Car rocks forward and back when using the transmission brake

[dhosh]

I am having what I believe is excessive forward and backward rocking, when I use the transmission brake. The harder the stop, the worse the rocking. Initially I thought this might be transmission issues, as I had "just' rebuilt the rear end and drive tube. After thinking it through, and manually rocking the car back and forth with the transmission inspection cover off... It is evident there is significant 'slop' in my rear end!

Here is a You Tube video showing manually induced rocking:

https://youtu.be/XrviqI_Cttg

You can't them it well, but there are marks on the tire and floor, to the extent of wheel movement, before there is movement noticed in the transmission (inspection plate off). The on-the-ground measurement from one end of the rock to the other, is 3".

About 3 years ago, I tore down the rear end of my '24 RPU, in order to inspect and replace the thrust washers, if needed. I expected there was an issue, given multiple shims necessary at the axle tapers. Good thing, as one was totally gone, and the other just about so. I should be able to find photos of the pinion and ring gear, etc. During the teardown, the following was done:

Dissmantled and cleaned everything.
Installed new Bronx thrust washers
Seems like I remember paper shims being replaced somewhere, as well.
Pinion gear bushing was cracked... Couldn't get it off, even with heat, so replaced 'drive shaft' with another that appeared in good shape
New bushings at both ends of shaft
Replaced pinion gear with what appeared to be a good used one
Replaced U-joint with good used or NOS
New gaskets
'tightened up' roller bearings at end of axles
Added modern grease seals at the of axles

Comparison between newly installed used pinion gear, and the old one in my hand:

Peek inside trans ... (Yes... Will be replacing bands before all is said and done) ... Does the clutch spring look normal here? I have no reason to think it's not working right.... Anything else look amiss?:

Thank you in advance...

[Lessumner]

Maybe the drive shaft nut needs to be tightened again? Les

[dhosh]

Hmmm.... The one holding the pinion gear on?

[walber]

In addition to checking for slop in the rear axle and the tightness of the nut on the pinion gear, check the u-joint for wear as well as the fit of the u-joint on the driveshaft and the fit where it enters the driven plate/output shaft of the transmission.

[dhosh]

Ok... Will do... Sounds reasonable!
I take it from other posts.... There should be bi
virtually no slop .... Rotating the tires should show immediate movement in the transmission?

[walber]

A little slack is not very worrisome in my view. Consider that a lit bit of slack in the gear fit, a little wear in the u-joint and where it fits the shaft or transmission, and a little wear in the bearings could add up even if all of the wear/looseness was in an acceptable range for the individual parts. A little here, a little there and there and pretty soon you can feel some slack at the rear wheel.

[Jerry VanOoteghem]

Just looked at your video. That looks like every T I've ever driven. Really, that's not much play.

Consider this, even if everything is in excellent condition & adjustment, you have normal play between the differential gears, plus the play between the ring & pinion, plus the play in the u-joint, (there's always a little), plus the play between the u-joint and the transmission shaft. It all adds up with no one thing being in bad shape.

No harm in checking things out however. Are your wheel hubs tight on the axle shafts? Is there any play in the rivets attaching your transmission output shaft to the drive plate? Are the bolts attaching your drive plate to the brake drum tight?

Also wondering what your braking technique is. Do you hold the brake pedal down tight, without letting up any just at the moment of stopping? If so, you can make any T rock back & forth after stopping. Try letting up on the brake at the very moment the car comes to a stop, or at least taper off the brake some just at, or slightly before, the moment the car stops. Even with no play anywhere, the drive shaft acts like a big torsion spring. While you brake, you're winding up that torsion spring. As the braking force ceases, (at the moment the car stops), the torsion spring will then unwind, driving the car backward slightly, until the torsion spring winds a bit in the other direction, then thrust the car forward a bit as it unwinds again. Letting off the brake slightly allows that first "wind-up" to be released without annoyingly pushing the car backwards and beginning the to & fro that you're experiencing.

[Jerry VanOoteghem]

Rotating the tires should show immediate movement in the transmission?

No. The additive effect of all the proper & correct clearances would dictate that there should be play between the wheels & the transmission. All you need to do, is to determine whether there's anything beyond normal clearances at work here.

[dhosh]

Hi, Jerry! How are things with the Casual T's??

As to your questions...

Are your wheel hubs tight on the axle shafts?
Yes... But will check..

Is there any play in the rivets attaching your transmission output shaft to the drive plate?

I did not check... but will do...

Are the bolts attaching your drive plate to the brake drum tight?

Ditto, here, although I couldn't get any of the bolts to move with casual attempt ... I.e., nothing 'rattly' loose.

When rocking the car back and forth by hand.... there is no movement on the drive shaft at all for those 3 inches.

It would be grand if it was normal! But... At the tail end of the season, it seemed to get quite noticeable. I have to say, though... I was doing a lot of driving last year, and might have gotten too 'comfortable' (ummmm i.e. lazy) driving, and got sloppy in my technique. I know in normal, planned stops, I do feather off the break at the end... Just as I'm coming to a complete stop. But, when I get too comfortable driving, I do know I tend to speed up, and, then, not properly anticipating light changes, etc. That would certainly be my bad.

I'll check the hubs again, and check movement on the axle ends with the wheels off, and see what I find.

I would love to see what shape the thrust washers are in after 3 years ... If nothing more than curiosity. That's a lot of messing around for fun, though!

[Jerry VanOoteghem]

OH! Hi Dennis! Didn't realize it was you. I just looked at the username and it didn't ring a bell. The Casual T's are hunkered down tight till this stupid virus lets up.

Sounds as if this is an issue that has worsened and has not been an ongoing thing? That bears some additional looking into then.

[dhosh]

Thanks, Walber and others as well, that have given me things to check!

Yes, Jerry... 'tis I. I get the emails from Steve, yet... I see several things, as here... Have been canceled. . I'm still working, as an 'essential', but just post retirement delivering parts for the local NAPA store. Other than that... We're pretty tight here, too. Better that than the alternative! Also been making face masks.. up ton80 so far.

Thamks, and take care!

[Shane Lach]

My Tudor did this. I'd come to a stop on a hill and roll back a bit. It was a smoked universal joint.

[Humblej]

I had that problem, loose pinion gear, tore up the driveshaft and keyway. New driveshaft and no more rocking.

[Allan]

Read Jerry's post keenly. On a modern car the brakes stop the wheels, and the car is STOPPED! On a model T the brakes work the transmission drum.Inertia, and the fact the wheels are NOT stopped, means the wheels are still allowing the car to go down the road. This force is soaked up by the axles, driveshaft, U joint, clear back to that slowing transmission brake drum. Stop the drum and there will be rebound in those components soaking up the braking force. Hence the rocking. It does intrigue the drivers of modern vehicles when you pull up along side them at the lights.

Jerry also hints at braking technique. If you have the time and space, use the brake pedal to slow the car right down, release the pedal before fully stopping, and re-apply it to make a full stop. That will dissipate the wind-up in the system, allowing a smooth stop.

Loose U joints, pinion nuts, axle nuts will all allow slop in the drive train, but they should reduce the effects of wind-up. Much of that wind-up should be taken out by loose components as they rock back to the other side of their movement range.

Allan from down under.