Gasser Engine Mounting Angle

[afx]

Many gassers have their engines mounted with a significant tail down angle.  Was this to induce more rear weight transfer (the prevailing theory of the day) or is it to eliminate drive shaft angle?

Edited December 24, 2017 by afx

[High octane]

No, the front suspension was raised to create more weight transfer to the rear wheels. The photo you show there is not a gasser but a street roadster and could also possibly run in an altered class.

[Snake45]

6 minutes ago, High octane said:

No, the front suspension was raised to create more weight transfer to the rear wheels. The photo you show there is not a gasser but a street roadster and could also possibly run in an altered class.

The Street Roadster class was the roadster equivalent of Gasser class, and had very similar rules. Among these were that the car had to be at least theoretically streetable, meaning fenders, headlights, seating for two, no "gutted" interior, and so forth. I think the Altered class allowed more engine setback than did Gasser/Street Roadster. 

[High octane]

58 minutes ago, Snake45 said:

The Street Roadster class was the roadster equivalent of Gasser class, and had very similar rules. Among these were that the car had to be at least theoretically streetable, meaning fenders, headlights, seating for two, no "gutted" interior, and so forth. I think the Altered class allowed more engine setback than did Gasser/Street Roadster. 

You mentioned fenders, and they must be "stock" fenders.

[Ace-Garageguy]

2 hours ago, afx said:

Many gassers have their engines mounted with a significant tail down angle.  Was this to induce more rear weight transfer (the prevailing theory of the day) or is it to eliminate drive shaft angle?

Nobody has actually answered the question.

So I will.

You are correct in your assumption that the significant tail-down angle is to eliminate much of the "drive shaft angle" that would occur if the engine was mounted level.

NOTE: The engine IS raised to enhance weight transfer, but the ANGLE the engine is installed at (within reasonable limits) has very little effect on where the center of mass of the engine / gearbox assembly is located, and that is the important issue relative to HEIGHT.

As far as the drive-shaft angle goes, it has to do with the motion of the elements of the universal joints, and the torque transferred through them.

In theory, there is no horse power loss due to angle. In practice, universal joints do not do well at any but the smallest angles (which is why CV joints are used in higher angle applications).

With universal joints operating at higher angles, the angular velocity of the output shaft (the element of the driveshaft behind each universal joint in this context) is varying above and below the angular velocity of the input shaft (the element of the driveshaft forward of each universal joint in this context). What that really means is that the output velocity and torque vary in opposition, cancelling any variation in horsepower....

BUT...the varying loads on the crosses, cups, bearing rollers, etc. tend to make them explode once the angles become large enough.

NOTE 2: The nose of the differential pinion shaft also needs to be corrected for everything to work right. The angles that both universal joints are working through need to be essentially equal. This requires rotating the rear axle assembly so that the angle of a line drawn through the center of the pinion shaft (relative to the horizontal) matches the angle of a line drawn through the crankshaft and gearbox output shaft.

                                                                                   

Anyone interested in the math?   Here you go...   https://en.wikipedia.org/wiki/Universal_joint

 

 

 

 

 

Edited December 24, 2017 by Ace-Garageguy

[afx]

Thanks Bill, keeping everything inline was my instinct but I found this diagram while researching the subject indicating that it's "Wrong" - go figure!

 

[Ace-Garageguy]

5 hours ago, afx said:

Thanks Bill, keeping everything inline was my instinct but I found this diagram while researching the subject indicating that it's "Wrong" - go figure!

This is one of those instances where insufficient knowledge and experience can lead to incorrect assumptions and interpretations.

The diagram you found is CORRECT the way it labels "wrong" and "right".

If you look closely, you'll read that the centerlines of the pinion shaft and the crank centerline are parallel, which I noted is necessary in my comment above in NOTE 2.

You will also notice that they are not excessively offset, which I referred to in this part of my response: "With universal joints operating at higher angles etc."

The car and engine in your photo in the opening post are both excessively jacked up, and to approach the relatively minor offsets of the shaft elements in your last illustration, the engine needs to be angled as shown.

I assure you, if you were to draw this all out to scale, you can get the relatively minor shaft offsets necessary for correct universal joint operation with the engine at that apparent angle.

HOWEVER...not ALL car builders appreciate all the subtleties of pinion-angle and driveshaft setup, and some pretty spectacular results have been known to occur, like when the front UJ snaps and the car pole-vaults on the driveshaft.

Edited December 25, 2017 by Ace-Garageguy

[afx]

Appreciate the information Bill.  Doing more research regarding getting the u-joints to "work" I found this in a Street Rodder article:

"We should mention here that the pinion on the rearend should be set at the same angle as the trans tailshaft, so if the latter is 3 degrees downward, the pinion will point 3 degrees upward. However, as noted in last month’s article on shimmy cures, the driveshaft should not be perfectly straight from trans to rearend, as the U-joints will run dry and fail. Some angle is advisable in the U-joints, but ensure the pinion and trans tailshaft are at equal angles."

Question: In addition to getting the u-joints to work does the slight offset also reduce drive shaft vibration?

Edited December 25, 2017 by afx

[Ace-Garageguy]

1 hour ago, afx said:

Appreciate the information Bill.  Doing more research regarding getting the u-joints to "work" I found this in a Street Rodder article:

"We should mention here that the pinion on the rearend should be set at the same angle as the trans tailshaft, so if the latter is 3 degrees downward, the pinion will point 3 degrees upward. However, as noted in last month’s article on shimmy cures, the driveshaft should not be perfectly straight from trans to rearend, as the U-joints will run dry and fail. Some angle is advisable in the U-joints, but ensure the pinion and trans tailshaft are at equal angles."

Question: In addition to getting the u-joints to work does the slight offset also reduce drive shaft vibration?

Having the shafts slightly offset, as referenced above, only serves to keep the joints "working", and lubricated. Obviously, as the suspension moves around (when the car negotiates a bump, for example), the offset angles will become more severe during the up and down movement of the vehicle. If the bearings in the joints have run dry because the shaft is too straight most of the time, they may not be able to cope effectively with the increased loads caused by greater offset angles, and early joint failure may result.

Three things are usually responsible for driveshaft vibration.

1) By far the most common in hot-rods or engine / trans swaps is a failure to match the pinion angle to the angle of the crank / trans centerline. That's one reason why the articles and illustrations you found make such a big deal about it. As noted in my first response, the angular velocities of shaft elements are different on either side of each joint at various degrees of shaft rotation. A properly set-up shaft, with the angles of the pinion and crank centerlines matched closely (with the vehicle at its normal going-down-the-road ride height) will cancel out these variations and both ends of the shaft will have the same instantaneous angular velocities at any given time. Mismatched pinion / crank centerline angles will have both ends of the shaft fighting each other as their angular velocities change at slightly different rates. Depending on the design of the rear suspension and the compliance of engine and drivetrain mounts, this may be felt as a vibration at certain road-speeds or loads. It will certainly hasten UJ failure.

NOTE: Not all rear suspension designs are equal when it comes to maintaining pinion angle under all conditions. Consider a live rear axle on coil springs, controlled by trailing arms. In this design, the rear axle will move in a slight arc as it moves through its bump / rebound travel, and will rotate around its centerline. The pinion angle will change during this movement. 

2) Another leading cause of driveline vibration in non-OEM installations is having the universal joints slightly "out of phase". This happens when an inexperienced or sloppy driveshaft builder fails to align the crosses of the UJs EXACTLY, for instance, where a shaft has been shortened or lengthened, or a yoke on one end of the shaft has been changed to accommodate a different trans or rear end. This will usually manifest itself in a moderate to severe vibration at all road-speeds. Competent driveshaft shops almost always get this right. Guys who shorten or otherwise modify their own shafts rarely do.

NOTE: It IS possible to do a fine job of modifying a shaft in the garage or home shop, even without a lathe. But it takes an amount of CARE and THOUGHT that is usually lacking among amateurs, and a lot of "professionals".

3) The third cause of driveshaft vibration is simply out-of-balance. Driveshafts need to be balanced, just like any other rapidly rotating component, and they usually have small weights welded to them to accomplish this. When a shaft is modified, it needs to be re-balanced by a competent shop. An out-of-balance shaft will often be felt as a cyclic vibration at some vehicle speeds, not others.

Getting hot-rods and customs to behave like OEM vehicles can be time consuming and frustrating, particularly when the original builder(s) failed to take things like these seriously.

NOTE: On a pure drag-racing car as shown in the opening photo, having the driveshaft running almost or dead straight is really OK. The car isn't going to be run for miles and miles on end with the UJs not "working", so with an occasional shot of grease, the joints will be just fine...but the crank / trans centerline angle and the pinion-shaft centerline angle still need to match exactly.

Edited December 25, 2017 by Ace-Garageguy

[dublin boy]

There's a set up like that in the Beatnick Bandit as far as I remember JC, it might be a start, but I'm sure you're already building one !  lol