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Old 12-17-2011, 08:19 PM   #1 (permalink)
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scrub radius

The distance (a) from the wheel axis to the kingpin axis on a vertical line that intersects the center of the tire's contact patch is the cotangent of the kingpin inclination angle multiplied by the distance from the kingpin axis to the vertical line through the center of the tire's contact patch along the wheel's axis (b).

The distance (a') from the intersection of the kingpin axis and a vertical line that intersects the center of the tire's contact patch to the point in the center of the contact patch is (a) minus the tire's effective rolling radius.

The scrub radius is the tangent of the kingpin inclination angle multiplied by (a').

To solve for the scrub radius by this method, the length of (b) can be derived from the measurement of the distance between the kingpin axis and the wheel or spacer flange, the wheel's backspacing, and the rim width. The center of the tire's contact patch would be approximately in the center of the rim, which is half the rim's width from the inside edge of the rim, which is the distance of the wheel's backspacing from the wheel or spacer flange.

Larger tire diameters affect scrub radius negatively (decreasing positive radii, or increasing negative radii).

Increased wheel offset affects scrub radius positively.

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Old 12-17-2011, 08:59 PM   #2 (permalink)
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Your point? I think most here understand the concept. The ones that don't are or should be on DWEB
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Old 12-17-2011, 09:08 PM   #3 (permalink)
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he asked the same question on Dweb and got the same response, he just forgot to add that he's running the stock tire at 20psi, and I'm not making this shit up...it's just more moronic bullshit.

All this coming from someone who doesn't know what a split washer is for

So I'll ask the same question, what is the problem, or is there a question buried in there somewhere
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1983 110 RHD Pick-up
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Old 12-17-2011, 09:48 PM   #4 (permalink)
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The tire pressure affects the effective rolling radius of the tire. I mentioned the size and pressure I was running when I initially tried the 30mm wheel spacers. What's gets you so worked up about 20psi? It's a good compromise on radials. I don't like to air up and down all the time. I'm not driving anything that requires less and if I drive the rocks on anything higher it rattles the dash to pieces.
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Old 12-17-2011, 09:56 PM   #5 (permalink)
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your seriously running your tires at 20psi on the street...wow

so you wonder why your steering is sluggish, not only does it increases tire wear, rolling resistance, cornering stability, builds heat faster...

whatever fills your bowl
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1983 110 RHD Pick-up

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Old 12-17-2011, 11:18 PM   #6 (permalink)
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yeah, like a mile and a half to the field or a couple miles to the trails... I didn't characterize the steering on the street. I'm not concerned with it.

There is no problem, but some questions. I installed the 30mm spacers to add some track width after lifting the Discovery about 3 inches over stock, and to keep turning radius to a minimum with larger tires (yet to come).

I characterized the steering after installing the spacer as having reduced effort and a tendency to wander, which is opposite what I expected. It caused me to speculate that the Discovery's stock scrub radius was negative (typical of ABS equipped cars) and that spacers could reduce it to near zero. Looking at the scrub radius today I can see that it's clearly a positive radius from the factory, and increased by the spacer.

I am not concerned with where the steering is now but where I'm going with it. I wanted to know what the kpi angle of the rover axle is, where my scrub radius was at, and where it will go with different spacers, backspacing, and tire diameters. Obviously, I answered my own questions.

A simple rule of thumb would be that for every inch of increased tire radius, about 3mm more offset is required to maintain the same scrub radius. For the most part, changes to the offset won't be compensated for by larger tires and we will be increasing the positive scrub radius significantly.

Widening the axles without additional wheel offset allows for the increased track width and clearances without increased bearing load and kingpin binding, but if the tire diameter is increased enough to cause a significant decrease in the scrub radius, the steering can be affected, and that is easily compensated with a small spacer.

Last edited by Nevada Ben; 12-17-2011 at 11:22 PM.
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Old 12-17-2011, 11:29 PM   #7 (permalink)
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just more mental masterbation...
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1983 110 RHD Pick-up
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Old 12-17-2011, 11:40 PM   #8 (permalink)
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Looks like your Google is working again.....
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Old 12-17-2011, 11:47 PM   #9 (permalink)
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Quote:
Originally Posted by uninformed View Post
Looks like your Google is working again.....
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1983 110 RHD Pick-up
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Old 12-18-2011, 09:27 AM   #10 (permalink)
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Has it got castor correction radious arms?

Forget ABS or otherwise, as its the same swivel housing back to 1972. Landrover arn't that technical
The only change is the challace bolt number, going from 6 to 7, with a few HD versions for 110 etc. (For that axel type).
I recon Rover have thrown Tyre and wheel combinations on this axel and built it as a production run if it seemed okay on test? For example, theres no change between 92", 100", 110" and 127" for Akerman.

Scrub radious is not normally mentioned. Sounds like it's just on the edge of being unbalanced for another reason?
Not that they ever steer well
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Old 12-18-2011, 11:58 AM   #11 (permalink)
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It does have caster adjusted radius arms. With respect to scrub radius, that keeps the king pin axis more or less unrotated from stock.
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Old 12-19-2011, 08:51 AM   #12 (permalink)
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Has it got castor correction radious arms?

Scrub radious is not normally mentioned. Sounds like it's just on the edge of being unbalanced for another reason?
Not that they ever steer well
yeah, driving around with 20psi in the tires and running castor correction on a stock tire I'm sure all does wonders for the already sluggish steering

I can't wait 'till he put's bigger tires on to see the irrelevance and wiki quotes that start spewing out then
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1983 110 RHD Pick-up
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Old 12-26-2011, 07:19 AM   #13 (permalink)
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I'm pretty sure 20psi is a lot less than what lead to the problems with Firestone/Ford Exploders.
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Old 12-26-2011, 10:01 AM   #14 (permalink)
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Going off of that Tom than shortly Darwinism will remove his presence from the forums...
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Old 12-26-2011, 03:28 PM   #15 (permalink)
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Out of curiosity I just looked, Ford said 26psi.
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Old 12-26-2011, 03:41 PM   #16 (permalink)
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Out of curiosity I just looked, Ford said 26psi.
no wonder people died........6psi more than they should have been running. Damn FOOLS!
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Old 12-28-2011, 08:10 AM   #17 (permalink)
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Quote:
Originally Posted by Nevada Ben View Post
I installed the 30mm spacers to add some track width after lifting the Discovery about 3 inches over stock, and to keep turning radius to a minimum with larger tires (yet to come).

I characterized the steering after installing the spacer as having reduced effort and a tendency to wander, which is opposite what I expected.
STFU!
Any car or truck, you widen the track you will wander, as in ratio you are shortening the wheelbase. You are turning your truck's track and wheelbase towards a square base, that's why the truck wander.

Get your truck back to the stock form then shut up and wheel it.
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