ltborg

just curious if anyone ever run aluminum or titanium bolts on a 3 piece wheel, specifically the ccw classic? ti seems to be too expensive to be worth it. aluminum might be ok but ive never seen anyone do it. is it safe on these wheels for a race only application? we do it on our wheels for the formula car all the time but thats a car + driver weighing less than a single corner on the vette. its mentioned in the weight reduction section of the sticky in c5 general but ive never seen anyone actual do it. any thoughts? tia

TedDBere

Titanium has been done, but I'm not sure about aluminium. FWIW

ghoffman

It is a good idea, especially since the fasteners are at the outer radius and would reduce the MOI of the rotatating wheel, and although Ti fasteners are great for this, good luck getting them at a price you can afford. As far as aluminum goes, 7075 has a yield strength of 70Ksi and an ultimate of 78Ksi and that is marginal at best for this. Besides, you would probably have to make the aluminum fasteners yourself in the size you need. That is alot of lathe time! The most common Ti alloy for fasteners is 6Al-4V, so if you want to compare material properties, see for yourself.
http://hardbarusa.com/Matpropw.doc

ltborg

thanks gary, thats a really good chart youve made up. i was trying to see what experience people have with this. i could do the calculations but i dont really have any good data to calculate wheel loads from. need to get some accelerometers on the car to get an estimate, or better yet some strain gages on the a-arms. oh well, im guessing even if it was doable with a reasonable factor of safety, at about half a pound a wheel, it probably wont make a difference i can feel. this is more to satisfy my engineering curiosity.

ghoffman

You could estimate the loads by assuming that the entire weight of the vehicle is on one wheel and then assuming that you can pull 1.25g's and then add some pothole guess and see if you are close, flunk, or have large margins. Then you can make a decision on what the next step is based on a long-term fatigue model.

Solofast

First you really need to stay away from aluminum for a fastener for this type of application, as Gary has said, the strength simply isn't there. If you don't have sufficient strength for a high preload, then your fastener goes into tension on each cycle, and we all know that aluminum has a relatively low fatigue capability compared to more conventional materials.

6-4 titanium, on the other hand, is fine, we use it often for fasteners, and you may be able to find some suitable aerospace fasteners that would work. Adjust your torque to get to the same percentage of yield as you would in a more conventional material and you will be fine.

Typically, you want to use a decent safety factor on any fastener that could be subjected to high cycle fatigue. The loading recommended (if my memory serves me correctly) by Colin Campbell in his books for this type of thing was a 5G factor on material yield. The logic being that a sharp pothole could put a short pulse into the suspension, that would be absorbed by the shock and spring, (and you wouldn't see more than two G's at the spring perch) but the inertia of the wheel, spindle and brakes would end up putting the equivalent of 5G's into the unsprung parts.

The last thing you want to find out is that you underestimated a load in this kind of case.

XPC5R

CCW used to have Ti hardware as an option, and it did save a pound or two per wheel. There are probably a few sets out there in the corvette community. Bolts now in the size the classics use cost from $3-$5 each. 26 bolts per wheel.

Try here: http://www.mettec.com/

ghoffman

Actually, I cannot take credit for this chart, it was from a public source and was originally a USAF/Mitre Corp document and is to a large extent Mil-HDBK-5. Since you are an ME student, this really needs to be looked at with a "Modified Goodman" criterion, not simple s/n fatigue or yield criterion. A Modified Goodman chart assumes a certain level of positive stress then cycles about that prestress. This is like an airliner wing that is supporting the entire weight of the plane so it is stressed to some level, then you start cycling the stress up and down, but rarely do you cross over into the zero or negative stress. In the case of a fastener, it is preloaded to some level that is (hopefully) never reversed, but is cycled in use.
http://www.fatiguecalculator.com/con...stresslife.htm
http://www.fatiguecalculator.com/def...fs.htm#Goodman