It is not often that a drive shaft breaks in the middle. They tend to twist there but not break. Especially when talking about 400 to 550hp cars. The weak links are the connection points at the ends. So either the front drops or rear drops based upon which end breaks..
On a one piece shaft you want your loop at the front or middle, not in the rear. This way if the shaft snaps up front the loop catches it before going to the concrete and causing you enormous back half damage by digging in in the same direction of your travel. If the rear snaps, the shaft falls to the concrete and since it is facing away from the direction of travel, it cannot dig into the concrete for more damage.
The loop is really mainly on the car to stop mass tunnel damage and possible passenger compartment entry. If you have ever seen one let go on the big end of the track, you know what I'm talking about. Just imagine having to refloor your mustang entirely.
Yeah, If I wasn't running late for work, i would have expanded and said something very similar. They usually fail at the connection point, and failing at the rear is not very dangerous for the driver or catapulting. But the front is dangerous for both...
Glad you were able to get confirmation from BMR. It's always good to get the confirmation from someone with more experience. I was using my general sense of physics and my information gathered from Myth Busters.
Makes perfect sense now. Not sure why I was thinking of it breaking in the middle. Things almost always break at the joints first. I guess having the rear loop helps protect in the rare case of a break in the middle.
Anyone having problems with the front BMR loop interfering with a Hurst shifter? Mine bangs against the loop from time to time and we're going to try to shim the mount area about 1/4" and see if that provides the clearance needed...
Anyone having problems with the front BMR loop interfering with a Hurst shifter? Mine bangs against the loop from time to time and we're going to try to shim the mount area about 1/4" and see if that provides the clearance needed...
Actually, the Hurst helped. The stock shifter really mashed into it with 1st, 3rd, and 5th. With the Hurst it only happens in first. But yeah, TacoBill's solution was a spacer and it worked just fine (I believe it was 1/4"). He has the MGW and had the same issue. I'm going to make a spacer, I just haven't had time, and with the Hurst, it really hasn't been as much of an issue. I don't know if this issue is exclusive to BMR or not. From what I can tell, their loop is a bit larger around than most (which to me, is better so if there's any flex, it has plenty of clearance so it doesn't hit the spinning driveshaft)
The NHRA rules require a driveshaft loop within 6" of the front U-joint when your car is capable of 13.99 quarter mile times with slicks or 12.99 with street tires.
One of our local tracks requires a loop at 12.99 or quicker with slicks or drag radials, while our other local track is much more conservative and requires a loop at 13.99 or quicker for d/r's or slicks.
As already mentioned, the critical (and most common) failure is a front u-joint, hence the NHRA mandating that it be within 6" of the front u-joint. I my 1968 Pontiac GTO, due to a custom exhaust I had, I mounted a loop about 8-9" back and they always gave me grief at tech inspections, but would eventually give me warning and let me run.
A buddy of mine had a front u-joint failure in his '71 Mustang Vert (351 Cleveland, 4-speed, and NO loop) while driving down the road at about 40mph. It dented the floor pan badly, catapulted his car about a foot into the air (luckily he wasn't going around a corner!), and after the shaft bent back it snapped the rear u-joint, bounced out from under the car and smashed through the windshield of the car that was following him! Talk about a mess! It made me a believer in driveshaft loops instantly!