Hi, 

 

Anyone good at maths? i think this has something to do with Pie.

 

I removed my rear wheels the other day to paint the drum brakes. 

 

When i put the wheels back on i made a note to line the center 'Skoda' badges up on each side. 

I then turned the car around and parked it up, i moved about 20ft in total.

 

When i got out the 'Skoda' badges were completely out of line! 

 

Why is this? surely the front and rear wheels rotate the same amount and the same distance and speed? 

 

If they dont then how come? can anyone explain it to me please

 

Wheels with tyres won't be exactly the same size - the front tyres typically wear quicker than the rears.

You said you turned the car around. During the turns, the inner wheels rotate less than the outers. All 4 wheels will be going at different speeds and will move a different distance.

 

This is why cars have differentials.

If you want the link to Pi (not Pie) the circumference of a wheel is diameter*Pi. The circumference is the linear distance around the wheel - i.e. the distance the wheel will travel in one whole rotation. Obviously though the diameter must include the tyre as well as the wheel so there will be some variation.

By the way = Pie = Fray bentos pastry and "meat" - Pi = 3.14.

 

And to answer the question - as rotation of a wheel is diameter x Pi - the diameter can vary due to tyre pressures, but more likely how much tread is left.  I actually notice on the speedo the difference between a brand new tyre and a well worn one, as the the diameter essentially drops by about 16mm when you have brand new tread, to one very worn out.

If you want the link to Pi (not Pie) the circumference of a wheel is diameter*Pi. The circumference is the linear distance around the wheel - i.e. the distance the wheel will travel in one whole rotation. Obviously though the diameter must include the tyre as well as the wheel so there will be some variation.

 

Apologies - Auto correct seems to like Pie more than Pi....

 

Wheels with tyres won't be exactly the same size - the front tyres typically wear quicker than the rears.

 

These wheels are 65 miles old....i only got the car friday! lol

 

 

That was bloody quick repling by you guys! 

The only way you can be sure, is with a piece of chalk marking start and finish, roll the wheels once (off the car) to exactly the same point-  this is your circumference.  If this matches to the mm between front & rear tyres on the same side, then maybe one is slightly underinflated and when the weight of the car is on them, one is ever so slightly shorter in circumference due to the larger flatter spot in contact with the tarmac?

Post #3 is all you need.

You get more deflection in the front tyres too because of the weight of the engine. In a turn the front and rear wheels don't follow the same kinematic envelope either, ie: the inner rear wheel travels less distance than all the others because of having one fixed axle and one steering axle, the difference in distance travelled depends on the radius of the turn in as the rear end of the vehicle tries to cut across the path of the front wheels in the shortest distance

the driving wheels of the car can rotate at different speeds if necessary via a epicyclic gearbox often just to referred to as a differential. Basically if there is more tractive load on one side it allows the planet gears to rotate which permits one side to travel faster than the other. It's difficult to explain in words, but there is bound to be some diagram on the interweb somewhere that will show it.

When i got out the 'Skoda' badges were completely out of line! 

 

 

GLENT - you appear to have a bad case of OCD - seek help.

You said you turned the car around. During the turns, the inner wheels rotate less than the outers. All 4 wheels will be going at different speeds and will move a different distance.

 

This is why cars have differentials.

 

Exactly.

GLENT - you appear to have a bad case of OCD - seek help.

There is someone at Skoda who has a similar affliction...

 

I noticed the other day while checking my tyre pressures that the 2 n/s wheels had both stopped with the valves right at the bottom at 6 o'clock.  I also noted that the Skoda badges (the new black/chrome style) were both the "right" way up with the arrow horizontal.

Checked the other side - valves were not both at the bottom - that would have been too much of a coincidence!  BUT the badges were aligned to the valves in exactly the same way.

 

Coincidence?  Machine Insertion? or someone at Skoda with a very fine eye for detail?

 

C'mon everybody - get out and check yours :-)

Post three is definitely the correct answer. But thinking about maths is definitely more fun.

There is someone at Skoda who has a similar affliction...

 

I noticed the other day while checking my tyre pressures that the 2 n/s wheels had both stopped with the valves right at the bottom at 6 o'clock.  I also noted that the Skoda badges (the new black/chrome style) were both the "right" way up with the arrow horizontal.

Checked the other side - valves were not both at the bottom - that would have been too much of a coincidence!  BUT the badges were aligned to the valves in exactly the same way.

 

Coincidence?  Machine Insertion? or someone at Skoda with a very fine eye for detail?

 

C'mon everybody - get out and check yours :-)

I've got better things to do thanks

I didnt take the wheels off so i could match the badges, i had them off to paint the rear drums and just though whilst putting them back on why not line them up.

Thrn i couldnt work out why they didnt match.

It was only a joke..

 

You know you could have popped the centre caps out and then put them in what ever direction you want - that way you could also make sure the bolt holes lined up. Not sure how you could do the bolt heads though..... :think:

 

Edited 23 September, 201312 yr by xman

Cars have 'differentials' rear wheel drive or front wheel drive or AWD, because as you see when you drive a car on snow or sand the wheel travel different distances as you steer around a circle.

As explained earlier on in the thread.

 

But even if the where all the same going in a straight line and a wheel was to spin, and that would be a driven wheel,

then that would be out by the amount of the spin.

 

These should look the part on a Monte Carlo Estate.

New Skoda emblem in the centre and a little weight at the bottom of the spinner so that it always stops with them the right way up.

Post three is definitely the correct answer. But thinking about maths is definitely more fun.

 

Something I heard about recently struck me as very clever, and faintly related to what we're discussing here:

 

Modern tyre pressure monitoring systems can tell if there's a problem by comparing the rotational speeds of the four wheels using the ABS sensors. Combined with sensing steering inputs and averaging suitably during times when the car is going straight, 'the car' can tell if one tyre is losing pressure because its effective radius decreases and so it has to spin faster to keep up with the other wheels.  Wish I'd thought of that!

By the way = Pie = Fray bentos pastry and "meat" - Pi = 3.14.

 

 

Pie = Fray Bentos = Jack Douglas   (I loved those ads)

the driving wheels of the car can rotate at different speeds if necessary via a epicyclic gearbox often just to referred to as a differential. Basically if there is more tractive load on one side it allows the planet gears to rotate which permits one side to travel faster than the other. It's difficult to explain in words, but there is bound to be some diagram on the interweb somewhere that will show it.

 

This is the best video I've ever seen explaining it. Even I understand it

 

This is the best video I've ever seen explaining it. Even I understand it

 

 

 

BRILLIANT!! that really does make so much sense! 

 

Thanks.

For my 8th birthday I was given a Lego 8860 kit -see this excellent webpage about it, I learned a great deal from this model, including changing the engine design to a V4 which ran much smoother as the orgional boxer got stuck occasionally.

For my 8th birthday I was given a Lego 8860 kit -see this excellent webpage about it, I learned a great deal from this model, including changing the engine design to a V4 which ran much smoother as the orgional boxer got stuck occasionally.

 

I must admit, I do end up being more of a kid than my nephew when he gets his Lego out.

 

Shame he hasn't got any Technic yet.

Something I heard about recently struck me as very clever, and faintly related to what we're discussing here:

 

Modern tyre pressure monitoring systems can tell if there's a problem by comparing the rotational speeds of the four wheels using the ABS sensors. Combined with sensing steering inputs and averaging suitably during times when the car is going straight, 'the car' can tell if one tyre is losing pressure because its effective radius decreases and so it has to spin faster to keep up with the other wheels.  Wish I'd thought of that!

 

Clever system in theory, except on the way home from work a few days back my light came on so I had to pull over and check my tyres. They were all fine and the system had just been falsely activated, I'm presuming by some weird bump in the road or a pothole or camber combined with some spirited driving. At least there is a handy reset button...

Clever but not fool proof it would seem.

Inevitable compromise between being fast enough and sensitive enough to give a timely warning of a real problem, and having good immunity to 'noise' in the measurements. Would rather have that compromise err on the side of false alarms than the other way round.  Got to get it about right though or customer complaints will be too high.

 

Not sure on Mk2s, but the <4°C ambient temp. audible warning from the cluster on the Mk1s is definitely in the 'more annoying than useful' category.