Carbon Fibre Wheel Issues

[AlexG]

Anyone else have issues with their carbon fibre/alloy centre wheels slowly losing pressure?

If we store ours at 30+psi then they are fine, then we drop them to around 11PSI when using the car, but if we don't pump them back up from 11 PSI after using the car then they slowly lose pressure and will be totally flat after about a day.

Any ideas? Have tried smoothing all the sealing surfaces so they are perfect etc and when dunking them in water there is no bubbles to suggest a leak!

[Zac]

I'd recommend double checking that your rim contour is correct. A lot of student manufactured rims I've seen I've gotten this part wrong. I've heard of at least one team unseating a tire mid corner.

[Drew Price]

You throw them in a tub of water at 11 psi, and see no leaks, but they bleed down after a day? http://fsae.com/groupee_common/emoticons/icon_confused.gif Something doesn't seem right.

Do you have caps on your valve stems?

I threw a year or two old set of slicks mounted up into a tub of water and was very surprised at how much air was oozing out of the sidewalls of the tires, are you sure it's the wheels and not the tires?

What about the mating surface of your wheel flanges where the multiple pieces are attached?

You may have to hunt pretty closely, but you should be able to find where they're leaking from.

Drew

[AlexG]

Definitley not having any bubbles out of it when dunking it, 100% confirmed.

However, just discovered that when measuing the diameter of where the tyre seals it is 327.8mm whereas our old succesfull alloy wheels were 330.087 mm, wondering if our carbon fibre wheels might be undersized too much for 13" tyres.

[Tim Gornik]

It is not unheard of to have air seeping through the actual weave. What type of cure process did you use? Auto clave? If the resin was significantly out dated it is possible for it to not flow correctly creating an issue. What does the surface finish look like? Pay close attention to this area under high pressure, and if it leaks try a couple coats of automotive clear coat.

You likely don't have the bead dimensions correct, as this is tricky to hold through out the cure process.

SAE /DOT publish information regarding allowable tolerance and offer profile suggestions for typical automotive wheels. Do a search.

[Adambomb]

I find it odd that they will hold 30 psi, but not 11. Almost sounds like sidewall deflection from supporting the car could be causing a leak around the bead. In other words, with 30 psi you don't have enough sidewall deflection to cause the bead to leak. Do they leak with 11 psi and not supporting the weight of the car? That would lend some more credibility to the idea that the bead diameter is too small.

[Neil_Roberts]

3M makes a variety of paint replacement films that fill the voids in prepreg enough to produce a water tight composite part. Air-tight sealing is of course harder to do. A monocoat epoxy wipe might help, and will make them shine.

Given the undersize bead diameter, I'd say that's where part of the problem is. The rest is likely air seeping through the thin sidewalls. It's normal for racing slicks to lose pressure much faster than road car tires.

[gaoleiHRT-China]

3M makes a very tough link betwen cabon figer and melt