Weakest Link

[flyboytroy99]

Everything I've read states that rear differential failure is the limiting factor in terms of horsepower increases. Just wondering if any one knows from experience what part of the rear diff is most prone to fail (what's the weakest link)? And how modified were you running when you trashed it? Thanks.

[RallyBob]

Lots of weak links. I have shattered the spider gears with a stock Opel with an automatic tranmission before. They are far too brittle, that is the problem. This is the weakest link by far, I've probably broken the spider gears on 12-15 Opel axles. I have 'stripped' the spider gears on rear axles from other cars, they usually just clean them right off. But Opels are particularly hard, and they will shatter into multiple pieces...if you are rolling along quickly while this happens the debris gets into the bearings and between the ring and pinion and the entire center section becomes junk in a matter of seconds.

The next step is to replace the differential with a limited slip, eliminating that weak link. But of course now you've doubled the strain on the torque tube and the pinion input spline, since you've doubled the drive traction. However I've also broken the input splines on stock-engined Opels before, without a limited slip. Higher HP cars will twist or break the torque tube shaft itself. But you can get a chromoly one from Finland or Sweden, or you can sleeve the stock one with some CDS or DOM steel tubing....I've always gone this route.

And I have seen a few axle failures at the outer flange, but that was in road racing or circle track use with a welded center diff. The stress from the non-differentiation breaks the fusion-weld at the flange (they are not one-piece axles), separating it from the axle itself. Car pulls hard to the side of the broken axle...crashes. Not good.

Those are the primary failures I've seen. But occasionally a high-hp car will break all the teeth off the pinion gear (about 210-220 hp). Depends on the application, the traction, the component wear, etc.

Generally, if you drive the car hard, even 100 hp can break an axle. But I've had a stock rear axle live with 190+ hp (no shock-loading). Big sticky tires make it worse, drag racing starts make it a lot worse. High mileage parts compound the problem.

IMO, unless you are absolutely adament about keeping it 'all Opel', think about a rear axle upgrade if you race, or drive hard, or simply have a lot of power. Aftermarket parts for a lot of other rear axles are soooo cheap compared to Opel stuff.
I am building a complete 8" Toyota rear axle for my turbo road racing Manta, and the cost of the used axle, the spool, the new ring and pinion, new center bearings, new seals, new axle bearings, AND the custom machining (convert to 4 x 100 bolt pattern) plus the custom rear disc brakes cost less than an Opel limited slip differential alone. And I know it will handle 500+ hp.

[madhatterpdc]

I would like to echo the recommendation of using a toyota rear end. There are a number of bits that need to be fabricated, but there isn't a lot of re-engineering required. The aftermarket support is almost as good as small block chevy's. Plus it's a good opportunity to start with a clean slate if you are looking to upgrade the rear suspension.

We need to preface Bob's reference to breaking driveline pieces with a stock Opel motor. He (and I) are extremely efficient at finding the weakest link in any vehicle.

YouTube - Ferrari engine Blown up!!

That being said, I have found that a critical *mechanical* link to be the stock clutch. Look into what it would take to upgrade to an S-10 clutch.

The *true* weakest link in every car I have ever driven has always been somewhere between the seat and the steering wheel. Haven't quite pinned the exact location yet, but I am working on it.

[namba209]

Originally Posted by madhatterpdc The *true* weakest link in every car I have ever driven has always been somewhere between the seat and the steering wheel. Haven't quite pinned the exact location yet, but I am working on it.

I think I've found or seen it once or twice during my years of being involved with motorsports. It's usually the "nut behind the steering wheel, or handlebarsl".