Technical DEEPER DISH WHEELS versus AXLE BEARINGS.

All the rules you guys have to follow on your builds makes me glad I don't live in Australia. But you still manage to build some awesome cars.

 

I understand that the content of this thread is directed mostly at legality. The title of the thread is what caught my eye. Assuming track width stays the same, there is no bearing load difference with rim dish changes.

 

If you change the 'dish' of a wheel you change the offset, which would have to change the track width, surely ???
Unless you mean going a wider wheel entirely with the same offset, Which would give more 'dish' outboard and inboard.

 

Understandably certifiers sometimes like to see floating axles for 10" (or more ) wide wheels

 

15x6 max offset - front
15x10 zero offset - rear

56K miles this way!!!

 

I'm going to say that wheel offset has little to do with solid rear axle wheel bearing wear. In all my years of working in automotive shops and teaching auto mechanics I never saw any pattern where deep reverse wheels caused excess rear wheel bearing wear.
On the other hand I have seen a number of cases where extra deep reverse front wheels caused excess front wheel bearing wear. The local two wheel half ton pickup guys with their ten and twelve inch wide deep offset front wheels that stick several inches out of the fenders have proven that one to me.
It's a bit like what an old priest told me 55 years ago when the subject of drinking came up. "All things in moderation son, all things in moderation" he said. That has proven true in many ways over the years.
The other thing is that if you choose to run deep wheels on the front it is prudent to inspect and repack the front wheel bearings at lot shorter intervals than you might normally do. That and do it correctly.

 

After mulling this over for the last couple of days, I agree with you. A solid rear axle should show the same bearing load regardless of wheel offset. The mass carried does not change.

The front bearings are probably a different story as they operate in a much more dynamic system. The action of the steering would place more load on the front wheel bearings.

 

My understanding for the thinking of the local rules is not so much bearing wear but axles breaking. With a semi floating axle the load is supported by the axle with the bearing acting as a pivot so to speak. With a full floating axle they have a double bearing setup which better supports the load and reduces axle flex.

When you run a deep dish you are moving the load further out from the bearing. By using the principles of leverage you will put more load on the axle. The main failure mode will be in cornering. You will be loading up the outside wheel.

To prove this to yourself, get a broom handle and secure it at one end. Now put a pivot point just in from the other end. You can stand on the end near the pivot and it will not break. Now gradually move the pivot closer to the fixed end and stand on the handle. At some point it will break. You weigh the same but the offset is changing.

Steve


Steve

 

I don't think the static load changes. Regardless of the wheel offset, the relationship of the wheel mounting plate, the hub, the bearing position, and the inner, splined end of the axle does not change. At rest, there would be no change in axle or bearing load.

I agree that there will be a dynamic change in the load during cornering and the wheel reacting to uneven road surfaces, but I wonder exactly how much extra load it would actually be for an offset change of, let's say, 1", keeping in mind that the offset change is the only thing that is different. The bearing has not been slid up the broomstick........

I am by no means an engineer, so don't take my word on it. Maybe some of our resident engineers can do the calculations and shed some light.
@Ned Ludd

 

Moving the “ center “ or the outside edge further from the axle bearing can put more leverage on the bearings. But the weight of the vehicles remains the same. But I assume you have a heavier axle under that car. If you have a nine inch rear end , it was put under pick up trucks and was rated for a load. You car should never reach that load.








Bones

 

You are correct the bearing has not been slid up the broomstick, but the broomstick has in effect been made longer. For every double in length from the pivot (bearing) I think the force is squared.

My take is the rules have been written for the lowest common denominator. If you have an original axle in a car and put deep dish wheels you will be pushing the design limit. Also putting deep dish wheels often results in wider tyres which puts strain on the axle due to extra traction.

If you put a 9" out of a F350 under a 1000 KG car you are still going to have a lot of margin for load. This is were an engineer can prove and certify stuff out of the normal.

Now for my personal opinion, I don't think you will not have an issue as you are obviously smart enough to think about what you are doing. As we have certain rules in this great country, more to protect us from the stupid, I am answering from a technical point of view. I have always found it helps to know what rules we are breaking and the effect. Than you can make an educated decision about your car and safety.

Steve

 

@Reidy , I have edited the title of this thread to read "deeper dish wheels" (instead of deep dish wheels). Some may think that the question was about stupidly large negative offsets, which was not my intention. I will give an example of what I am meaning, although it uses Australian vehicles.

1998 onwards Ford Falcon wheel offset is 36mm positive (less dish).
1969 to 1998 Ford Falcon wheel offset is 10mm positive (deeper dish).
That's a 26mm (or 1") difference. So, were there more failed bearings / snapped axles pre 1998 ???
Does this much extra dish really make that much difference? I am not questioning our regulations in any way. I would just like to know the actual science of it..........

 

Interesting question, a quick look on the net indicates that the au Falcon (1998) had a different diff housing and therefore possibly different axles and bearing setup.

The attached link https://www.wheel-size.com/size/ford/falcon/1998/ shows there were a variety of offsets available for the 98 falcon depending on wheel diameter. At a guess a larger diameter wheel would put more of a bending force on the axle under cornering. The lever length from the ground to the wheel centre in longer. Under cornering the tyre is gripping the road under trying to bend the rim under the car (think tyre rolling off rim). Logic tells me for a larger diameter rim you would need greater positive offset to keep the bending force on the axle identical.

From memory there was also a power increase with the AU Falcon. Therefore more stress on the axle. I am sure some boffin with a computer and a lot of very expensive modeling software worked out all of the calculations and choose an axle and wheel offset combination that met the design spec using the least amount of materials.

I suspect the rules were written with a large safety margin in mind to cover most applications.
So back to your original question on how much of a difference does it make. It makes enough difference for car manufactures to specify different offsets for different wheel diameters. For you application the rules say 1/2" a side is fine with no engineering. The true safe limit is therefore somewhere between 1/2" and a ridiculous amount of offset.

Steve

 

You are correct.