Does this car look like it's evil? I think it is. I may have recently posted about my struggle with the '52 DeSoto, and (what I thought was) a leaking power steering hose. That turned out to be, after I spent all day getting it out, making a new one, replacing it, a transmission cooler line, which I'd unknowingly damaged (probably) putting the engine in. Fluid was coming from that wound, creeping up the tube (capillary action is fascinating), until it got to the power steering hose it touched, and then ran down that, right where the crimped end goes into the hard line part into the steering box. It's kinda funny now... I'd gotten the car all ****oned up, everything was great, so we jumped in it last night and drove the 100 miles over to Ann Arbor to watch our grandson play tennis. So far, so good. We got on I-94, I accelerated off the ramp, and at roughly 75 mph (I wasn't looking, but keeping up with traffic) heard, and felt, a deep, rumbling vibration. Now, there's construction everywhere on this highway, and where there isn't it's in TERRIBLE shape, so I first thought, "This is the road.", because it felt like driving on a rumble strip. I slowed down to about 65, smooth as silk, and soon came up behind a slower moving car, accelerated to p***, and as soon as it hit 75 (looking this time), same rumble strip. Only there was no rumble strip. Slowed back to 65, and decelerating felt the rumble briefly increase. "This has got to be a driveshaft thing", I thought, remembering the guy at the driveline shop who made it told me, when I picked it up, that it was about 2" longer than he'd have liked to have made a one piece shaft. "Great", I said, "Now you're telling me?", but since we'd had no trouble, I thought it was OK. So, I called the shop and talked to one of the guys about it, described what it does, and told him that in three years of driving it, this is the very first time we've driven it on the freeway , NOT TOWING OUR TRAILER, at highway speeds. "A driveshaft with a tube over 60" will oscillate", he said, "DON'T DRIVE IT OVER 70, IT'LL COME APART!" "Bring it in, I'll make you a new one in large diameter aluminum tube, that'll solve the issue. Or, I can make you a two piece shaft, but you'll have to make a mount for a carrier bearing." So, my hunch was right, it's the driveshaft. The angle of the pinion is 3 degrees up, the trans is 3 degrees down. He said the offset center section in the Ford 8.8 isn't an issue (I had offset the engine and trans about an inch to partially compensate), so all I have to do is have another driveshaft made. I kept the speed to 65 the way over and on the way home, so I think we can drive it moderately until I can have the new shaft made, but for now we're towing the trailer with the Diamond T this weekend.
3.73's. Overdrive trans, so it should be happy at 80. It's kinda embarr***ing to say that I haven't tried to drive it at that speed before.
I was told 60 inches as well. But keep in mind, the OD is for the engine, DS still spins the same at speed w/o OD.
Drive shafts have what is known as critical speed. For every tube diameter, there is a critical speed that if exceeded will cause catastrophic failure. The critical speed is dependent on tube length. Like your driveline shop mentioned, the fix is to go to a two piece shaft or a larger tube diameter. I’m sorry I couldn’t provide more details, but it’s been 25 years since I worked in a driveline shop and built shafts.
Yeah, I know, I meant the car should be happy, considering the suspension and engine. It's just the driveshaft that isn't happy. It's not happy AT ALL.
I recall the ‘42 Ford PU I had and the ‘48 F1 both used the same DS, 60 inches long and 3 inches in diameter. But 70 mph was not an option
Oh, and to top it off, the right brake light is not working. AGAIN. Looks like I'm taking the wheel off and diving into that turn signal switch. Might as well do that while the driveshaft is out of the car...
https://****erparts.com/calculators/critical-speed-rpm-calculator As others said, larger tube diameter can raise the critical speed a lot. Thicker tubing does not improve things at all. With regular type U-joints and heavy throttle "secondary couple" can excite the critical speed at 1/2 the normal rpm. A real flexible transmission mount or rear axle mount can lower the calculated critical speed quite a bit. If the shaft was made of tubing with uneven wall thickness ( unusual) or has more than about .04" bend/bow there can be a heavy spot in the middle of the drive shaft that can not be balanced with weights at the end. Then the shaft will tend to "whip" at higher rpm. In severe cases I've seen unacceptable whip at less than 1/2 the critical speed.
It's a Kirker color, I used their base coat converter and clear. It's VERY transparent, kinda like a candy, rather hard to spray and get an even coat. The result is OK, but in retrospect, I probably shoulda spend a bit more on materials.
I'm going to go with their recommendation to simply have a larger diameter aluminum tube shaft made. The only bad thing about this is that I'll need to modify the back end of the driveshaft tunnel to accommodate the offset center section of the 8.8. I THINK a heavy hammer and a Port-O-Power will get the job done.
The Jalopy Journal
