Scrubbing Tyres - new information - last post September 2008

[UK-Corlett]

Hi

[edited] naming reversed by jss06

Are there any tyre specialist out there !!

I have a problem with scrubbing tyres
As you can see the inside tread is wearing much faster than the outside.



I have measured the angles.
I get a [edited] camber] (bottom of wheel outward) of 2 degrees.
6.5mm further out at the bottom of the steel rim.
And [edited] toe-out] (twisted about the vertical axis) of 0.5 degrees.

I am baffled. The [edited] camber] is not very much, 2 Degrees, common in many motor cars and would not its self result in scrubbing.
The [edited] toe-out] would scuff but its only 0.5 degrees.

I can correct the [edited] camber], by adding a hardwood wedge between the suspension unit and the frame, but not the [edited] toe-out].
For now I intend to swap the tyres over and get my money's worth but I need to solve this for the next set not to wear out so quick.

OTHER INFO
I made the TD 2 years ago
I have run say 6000miles.
She weights 340kg
She bobs about a bit when going slowly over rough roads but at 60mph she' as solid as a rock. I wonder if I am seeing down force at speed and the apparent weight of the TD increases and the wheel geometry gets worse.
I also let the tyres down to limit the bouncing, which worked. And then I noticed the wear so they are back at their recommended pressure, 40 PSI ish.
The tyres are 20.5X8.0-10 and £45 each (ouch !)
I made the frame, its not an HF or kit trailer
In the UK we use suspension units like these.

[jss06]

I think you have toe confused with camber. toe is how much the tires point in toward the frame. Camber is how much the tires lean.

Measure the distance between the front of the tires and the rear of the tires to detwermine the toe angle. I am willing to bet you have a substantial bit of negative toe (tires point out from the frame at the front) that is causing the excessive wear pattern. To adjust it you may need to reposition your axle mounts or may need to replace them if they are loose and causing the tire to twist slightly while being towed.

http://en.wikipedia.org/wiki/Caster_angle
http://en.wikipedia.org/wiki/Camber_angle
http://en.wikipedia.org/wiki/Toe_(automotive)

[Arne]

I've never liked stubs for the reason of the problems you are having... Can you get a straight edge (or line) between the 2 stubs to see if they are mounted correctly.....

Even with 6 bolts, the stubs can/will move.

If you find they are not in alignment with each other, can you run a piece of angle iron across between them to line them up (and leave it there permanently)..

also, are they mounted to wood (poor design) or metal?

[UK-Corlett]

OK OK

I used a common term incorrectly but defined what I meant by it so!!
Forget the terms then.

I used a laser and projected the parallness (toe out if you like) of the wheels onto my garage door 5 metres away. This calculated to an angle of 0.5 degrees for each wheel.

As I said (forgiving terms) not enough I think to cause scrubbing.

[Volvord]

What you describe as your Camber @ 2 deg and toe-out of .5 deg will both cause the tire wear you are experiencing. Try to adjust your camber to as close to zero as possible (+- 0.5 deg)

Set you toe to about 1/32" toe-in

[brian_bp]

... I wonder if I am seeing down force at speed and the apparent weight of the TD increases and the wheel geometry gets worse....

I can see how increased vertical load would cause suspension deflection which would increase the negative (top inward) camber...

but think the chance that anything the shape of a teardrop trailer has aerodynamic downforce has to be very very small. Lift, I would believe.

[angib]

2 degrees negative camber (top of tyre leaning in) seems fairly small to cause what looks like pretty severe tyre wear - though photos can be misleading.

However any negative camber on a trailer isn't good for stability. Something like 1-2 degrees positive camber (top of tyre leaning out) gives the best towing behaviour - it's all to do with 'camber thrust', but I'll not discuss that further in mixed company....

Fixing the camber would be pretty easy - a washer or two under the middle mounting bolt hole and double that number under the outer mounting bolt hole would do it.

Andrew

[madjack]

...this is what I would do to check things out...
1) on one side, measure from the front of the steel frame to a point around 1"(2.5cm) in from of the axle bracket...

2) take that measurement and transfer it to the other side...

3) measure from the front of the coupler to that mark on both sides...they should be the same...

4) place some sort of straight edge across those two marks and measure back to the axle mounting bracket at either end of both brackets...are those measurements(all 4) the same...

most straight axles are bowed up in the middle to account for weight deflection so that when loaded, you get near neutral camber...don't know how that applies to stubbies.......
madjack

[brian_bp]

...most straight axles are bowed up in the middle to account for weight deflection so that when loaded, you get near neutral camber...don't know how that applies to stubbies...

I realize that the suspension units in question are not exactly Flexiride, but they're similar.
Flexiride's technical data page includes a Camber and Toes Angles drawing which illustrates Madjack's point using full axle, and at least some models (FR-1400, FR-2000, FR-3500) of their "half axle" are shown with a 1.5 degree camber built into the mounting bracket... the diagrams for other models (FR-5200, FR-7000) show shimming similar to Andrew's description.

I assume that this information from UCF suggests that the lighter-duty models, which are similar to the units in this topic and appear to not have camber built into them, should be shimmed as Andrew suggested.

[rainjer]

I had the same problem on my trailer. The person that owned it before me backed into a stump and bent the axle forward in the middle. Only one of the tires wore like that. The problem is that both wheels are actually toed out in the front and are not running parallel to the tongue. The trailer when towed will not track straight behind the car and will drag one tire more than the other.

Also check you tire pressure. I will bet the one in the picture is lower than the other one.

Jeremy

[UK-Corlett]

Hi all
Thanks for the contributions

This camber (in at the bottom) can't be the whole problem. I remember a 1970's small BMW. It's rear wheels stuck out at the bottom an enormous amount.

Thanks rainter
Sorry but this is something I did not include in the post. The wear is exactly the same on the other tyre.

Thanks Madjack
I will do some more measurements this week end. You make an interesting point.

Thanks bian_bp
This is what I was looking for, a written definitive answer.

Thanks angib
The good news is that I can easily alter the camber. I think I favour wooden wedges over washers. I have already selected the bit of teak I will be making the wedges from and with bian_bp help I know how steep to make them. This weekend I will be the swapping the tyre on the rims to get my moneys worth out of the rubber.

I am still left with a doubt however about this BMW with its massive camber and that the wear could be from the toe out, which I can't easily alter.

Clive

[brian_bp]

This camber (in at the bottom) can't be the whole problem. I remember a 1970's small BMW. It's rear wheels stuck out at the bottom an enormous amount.

I'm guessing that this was a BMW 1600, 2002 or similar 4-cylinder sedan; they had a semi-trailing arm independent suspension.

Both the trailer suspension and the automotive semi-trailing arm have one arm on each side, simply pivoting like a hinge. The big difference between the two is that the trailer is a "fully trailing" design, which means that the pin of the hinge runs straight across the frame, while those cars use a "semi-trailing" design, which means that the pin of the hinge is angled back.

The angle of the pivot matters because while the camber and toe of a trailing arm doesn't change as the arm moves up and down (the trailer), those angles in a semi-trailing setup change signficantly with travel. The BMW has to start with negative camber a normal height to avoid cambering positive or even toeing out when the spring compresses under cornering, accelerating, or (especially) braking.

The trailer only needs a bit of camber and toe to compensate for the bending and rubber deflection which happens under load; there's no need to allow for any result of the arm swinging, so they don't start with negative camber.

The AutoZine Technical School - Suspension page has a reasonable summary of the two with a nice illustration.

My wife and I have a Triumph Spitfire with "swing axles"; the angles work the same way as any semi-trailing arm with the hinge axle way off of trailing, and it has huge negative camber - it's also a blast to drive (when the engine works) and doesn't get enough use for us to know much about how the tires wear.

[Alphacarina]

Your picture shows wear on the inside of the tire - That means the wheel is leaning in at the top and out at the bottom . . . . negative camber

2 degrees of negative camber by itself wouldn't necessarily wear the inside of a tire like you have and neither would half a degree of toe . . . . but if you combine them both, you could see some real problems. No trailer axle should have any built in negative camber, because that will only get worse as you load the axle

I have a pair of stub axles very similar to the ones you pictured (mine are Flexirides) and their mounting plates have about 2 degrees of positive camber built into them because as you load a trailer axle, the camber typically goes more negative, so with mine it would even out at near zero camber when the axle is loaded

If you have negative 2 degrees static, you could have a couple more than that in a loaded configuration and maybe even a couple more in a jounce situation, so your negative 2 degrees might actually be as much as 5 or 6 degrees in some situations. Are your axles mounted right side up (under the frame)? You can't use them reversed or they will have negative camber to begin with

Don

[angib]

I am still left with a doubt however about this BMW with its massive camber and that the wear could be from the toe out, which I can't easily alter.

In the 70s, a mate of mine had a Renault 8 Gordini or Rallye (rear engine, twin Weber, swing axles) - a seriously quick car in its day - which had the same massive negative camber on the rear wheels. It made the rear end stick 99% of the time (the other 1% involved sentences including words like 'bushes', 'cliff' and 'arse-first') but it did wear out the inside edge of the rear wheels and made it very 'fidgety' in a straight line.

However he was a master brewer and improved the weight distribution by carrying a crate of beer in the trunk at the front - the car lost stability if you emptied the bottles but you were then too drunk to notice......

Andrew

[UK-Corlett]

Hi angib and brian_bp

These is the kind of camber story's I was thinking about. I understand that the camber was there to improve cornering etc but it did not lead to high tyre wear.

The wedge I intend should bring it from 2 deg camber (out at the bottom) to 1.5 Degrees (out at the top). see technical date ref from brian-bp.

Hi Alphacarina
He wrote "Are your axles mounted right side up"

They are the right way up. The trailing arm would not load up properly if not. But thinking out of the box is good.
This picture is upside down, right.