Thread: Wheels and Tyres for a Puma

Hi,
The Disco rims I had on my 110 County were 7 inches wide had +33 offset, the 255/85R16 BFG KM1 Mud Terrains hit the radius arms, and the side wall of the tyre just kissed the upper rear spring mount, not enough to damage the tyre but enough to polish the mount, they were inside the flare.
The touching the spring mount is also caused by the taller stiffer springs I had in it, with the standard springs they cleared.

The 130 rims on the 130 are 6.5 inches wide with a +20 (or 20.6 according to Rick) offset the 255/85R16 BFG KM1 Mud Terrains hit the radius arms but clear the rear spring mount and are inside the flare at the front but just stick out past the tub at the back (only an issue on a 130)

I reckon 10 or 15 mm less offset would have the tyres out past the flare.

I have no idea how much less offset I would need to be able to have my steering stops wound all the way in.

Why don't you fit the tyres to your standard wheels and see how they go.
This "steel wheels are better in the bush" is as nonsensical as my preference for a 300Tdi over something with electronics
Learn how to change a tyre on the alloy and move on.

Sorry I can't tell what will clear every thing but hopefully this will help you get there.

Tony

Originally Posted by TonyC

Hi,

[snip]

The 130 rims on the 130 are 6.5 inches wide with a +20 (or 20.6 according to Rick) offset the 255/85R16 BFG KM1 Mud Terrains hit the radius arms but clear the rear spring mount and are inside the flare at the front but just stick out past the tub at the back (only an issue on a 130)

I reckon 10 or 15 mm less offset would have the tyres out past the flare.

[snip]
Tony

Yep, my rough measurement tonight with the spirit level and tape showed 10mm further out from the tyre was pretty much the edge of the flare.

Call, les Richmond automotive, they are good to deal with. I would run a zero offset or close to on a 8 inch rim for the 255/85/16.

16" x 8" 0mm BLACK Triangle style patern, you can get this from the above company.

Originally Posted by lambrover

Call, les Richmond automotive, they are good to deal with. I would run a zero offset or close to on a 8 inch rim for the 255/85/16.

16" x 8" 0mm BLACK Triangle style patern, you can get this from the above company.

Why an 8 inch when 7 is the recommended width?
6.5-8 are the allowable widths.

Tony

You also need to consider using less than 20mm offset on a 6.5" rim (standard 130 rim) or less than 30mm on a 5.5" rim will have a effect on the steering geometry, in that your Caster angle will no longer intersect with the center of the tyre where it touches the floor.

Diagram below might help solve some confusion.

Pls excuse picture does not show Landrover but the principle is the same, instead of the top & lower balljoints on the wishbones you will be imagining the top & lower swivel bearings on your CV housing



Less Offset needs taller tyres to bring the geometry back into spec.

Altho My 18mm offset 5.5" bush rims are Great offroad & give a really good turning circle but are horrible on the highway & get the speed wobbles when I go over a bump.

Originally Posted by goingbush

You also need to consider using less than 20mm offset on a 6.5" rim (standard 130 rim) or less than 30mm on a 5.5" rim will have a effect on the steering geometry, in that your Caster angle will no longer intersect with the center of the tyre where it touches the floor.

[snip]

You mean King Pin Inclination (KPI) or Steering axis Inclination (SAI)

Castor is the tilt backwards (at the top) of the steering axis when looking from the side.

Anyway, AFAIK no vehicle on the road uses a zero scrub radius, they either use a negative srcub radius (front wheel drive cars) or positive (rear wheel drive and solid axle full time 4WD)

Zero scrub radius while seemingly ideal actually reduces feel and feedback in the steering, positive (and negative) scrub radius give a little more feel and feedback, weighting the steering as the tyre contact patch scrubs a little while turning.
Some books say zero scrub can make the steering inconsistent in feel.
AFAIK I've never driven a car with zero scrub radius.

If the scrub is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a welded differential, inducing a condition called 'squirm'. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the contact patch. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes 'grabby' causing tire wear to increase and the steering to become unstable.

Positive and negative scrub radii have benefits in different types of suspension. A MacPherson strut assembly typically performs well with a lot of SAI and caster, a system negative scrub works well in. Because both SAI and caster increase the amount of camber on the outside wheel when steering, the fulcrum pivot point is at a point that has more leverage, requiring less steering effort. Negative scrub also helps reduce torque steer in front wheel drive cars. Positive scrub radius works well with suspensions that use dual control arms that use less caster and SAI to optimize geometry.

Scrub Radius | Technical || H&R Special Springs, LP

[ame="http://en.wikipedia.org/wiki/Scrub_radius"]Scrub radius - Wikipedia, the free encyclopedia[/ame]

Too much positive (or negative) scrub radius and the steering becomes heavy and with miles too much kickback when hitting bumps and can really accentuate any play in the swivel pins or steering system.

Also, we have to remember that the steering is a dynamic situation, and as the car is cornering the contact patch of the tyre moves as the carcass flexes, and a tall 85 aspect ratio tyre has a lot more sidewall flex than, say, a 45 aspect ratio tyre, so as the cornering loads increase the contact patch on the outside/working tyre can come back towards the SAI intersection point, actually reducing the scrub radius.

Originally Posted by lambrover

Call, les Richmond automotive, they are good to deal with. I would run a zero offset or close to on a 8 inch rim for the 255/85/16.

16" x 8" 0mm BLACK Triangle style patern, you can get this from the above company.

Originally Posted by TonyC

Why an 8 inch when 7 is the recommended width?
6.5-8 are the allowable widths.

Tony

I agree, 8" would be fine with a 285/75 tyre but miles too wide with a 255/85 tyre, that's if you could even get the bead out onto the rim to mount it.
I've seen most tyre places struggle to get 265/75 tyres mounted onto 8" rims.

With the wider rim the tyre sidewalls will be more vulnerable and the chances of getting crap between the sidewall and rim probably increase dramatically too as the rim is seemingly wider than the tyre.

Originally Posted by rick130

Standard 110/Disco I rims (at least pre TDCi) are 33mm positive offset.

POSITIVE OFFSET
Refers to wheels where the mounting face is (outboard) of the centre line of the wheel. Most often found on front wheel drive vehicles.

from here Performance Wheels | Technical Information

130 rims are 20.6mm positive offset.
This concurs with the backspace of 4"/103mm

In this case, my modded disco 1 rims standard 20mm out.. 13mm. If you get a 7inch rim, 255/85 with this you won't rub, and you'll fit under the guards... I run this setup.

I worked it out with a metal ruler, some string and tape.