Jumping in reverse

[dannyteal1990]

I wondered if anyone could shed some light on an issue I’m having with my Austin 7 Cambridge special. 

Whenever the car is in reverse it jumps up and down/judders when setting off. Once it’s rolling it seems to be ok but as soon as a slight gradient happens it’s back to jumping. 

Some background:
The cars rear axle was in pieces when I got it. I’ve put this back together using the woodrows red book for reference and think this is all okay. It’s got new lip seals and bushes on the diff carrier. 
The car does work perfectly in forward gears. 1st gear does seem a bit “sharp” if that’s the right word. 

It also pushes forwards and backwards perfectly with no resistance or jump so it’s only when it’s driven. 

The car has hydraulic Morris 1000 brakes. The hoses, lines, cylinders, master have all been replaced with new. The  shoes have been given a chamfered edge to try and stop. 
We’ve also removed the hand brake rod to see if they brakes were being held on with no change to the jumping. 

It also doesn’t jump if the rear wheels are jacked up and run in reverse so only when there is pressure/weight on the wheels.

It’s not a massive issue but it would be nice to be able to reverse rather than get out and push 

At a bit of a loss as to what to try next so suggestions welcome!

[John Mason]

I am no expert on this but when you put your rear axle back onto your car did you adjust the fixing point of the torque tube to the body correctly the (I just forget the technical name for it) just before it bolts to the prop shaft, if this has too much play it can cause similar juddering like you describe.

John Mason.

["Slack Alice" Simon]

If you look back over the Forum, this is not uncommon.

If you are tentative on the clutch, it make it worse.

If you just let the clutch in, the juddering usually stops, but then you are going too quickly for comfort.

I guess that if everything that should be tight, is tight, the juddering goes away.



Worst case, though: you haven't put the back axle back together properly and a reversal of direction means something horrible is happening within.

Unlikely, though.

[Malcolm Parker]

I think your problem is common to many Austin 7 specials and does not imply that there is anything wrong with the rear axle.  Check that the torque tube mounting has no undue play and the rear shock absorbers might benefit from adjustment.  If you are at a gathering of Austin 7's and it involves reversing up a grassy slope you will see all sorts of  jumping about by the rear axles.

[andrew34ruby]

My ruby is the same. Setting off forwards is better in second than first gear. Setting off backwards we don't have that option.

[Chris KC]

It's called rear axle tramp, and pretty normal for A7's I think.

[dannyteal1990]

Thank you for all the reassurance that this is a common issue. I will recheck all bolts for tightness but pretty sure they are all done up. 

I’ll just live with the quirks that is Austin 7 driving.

[Bob Culver]

It seems to arise largely from the inherent variation of wheelbase somehow transmitting to the clutch.
The torque tube ball is invariably worn and unless somewhat slack in the normal postions is likely to jam at the extremes, with risk of something breaking.

[Steve kay]

Nobody has yet mentioned the basic step of checking the discs in the rear shocks, and when reassembling having cleaned or replaced them, put slightly shortened valve springs inside the original springs for additional friction.

I should add that valve springs were mentioned as one assumes that every workshop sheltering a Seven will have old valve springs in a mayonnaise jar on the shelf, but any spring of suitable size will do.

The further edit is to thank Bob for this evening's oooh matron contribution!