Rear axle tension pins are sheared immediately

[dstewart@peak.org]

Anyone advise the purpose of those rear axle tension pins? I've replaced the inside pins (at transmission output shaft) a couple times and they alway shear immediately upon being driven. The axle is floating on the splines so I see no purpose in having the pins in place at all. Just wondering if anyone with engineering skills has looked this over and can advise.

[4129R]

Are you talking about the C shaped roll pins, about 1/8" in diameter you have to carefully knock out with a long drift to get the drive shafts disconnected from the gearbox output shafts each side?

I have never had a problem. Are you using the right pins?

[EuropaTC]

Do you mean the pins circled in red on this drawing ?

If so, then the idea is that the yoke is shimmed hard up against the transmission and this is critical to the life of the gearbox/differential, there should be no play.

The yoke is a loose fit on the output splines and if the pin wasn't there would slide around, which is what some folks do when they adopt a twin link rear suspension.  But in the Lotus design it should all be locked up solid, normally when I fit the pins in some force is needed to drive them in with the shim packing in place and getting them out is even harder.

If yours are shearing my first guess would be that you're missing some shims  (also shown in the drawing) which is allowing the drive shaft to move in and out as the suspension rises & falls and that's putting some shock loading on the pins.  With the shims in place all the loads will go into the transmission housing/bearing, which will in turn support the pin ?  (that could be a load of rubbish, my excuse is that it's late here and I'm tired....    ) 

Brian

[mikec]

If you have a standard set up, then the shims are very  important to avoid this happening.  If you have for example the Banks twin link set up then pins are not required.

[Arizona]

Yup, sounds like you're missing some shims. Go to the Europa Knowledge Base http://www.lotus-europa.com/ekb.html, search for "shims" and you'll find more info than you'll want to know.

[Grumblebuns]

As everyone else has said, your axles not shimmed correctly. I just finished shimming my TCS and it's a somewhat long and drawn out process but not difficult to do. This is how I checked my shim requirement.

With the axle pins removed and the car sitting normally on the ground, see how the two holes are aligned in relation to each other. I suspect your holes will be greatly misaligned. The end result of all this shimming is to get the holes aligned so that when you drive in the roll pin, you put some tension with the yoke against the transmission on both side. It took me several tries per side until I got the shim clearance close.

Read up on the several hundred posts on the knowledgebase under "shims", "roll pins" and anything else you can think of. The posting by Tim Engel are especially helpful.

How tight are you splines on the yoke and shaft?

Joji Tokumoto
Fallbrook, Ca 

[andyelan]

Hi there

While I agree with all the above comments, I would also suggest that you check the drive shaft splines for wear. If this is excessive then you'll find that the drive is being taken on the roll pins rather than the splines and this will also likely result in a sheared pin.

Regards
Andy

[EuropaTC]

Hi there

While I agree with all the above comments, I would also suggest that you check the drive shaft splines for wear. If this is excessive then you'll find that the drive is being taken on the roll pins rather than the splines and this will also likely result in a sheared pin.

Regards
Andy

A good point there, I'd never even considered the splines would be so worn that the drive is entirely through the pins. On reflection, if they are snapping instantly, that's the most likely culprit ?

Brian

[dstewart@peak.org]

Thank you everyone! It was a combination of both things mentioned here. The shims were missing (that would do it all by itself) but also the yokes were both very loose on the transmission output shaft. Ordered new yokes and the fit is now snug. Will be milling the old yokes to make custom shims.

[BDA]

You don't have to make custom shims. They are available from the usual suspects (r.d., DBE, etc.).

[jbcollier]

... and are cheap as chips.  Make sure the pin puts the whole assembly under some PRE-load.