Need Ford 9 inch rear advice.

[DaveR]

I decided to go with the Wilwood internal drum parking brake/disc brake combo on my Ford 9" rear. My rear end house (a nationally recognized name) informed me that the drum brake set up made it impossible to reach the four axle retention plate bolts with a wrench, so they needed to drill an access hole in the axle flange (at an additional charge) so that a socket will go through the flange and mate with one nut a a time.

Here's my question. As soon as I saw the single 1 inch hole in each flange it occurred to me that this will cause an out-of-balance condition resulting in vibration in the drive train at high speed. It is only 2 inches off-center, but a hunk of steel 1 inch in diameter is a lot of weight.

When asked about this, they responded that Ford has done this for years. I said that's fine on a common passenger vehicle, but doesn't seem appropriate on a set of 31 spline custom made forged racing axles to be used in 150 MPH road racing. They said they would drill a counter balance hole if I wanted, but they woudn't know what size, and aren't set up to balance axles. This from the best in the nation!

I'm thinking that I will have to do it myself. Anybody ever faced this problem or have an opinion?

[Mr.Fixit]

Currie ?

[Dirty Harry]

My 9" utilizes some stock Ford axles from Moser that were re-worked to fit my shortened housing. The flanges both have the holes you describe. If those holes cause a vibration, I am not aware of it. I hope this helps.

[DaveR]

I was trying to be discreet, I don't need to disparage, I just don't want to end up fighting a vibration problem and having to go through the routine of: Is it the tire? Is it the wheel? Is it the axle? Is it the drive shaft?

Actually I feel a little better after deciding that it's well within my capabilities to drill it myself, because I thought it would have to be done before final assembly. I didn't want to pull the axles back out for drilling when it would be so easy now.

I think I can drill it after assembly, because installed is probably the best method I have to hold it in place, and it doesn't have to be machinist perfect, even hand drilled would be an improvement. I can drive it, then drill it in place if it's a problem, the ole trial and error method.

Fixit, you're an expert, what's your opinion, will this cause perceptible vibration?

[tommachine73]

Dave R

Although you have a valid point there is no need for concern. The amount of material removed is not great enough to cause a vibrational problem.Simply put the the hole which is located on a diameter of of say less than 3.00 inches spun at 150 mph has not enough mass at that diameter to cause the wheel to vibrate.However take that same amount of weight and remove from the tire area and you will have a significant problem. I have a Ford 9 inch in my cobra and there is no noticeable vibration at 150 at the track.

[tommachine73]

Dave R

p.s do not try to drill that flange.The axels themselves are hardend.The flanges are slighly hard.Leave it alone if they needed to do it they would.

[DAVID GAGNARD]

Dave R;

My Versailles rear end has that same exact large hole in each flange from the factory for access to the backing plate nuts.......... I have worked on a few 9" p/u truck rear ends and they all had it too... Do not worry about a vibration or out of balance,they came from Ford like that since the begining of the 9" and have worked just fine...........

David

[David Kirkham]

I have seen lots of Ford 9" axles drilled with that same hole. Doesn't seem to bother the Mustangs the race at Willow and they reach some pretty high speeds.

I think you will have a very hard time drilling the axle. I would leave it alone. It is probably hardened 4340 and you would be very hard pressed to drill a 1 inch hole in the stuff. Even clamped in a CNC it is miserable stuff to machine.

Hope this helps...

David

[kountzecobra]

No problem drilling the axles. I drilled my Moser axles. 7/8" ? It just takes a decent drill bit. Only the axle shaft is hard, the flange is not near as hard, otherwise breakage would be a problem.

[zipzip]

Assume the flange is 1/2" thick, then the unbalance force at 150 MPH with a 26.6 dia tire is 22 lbs....Not too significant. If it makes you feel better, drill out the opposite side, but be careful not to "swiss cheese" the flange. Remember, it has to take those high torque starts with hot slicks!

As tommachine73 says, if it were on a bigger radius it will become more significant. And actually, the force is relative to the square of the speed, so make sure you're tires are fully inflated...

BTW: if you're interested, the formula is F=1.77WR(n/1000)^2 where W=unbalance mass in ounces, R = unbalance radius and n = rpm of shaft.

[DaveR]

All,

Thanks for your input, sounds like nobody has had any problems, plus it also sounds like I might be able to drill them in-place if I have vibration. I'll find some other assembly step to worry about.