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Pressure loss - steel vs alloy rims - Andrew-T

My 1991 Pug 205 still has its original steel wheels, which were given new tyres 3½ years ago by the previous owner. Since I got it at the end of 2022 I can only remember topping up the pressure once or twice by a psi or two. The 207SW, which is half its age, needs more frequent attention despite having its rims thoroughly cleaned with each new set of tyres, and occasionally in between if bead-leaks become tiresome.

It seems a shame that alloys corrode less visibly than steels - which unfortunately always rust at points of high curvature - rims, bolt holes, etc. - while alloys seemingly always suffer corrosion at the bead. Is this because of reaction between alloy and rubber compound ?

Pressure loss - steel vs alloy rims - paul 1963

Good question Andrew, wonder if it's anything to do with the sulphur used in curing rubber?

Pressure loss - steel vs alloy rims - edlithgow

I understand carbon black in rubber can cause electrolytic corrosion, due to its position in the electrochemical series.

I've never had any trouble with wheel corrosion (I've only ever had steel wheels) apart from a failure to seal the bead once with a newly fitted remould,(which they eventually addressed by putting an inner tube in it..) but I did have corrosion of a radiator pipe once that looked as if it was caused by reaction with the hose.

I attempted to isolate it with polythene film (and I think maybe aluminium foil, though I'm not sure now) and changed the coolant, and it held until the cops got the car maybe 5 or 6 years later.

Edited by edlithgow on 05/07/2024 at 17:12

Pressure loss - steel vs alloy rims - edlithgow

"Unacceptable material pairs include a rubber seal with a carbon black loading so high (for UV resistance) that it is conductive and causes galvanic attack of a stainless screw or pin. Gaskets incorporating graphite have caused similar problems for stainless steel flanges and must not be used for seawater regardless of the stainless steel alloy. "

www.assda.asn.au/publications/technical-faqs/galva...n

Chart immediately below, at the bottom of the page, shows a bigger difference between graphite and aluminium than between graphite and low-carbon steel.

Then I suppose the UK climate will add salty water, which will form hygroscopic deposits in the wheel/tyre interface.

Here's a doode with his hat on backwards, to add more cool cred to this explanation

. (Havnt actually watched this but I assume its cool and credible, since hes got his hat on backwards)

www.youtube.com/watch?v=yRpMZaU8zKw

I suppose you could attempt to address the problem with a sealant, though you'd want to avoid anything that might attack the rubber. Maybe a silicone spray or grease.

Alternatively, or in addition, you could attempt to isolate the rim with something tough that would stand up to the stresses of tyre mounting, maybe an epoxy, though I dont personally like epoxies.

Perhaps an alkyd resin like Penetrol, which I havn[t used, but it sounds useful

Failing that I would probably try my automotive panacea, vegetable oil (boiled linseed in this case) and rubbing with crumpled aluminium foil.

I'd let it set for a day or two to minimise the possibility of rubber attack, then I;d go over it with a thin smear of silicone grease, (which shouldn't be unobtainium in the UK), before fitting the tyre.

Edited by edlithgow on 06/07/2024 at 01:48

Pressure loss - steel vs alloy rims - edlithgow

Come to think on't, the rim coating isnt likely to be practical if someone else is mounting your tyres, as is likely.

You might be able to do it by just breaking the bead, which is a lot easier than complete removal, but still probably too much trouble

Pressure loss - steel vs alloy rims - bathtub tom

I wonder what effect, if any, tyre soap has? They do seem to slather it on liberally.

Pressure loss - steel vs alloy rims - Andrew-T

I wonder what effect, if any, tyre soap has? They do seem to slather it on liberally.

Soap obviously helps to get the bead over the rim, and probably also to seal between tyre and wheel, tho maybe that slowly dissipates with water penetration. Without knowing the composition of the 'soap' I have no idea whether it has a chemical effect, but if it is at all alkaline it might ?

Pressure loss - steel vs alloy rims - edlithgow

I wonder what effect, if any, tyre soap has? They do seem to slather it on liberally.

Soap obviously helps to get the bead over the rim, and probably also to seal between tyre and wheel, tho maybe that slowly dissipates with water penetration. Without knowing the composition of the 'soap' I have no idea whether it has a chemical effect, but if it is at all alkaline it might ?

Dunno about that stuff, but I believe most soaps and detergents are alkaline, and many contain salt.

Pressure loss - steel vs alloy rims - Engineer Andy

I wonder how much physical damage (misshapen wheels) is a factor with modern alloy-wheeled cars, even very minor damage from kerbing and from speed humps, especially on cars with lower profile tyres.

Bear in mind that steel wheels are stronger than alloy wheels, and thus are less susceptible to getting misshapen.

Pressure loss - steel vs alloy rims - Andrew-T

.... steel wheels are stronger than alloy wheels, and thus are less susceptible to getting misshapen.

Steel wheels are not only 'stronger', they are less brittle. That's why alloy wheels are much thicker, but alloy is a lighter material so equivalent wheels weigh about the same. Steel wheels are pressed and welded, while alloys are cast (IIRC).

Pressure loss - steel vs alloy rims - Palcouk

The pressure loss on my wifes 2005 Rav was resolved when I went to a decent Tyre shop who cleaned the wheels properly

Pressure loss - steel vs alloy rims - edlithgow

.... steel wheels are stronger than alloy wheels, and thus are less susceptible to getting misshapen.

Steel wheels are not only 'stronger', they are less brittle. That's why alloy wheels are much thicker, but alloy is a lighter material so equivalent wheels weigh about the same. Steel wheels are pressed and welded, while alloys are cast (IIRC).

BUT THAT would mean the "reducing the unsprung mass" thing is baloney, and its all about the bling!

AND that would mean people are shallow and stupid!

Imagine my surprise!

Surely some mistake?

Pressure loss - steel vs alloy rims - Engineer Andy

.... steel wheels are stronger than alloy wheels, and thus are less susceptible to getting misshapen.

Steel wheels are not only 'stronger', they are less brittle. That's why alloy wheels are much thicker, but alloy is a lighter material so equivalent wheels weigh about the same. Steel wheels are pressed and welded, while alloys are cast (IIRC).

BUT THAT would mean the "reducing the unsprung mass" thing is baloney, and its all about the bling!

AND that would mean people are shallow and stupid!

Imagine my surprise!

Surely some mistake?

And all to sell more alloy wheels and tyres, because both don't last as long as higher profile tyres shod on steel wheels, for many reasons, and cost vastly less.

The better handling abilities of the former are relatively minor unless you jump up several sizes (min. 2in on the wheel diameter and 30mm on the tyre width). The loss in comfort is noticeable though.

Pressure loss - steel vs alloy rims - focussed

.... steel wheels are stronger than alloy wheels, and thus are less susceptible to getting misshapen.

Steel wheels are not only 'stronger', they are less brittle. That's why alloy wheels are much thicker, but alloy is a lighter material so equivalent wheels weigh about the same. Steel wheels are pressed and welded, while alloys are cast (IIRC).

I suspect it may be that alloy wheels are actually easier to produce at scale on automatic CNC machinery than steel wheels.

With my production engineer hat on:-

Steel wheel production. Rim

Steel strip cut to length - rotary bend to form a hoop - weld the join in the hoop.

Form the rim shape in forming rolls punch out and form valve hole, possibly anneal rim afterwards.

Punch out centre blank from strip and press out centre profile including punching and forming holes and profiles for wheel bolts/nuts - one operation.

Weld centre to rim - weld. Clean up welds, straighten rim to run true to centre.

Gritblast and paint.

Alloy wheels.

Pressure or gravity die cast whole wheel at one operation.

CNC 5 axis machine the whole wheel -turn and face the back, All automatic with a robot to load/unload and turn the wheel blank

Gritblast and paint - machine the outer face if design needs it.

It's just easier to automate.

Pressure loss - steel vs alloy rims - edlithgow

.... steel wheels are stronger than alloy wheels, and thus are less susceptible to getting misshapen.

Steel wheels are not only 'stronger', they are less brittle. That's why alloy wheels are much thicker, but alloy is a lighter material so equivalent wheels weigh about the same. Steel wheels are pressed and welded, while alloys are cast (IIRC).

I suspect it may be that alloy wheels are actually easier to produce at scale on automatic CNC machinery than steel wheels.

With my production engineer hat on:-

Steel wheel production. Rim

Steel strip cut to length - rotary bend to form a hoop - weld the join in the hoop.

Form the rim shape in forming rolls punch out and form valve hole, possibly anneal rim afterwards.

Punch out centre blank from strip and press out centre profile including punching and forming holes and profiles for wheel bolts/nuts - one operation.

Weld centre to rim - weld. Clean up welds, straighten rim to run true to centre.

Gritblast and paint.

Alloy wheels.

Pressure or gravity die cast whole wheel at one operation.

CNC 5 axis machine the whole wheel -turn and face the back, All automatic with a robot to load/unload and turn the wheel blank

Gritblast and paint - machine the outer face if design needs it.

It's just easier to automate.

While that sounds reasonable, it also sounds as if those advantages would have existed for quite a long time.

And yet a few (maybe 4 or 5) years ago another foreign lecturer asked me if he should pay extra for alloy wheels for his new Toyota Yaris, so alloys were an extra cost option here then, at least for that relatively modest car.

I'd suspect steelies are no longer an option, or at least no longer the no-cost-default-option, though I dont know this for a fact.

Pressure loss - steel vs alloy rims - galileo

Back in the early 1960s, as part of my Metallurgy Degree course, we had a visit to a Rubery-Owen plant making steel wheels exactly as described by focussed.

They were also pressing truck side chassis members using a hydraulic press which leaked water copiously on the press stroke, and BMC Cambridge/Oxford rear axle casings.

Final quality inspection of the axle casings involved being supported at the ends and the differential housing being struck with a sledge hammer: no cracks = OK.