revolution.

 

Put the tracks back on the tank, and both ride at the same height, but one is longer front to back.

 

If you had no differential on the tank and both drive sprockets rotated at the same RPM, the tank would be forced to steer in a radius, with the longer track on the outside of the turning circle.

 

If you have a 15.00 and a 14.00 tyre the circumferences (length of tank track) are unequal. Altering tyre pressure to raise of lower the wheel centre height does not alter now far down the road the wheel wants to move for each revolution. (nor will this distance change if the rolling radius does change because the sidewall naturally flexing more on the bigger tyre)

 

But even if you do get greater side wall deflection on a 15.00 than a 1400 at normal inflation pressure, (and there is no question of under inflation) comments about side wall deflection and consequent rolling radius do not seem to consider that a 15.00 has a greater circumference than a 14.00. and therefore irrespective of rolling radius the 1500 travels further doen the road each rev than the 1400.

Edited March 13, 2010 by antarmike

[antarmike]

Yes the other leak IS IN THE SAME EFFING PLACE!

 

What a pi55 poor Scammell design. And as for their manufacturing, it is shyte.

 

The last fulcrum pin was full of machining swarf, down the drilled and tapped extraction hole, and in the end of the shaft that carries the split pin. I reckon the problem is the brake shoe Fulcrum pins have been put in trapping swarf in the tapers, and over time the swarf has compressed and the pins come loose. I am sorry to be rude about the much loved and admired Explorer, but faced with another walking beam end casting to take off and another effing castle nut to take out I am left completly underwelmed by their abilty to design or manufacture anything. LONG LIVE THORNYCROFT.

 

The reason the fulcrum pin comes loose, is the taper on the pin does not match the taper in the hole. When fitting the clean pin into the hole, the small dia goes tight but there is still clearance at the large diameter end and the pin can be wobbled. The problem is down to lousy engineering by Scammell. A I say long live AEC and Thornycroft. both were far better at engineering than Scammell. It is a pity that Scammell outlived both AEC and Thornycroft because both of these companies were far better designers and manufactureres than Scammell ever were!

[N.O.S.]

Assume you have a tank with different numbers of links in the each track.

 

The tracks could be taken of the tank formed into to round wheels. Obviously the one with more links in it will travel further if you roll it along the road one revolution.

 

Put the tracks back on the tank, and both ride at the same height, but one is longer front to back.

 

If you had no differential on the tank and both drive sprockets rotated at the same RPM, the tank would be forced to steer in a radius, with the longer track on the outside of the turning circle.

 

 

Take your tank track thing. The only dimension that affects how far the tank travels with a track of variable length is the radius of the drive sprocket, which equates to rolling radius of a tyre.

 

The drawing below shows a short track, a long track, and a track held in the shape of a radial tyre with one mother of a sidewall bulge :-D They all travel at the same speed ('R'is the same).

 

Hey, here comes uncle Bob again!

[antarmike]

Take your tank track thing. The only dimension that affects how far the tank travels with a track of variable length is the radius of the drive sprocket, which equates to rolling radius of a tyre.

 

The drawing below shows a short track, a long track, and a track held in the shape of a radial tyre with one mother of a sidewall bulge :-D They all travel at the same speed ('R'is the same).

 

Hey, here comes uncle Bob again!

 

Yes you are right, wrong thinking on my behalf. What I meant to say was put two long tracks on one tank, two short tracks on another tank, tension them up, and both have the same rolling radius. Mark the link on each tank that has just been laid on the ground, and move the tanks forward until the same marked link is again just laying onto the ground, and the one with the longer tracks will have moved forward further.

 

Ride height, (rolling radius) is the same for both but the one with the greater length (circumference) travels further per complete revolution of the track (or wheel) Ie ride height/ rolling radius has has no direct corelation with the distance a tyre moves forward per revolution.

Edited March 13, 2010 by antarmike

[AndyFowler]

Take your tank track thing. The only dimension that affects how far the tank travels with a track of variable length is the radius of the drive sprocket, which equates to rolling radius of a tyre.

 

The drawing below shows a short track, a long track, and a track held in the shape of a radial tyre with one mother of a sidewall bulge :-D They all travel at the same speed ('R'is the same).

 

Hey, here comes uncle Bob again!

 

I hope you own the copyright for those drawings Tony because we are not allowed to post drawings taken from technical manuals without the owners permission ! :-D

[gritineye]

Well....unable to do the maths, I resorted to redneck principles and measured the circumference of a tyre jacked off the ground fully inflated and then at a much lower pressure bearing the weight of the vehicle, a couple of times to make sure.

 

This proved Mike absolutely right on the circumference part.

 

This begs the question, why have generations of people involved in designing drive-lines and working out gear ratios including those that re-calibrate speedos, insisted that the rolling radius is so critical?

 

I had a 130 mph speedo (a few people where impressed) recalibrated a long time ago for my Series 1 V8, and was asked to check the pressures, mark the tyre wall and road, roll the vehicle forward for ten revolutions and then mark the road again. Then measure the distance, this gave them the rolling radius to work from.

 

I'm beginning to liken this question to the "Why do we never see the other side of the moon?" one.

[antarmike]

Well....unable to do the maths, I resorted to redneck principles and measured the circumference of a tyre jacked off the ground fully inflated and then at a much lower pressure bearing the weight of the vehicle, a couple of times to make sure.

 

This proved Mike absolutely right on the circumference part.

 

This begs the question, why have generations of people involved in designing drive-lines and working out gear ratios including those that re-calibrate speedos, insisted that the rolling radius is so critical?

 

I had a 130 mph speedo (a few people where impressed) recalibrated a long time ago for my Series 1 V8, and was asked to check the pressures, mark the tyre wall and road, roll the vehicle forward for ten revolutions and then mark the road again. Then measure the distance, this gave them the rolling radius to work from.

 

I'm beginning to liken this question to the "Why do we never see the other side of the moon?" one.

 

Measuring the length of travel along the road for exactly 10 revolutions of the wheel gives 10 times the circumference of the wheel, so the person who calibrated the speedo was working on circumference of the tyre, not rolling radius.

 

Normally for a given tyre construction, at normal pressure there is a direct relationship, (ratio,proportion call it what you will) between the rolling radius and the circumference. However for different tyre constructions and infaltion presures there will not be the same ratio/ prortionallity between rolling radius and circumference.

 

This is why you are told when matching part worn tyres to a double drive bogey with no centre diff, measure the circumference of the tyres and try to, for both axles, pick two tyres, the sum of whose circumfrences are as near equal as possible to the sum of the circumferences of the tyres on the other axle, or for a walking beam rear bogey choose the two tyres that are closest in circumference measurement to each other for the tyres on each side of the bogey.

 

Circumference matters, rolling radius doesn't. (or so my wife tells me!)

Edited March 13, 2010 by antarmike

[antarmike]

Second brake fulcrum pin removed, cleaned and refitted complete with split pin, without having to strip the shaft/ bearing/gear assy out of the walking beam end casting. This one was tighther and I had to reduce the overall width of the manufactured spanner by a further 3mm to slip it between the gear and the casing!

 

But it is done now, Hopefully we will cut the shim gaskets tomorrow and have the casing back together without too much trouble.

[antarmike]

Well....unable to do the maths, I resorted to redneck principles and measured the circumference of a tyre jacked off the ground fully inflated and then at a much lower pressure bearing the weight of the vehicle, a couple of times to make sure.

 

This proved Mike absolutely right on the circumference part.

 

This begs the question, why have generations of people involved in designing drive-lines and working out gear ratios including those that re-calibrate speedos, insisted that the rolling radius is so critical?

 

I had a 130 mph speedo (a few people where impressed) recalibrated a long time ago for my Series 1 V8, and was asked to check the pressures, mark the tyre wall and road, roll the vehicle forward for ten revolutions and then mark the road again. Then measure the distance, this gave them the rolling radius to work from.

 

I'm beginning to liken this question to the "Why do we never see the other side of the moon?" one.

 

For radial construction there is a fair chance there is steel belting under the tread, and synthetic fibre in the walls. If this is so, it would be virtually impossible, by altering pressure or by any other means to alter the circumference of the tyre. (this being fixed by the length of the steel belt). Irrespective of rolling radius it is the fixed circumfrence that determines how far the wheel will move forward per revolution, and therefore how accurate the speed and distance recording device was.

 

The only way circumference changes is through wear of the tread, which over time reduces the circumference.

Edited March 13, 2010 by antarmike

[abn deuce]

Antarmike ,It does seem they could have engineered the pin to have been threaded into the casting from the outside complete with a taper area to keep the lubricant inside instead of having to split the case off the rear assembly, and without the fantastic help of that modified wrench , you would had to remove that gear to get access !!!

If it's a pivot point only there should not be that much torque being applyed that would need to be a worry , or baring that why isnt the shaft pinned to prevent its rotation once its installed ?

[antarmike]

Antarmike ,It does seem they could have engineered the pin to have been threaded into the casting from the outside complete with a taper area to keep the lubricant inside instead of having to split the case off the rear assembly, and without the fantastic help of that modified wrench , you would had to remove that gear to get access !!!

If it's a pivot point only there should not be that much torque being applyed that would need to be a worry , or baring that why isnt the shaft pinned to prevent its rotation once its installed ?

 

The pin's taper is about 3 inches long. It would surfice if it was only 1 1/2" long in which case the pin could pull into a lug on the outside of the casing, and the casing to have no hole through to the inside.

 

Imagine what the casing looks like where the brake expander cam is. Imagine a taper in the part nearest the brake shoes, and the nut pulling the pin into place being where the actuating arm is. Everything is external and easy to work on. If the pin does loosen, there is no oil leak.

 

As Aleksadr Orlov would say "simples" but "simples" was not in Scammell's vocabulary, everything had to be done in a daft, stupid fashion, with no thought as to how to service what they had made, or how reliable it would be over time.

 

Simples?? no Simpletons....

Edited March 13, 2010 by antarmike

[N.O.S.]

Not so fast........:nono:

 

Yes the circumference of the tyre is to all intents and purposes uncompressible. Those bl%%dy army 12x20 cross ply sidewalls are not much better. But a radial tyre sidewall is immensly flexible by comparison, and can tolerate a fair degree of torsional (rotational, radial) wind-up, as well as axial flexing (sidewalls bulging under load).

 

Try another one of Professor Crump's illustrations (this is highly simplified but serves the purpose here):

 

Both tyres here have the same cirumference. The one below demonstrates that when compressed under load the effective rolling radius R2 is reduced.

 

But note also that the equivalent rolling radius at the top of the tyre R3 (not you Richard) becomes greater. This is because the tread cannot be compressed, so the sidewalls flex to allow it to move away from the rim to retain its 'length'. At the top the velocity of the tread relative to the hub V2 must therefore be greater than at the bottom V1. So the tyre tread effectively plays "catch-up" as it rolls around to the road once more, where it slows down to pass between the rim and the road.

 

So the velocity V of each point of the outside of the tyre is constantly varying - but the average velocity must be equal to that of the tyre shown at the top unless the tyre spins on the rim.

 

If you don't believe this, try sticking 8 tonnes in the 4t capacity bucket of a 4wd artic loader with soft radial tyres (which were admittedly 'a bit slack' :embarrassed: ) and see how the rear tyres scrabble around trying to push the machine faster than the fronts (which were doing the gripping) will allow. Come to think of it the fronts looked just like Prof. Crump's lower tyre :wow:

 

In fact large earthmover tyres are a good example of how different tyre pressures - for front / rear on multi axle drive machines with differing axle loadings and no centre differential - are critical to achieving the same rolling radius, and resulting smooth non-kangaroo ride.

 

When matching truck tyres, the cirumference is always going to be proportional to unloaded diameter. Maybe some people measure the circumference (3.142 x dia.) as it gives a much more accurate way of matching tyres of the same diameter (and hence circumference) than by trying to accurately measure the diameter. You have to assume that two truck tyres of similar circumference / diameter and pressure will have the same rolling radius. You can't play around with truck tyre pressures to jiggle rolling radius, because truck tyres require a specific pressure to carry the load safely.

Edited March 13, 2010 by N.O.S.

[Richard Farrant]

 

Yes you are right, wrong thinking on my behalf. What I meant to say was put two long tracks on one tank, two short tracks on another tank, tension them up, and both have the same rolling radius. Mark the link on each tank that has just been laid on the ground, and move the tanks forward until the same marked link is again just laying onto the ground, and the one with the longer tracks will have moved forward further.

As the discussion was on speedo readings, ( I think ), the length of tracks are irrelevant, what is relevant is how many links are laid on the ground for one turn of the drive sprocket. What must be remembered is that a tracklaying vehicle is much like a railway vehicle in that it runs on tracks, but carries them wherever it goes. So, speedo is calibrated for number of track links laid in a turn of the sprocket, not a good comparison when discussing tyres of varying diameters or circumference.

[antarmike]

As the discussion was on speedo readings, ( I think ), the length of tracks are irrelevant, what is relevant is how many links are laid on the ground for one turn of the drive sprocket. What must be remembered is that a tracklaying vehicle is much like a railway vehicle in that it runs on tracks, but carries them wherever it goes. So, speedo is calibrated for number of track links laid in a turn of the sprocket, not a good comparison when discussing tyres of varying diameters or circumference.

I agree that is why I struck through that part of my original post that related to tank tracks.

[Richard Farrant]

I agree that is why I struck through that part of my original post that related to tank tracks.

 

Ah well having read this later on and replied to a subsequent post, it appeared that you still thought that track length had a bearing on it. That is the trouble with deleting or altering past posts.

 

Thanks anyway.

[antarmike]

Not so fast........:nono:

 

Yes the circumference of the tyre is to all intents and purposes uncompressible. Those bl%%dy army 12x20 cross ply sidewalls are not much better. But a radial tyre sidewall is immensly flexible by comparison, and can tolerate a fair degree of torsional (rotational, radial) wind-up, as well as axial flexing (sidewalls bulging under load).

 

Try another one of Professor Crump's illustrations (this is highly simplified but serves the purpose here):

 

Both tyres here have the same cirumference. The one below demonstrates that when compressed under load the effective rolling radius R2 is reduced.

 

But note also that the equivalent rolling radius at the top of the tyre R3 (not you Richard) becomes greater. This is because the tread cannot be compressed, so the sidewalls flex to allow it to move away from the rim to retain its 'length'. At the top the velocity of the tread relative to the hub V2 must therefore be greater than at the bottom V1. So the tyre tread effectively plays "catch-up" as it rolls around to the road once more, where it slows down to pass between the rim and the road.

 

So the velocity V of each point of the outside of the tyre is constantly varying - but the average velocity must be equal to that of the tyre shown at the top unless the tyre spins on the rim.

 

If you don't believe this, try sticking 8 tonnes in the 4t capacity bucket of a 4wd artic loader with soft radial tyres (which were admittedly 'a bit slack' :embarrassed: ) and see how the rear tyres scrabble around trying to push the machine faster than the fronts (which were doing the gripping) will allow. Come to think of it the fronts looked just like Prof. Crump's lower tyre :wow:

 

In fact large earthmover tyres are a good example of how different tyre pressures - for front / rear on multi axle drive machines with differing axle loadings and no centre differential - are critical to achieving the same rolling radius, and resulting smooth non-kangaroo ride.

 

When matching truck tyres, the cirumference is always going to be proportional to unloaded diameter. Maybe some people measure the circumference (3.142 x dia.) as it gives a much more accurate way of matching tyres of the same diameter (and hence circumference) than by trying to accurately measure the diameter. You have to assume that two truck tyres of similar circumference / diameter and pressure will have the same rolling radius. You can't play around with truck tyre pressures to jiggle rolling radius, because truck tyres require a specific pressure to carry the load safely.

 

I am sorry but both your arguement and your drawings are flawed.

 

 

The centre of the wheel stays centred in the tyre at all point except at the bottom (flat part) where the radius from the hub to the outside of the tyre does vary.

In this sketch, r1=r2=r3=r4=r5=r6 and the speed at the outside of the tyre remains constant throughout this part of the revolution.

At r7 the tyre first lays onto the road, at this point the distance from the hub to the outside of the tyre starts to reduce, r9 being shorter than r7, r11 being shorter than r9.

 

R11 is the smallest radius, r10 sees it increase, and r8 sees it increase futher still.

 

If r7, r9,r11,r10 and r8 are equally spaced angularly, then length a = length d, but these are longer than length b and length c. The tyre squirms to accomodate the varying lenths, the tyre ahead of r11 compresses, the tyre behind r11 extends.

 

At r11, directly below the centre the tyre is at rest, neither streching nor compressing. At this point there is no slip and velocity v2 relative to the hub is equal and opposite to v1 the velocity at the very top of the tyre,or at any point where the tyre is round.

 

 

If there is a larger tyre fitted with more sidewall flex, the hub may be at the same height from the road as before, but a greater length of the circumference will lay flat onto the road.

 

Again the tread against the road will have to compress by squirming, as it moves along the straight part of its route from where it first lays onto the road, until under the hub, then it will expand again as it passes behind the hub. The squirming will be more pronounced than in the first diagram, but again the velocity V1 of the outside of the tyre, relative to the hub, measured at any point on the round part of the tyre will be constant, and it will be the same as the velocity directly under the hub V2 where the tyre is at rest length wise, neither stretching nor compressing and not slipping on the road.

 

The questions to ask are

 

Qa) has a 1500 x 20 tyre got a larger circumference than a 14.00 x20? Aa YES

Qb) Directly under the hub, is the tread slipping on the road? Aa NO THE TYRE TREAD IMMEDIATELY BELOW THE HUB NEVER SLIPS ALONG THE ROAD.

Qc) if a 15.00 tyre has a bigger circumference than a 14.00 tyre and at one point on the road it never slips. how can the wheel do anything other than travel further in one rev with a 1500 tyre fitted , than it does with a 14.00 tyre? Ac IT CAN'T A 15.00 TYRE HAS TO TRAVEL FURTHER ALONG THE ROAD FOR EACH REVOLUTION THAN A 14.00 TYRE.

 

Quod erat demonstrandum, as my latin teacher used to say.

 

Yes and for the purists I know velocity is a vector and speed is a scalar quantity, so don't bother pulling me up on that one,(I should be saying distance travelled per unit time, but I really can't be rrsed.)

Edited March 13, 2010 by antarmike

[Richard Farrant]

Ha, think I see a flaw in all this. Now most of us run on bargrip, trackgrip or other cross country type tyres. They are not flat across the tread, so circumference is measured around the centre ( mould line ), OK all well and good, but.....because of the heavily radiused profiles, when running at lower pressures you get the outer flanks of the tread profile in contact with the road, when this happens, the centre of the tread actually inverts as sidewalls bulge out, therefore the maximum circumference is reducing to the smaller circumference of the outside of the treads.

 

No matter how much theory and calculations you can come up with, there is nothing better than practise to prove a point.

 

:yawn:

[N.O.S.]

Oh dear, I can't cope with Scalar Quantities.

 

Mike - how about we have a beer (Bernard's getting a round in at the clubhouse right now) and call it a draw? :-D

[antarmike]

Oh dear, I can't cope with Scalar Quantities.

 

Mike - how about we have a beer (Bernard's getting a round in at the clubhouse right now) and call it a draw? :-D

 

I'm always ready for a beer!