Thread: Barn Find

I suspect this has been around the traps before... and this is my version of The Reason. - Utilise this theory at your own peril !

We're driving an all-wheel drive car, fitted with a centre-diff. In my case, 95 Vogue, this also has a viscious coupling as well as the metal cogs. - but I digress...
Because of a host of reasons, it is a Really Good Idea if the front and rear axles turn at the same rate, at least when averaged over time & distance on solid roads.
The difference F-R suggests to me that the backside is a tad heavier than the front/ Or will be with a full tank and a pax in the back. Or your lunch...

What we're concerned about is the rolling radius. This is the distance from axle centre to the ground. We would like this to be the same F to R, so that the super-strong LR centre diff has an easy lift... very little 'difffing' required. VERY important when a BW transfer case is in the mix... the VC self-destructs if it works too hard.

LR engineers have worked out that under the specified load conditions, using these odD tyre pressures will give you the required R/radius's at both ends. At the same time. - Ideal.

Individual driving habits and loads/speeds/tyre size and type can pull the pressures either way, but for my money, starting off with 'suggested' pressures is a short cut.

Fine-tune with the "Four-pound Rule" or a chalk-line across the tyre, or the wearing appearance and trends.

And if you have another sized tyres fitted... measure the axle-centre to ground and adjust pressures F/R to get the same result, for starters...

Rolling radius of radial tyres does not change measurably over the entire range of practical pressures. It is a misconception that pressure changes the effective radius - unless the tread is slipping on the road, every revolution of the tyre covers the same distance, as the belt behind the tread is essentially incompressible. The actual effective radius is a little less than the apparent radius, as the tread rubber outside the belt is compressible - how big an effect this is depends on the tyre construction and materials and is different for each type of tyre, but the effect is small for any normal tyre. (crossplies are more affected by pressure)

The difference in pressure specified is a compromise between ride, tyre life, and handling, with recommendations changing over time as ideas and priorities change.

For example, the Series 2a tyre pressures specified in the owner's manual are a lot lower than those for the same tyres on the almost identical Series 3 twenty years later. Also, I note in my 110 owners manual, the Australian Supplement lists pressures higher than those in the original English manual, again for the same tyres.

John

Hi John,

I have noticed the same with the Defender, as my old TD5 suggests 28 fronts 48 rears from memory, & the fronts look really bagged at those pressures. The Series vehicles were 20/25 psi for the 80" & 25/30 for the 107". At these pressures the steering seems overly heavy - not noticed on the Range Rover with it's power assistance. But I always feel sorry for the poor old worm & nut in the series at low speeds.

With Series 1's loaded for a trip I have noted that the front is considerably lighter, hence why we install jerry cans on the front bumper!

Regards Nigel

Originally Posted by JDNSW

Rolling radius of radial tyres does not change measurably over the entire range of practical pressures. It is a misconception that pressure changes the effective radius - unless the tread is slipping on the road, every revolution of the tyre covers the same distance, as the belt behind the tread is essentially incompressible. The actual effective radius is a little less than the apparent radius, as the tread rubber outside the belt is compressible - how big an effect this is depends on the tyre construction and materials and is different for each type of tyre, but the effect is small for any normal tyre. (crossplies are more affected by pressure)

The difference in pressure specified is a compromise between ride, tyre life, and handling, with recommendations changing over time as ideas and priorities change.

So, assuming thr rolling radius is consistant (?) how about rolling resistance? - Would this impact on the centre-diff regarding loading.?

Originally Posted by Avion8

I managed to find some new tyres for the old Rangie. The old Michelin X M&S had plenty of tread left, but most were cracked in the sidewalls. In the end I went with Dunlop Road Gripper S - could not find many 205R16 tyres in WA!



The old rusty wheels with tired Michelins.



New Dunlops on tarted up Rostyles. Last set of Rostyles I painted was for my then MGBGT back in the early 70's.



Comparison of the Michelin & Dunlop treads.

Hi Nigel,
Are the Road Grippers 235/85/16? If so is there any rubbing? I'm trying to get my gearing a bit higher and notice d that these tyres are about 800mm in diameter as opposed to the standard size (205/75/16) of about 750mm.
If your not carrying much weight 30psi front and far will be good.
Ta
Jools.

Originally Posted by superquag

So, assuming thr rolling radius is consistant (?) how about rolling resistance? - Would this impact on the centre-diff regarding loading.?

Rolling resistance changes with pressure, as more energy is dissipated flexing the tyres as pressure is lowered. But this will not affect the differential, as the problems with it relate to speed differences rather than torque differences.

John

Originally Posted by discojools

Hi Nigel,
Are the Road Grippers 235/85/16? If so is there any rubbing? I'm trying to get my gearing a bit higher and notice d that these tyres are about 800mm in diameter as opposed to the standard size (205/75/16) of about 750mm.
If your not carrying much weight 30psi front and far will be good.
Ta
Jools.

Hi Jools,
The Road Grippers are 205R/16. There is no aspect ratio quoted as is true of the original Michelins. I have measured the diameter of both & they come out almost equal at 734 mm's or so.

Dragging through a lot of internet stuff informed me that the original 205R/16 were probably 80 from an aspect ratio standing. So I tried to order some 205R-80/16 tyres and all you can get in Australia is a space saving spare wheel with an 80kph speed limit.

Cheers Nigel

Nope, nyet, nein... a 206R16 has an aspect ratio of 82%.

- It is an old convention, when there was one aspect ratio... so it did'nt need to be stated, all radial tyres were 82%.. unless otherwise marked. eg, 205/70 R-16

So, in modern and full notation, your original 205R-16 tyres are ... 205/82 R-16.- with 205/80 R-16 being pretty close.

But nowhere near if you're OCD.