I would bet on the housing flexing. Look at any differential and even with cast iron they have reinforcing ribs.
After the $2008 Challenge, we knew something wasn’t right with our Berzerkeley’s driveline. During our first run in the autocross competition, the car lost the ability to drive. We had spent quite a bit of time on testing for the car, so suffering a component failure was especially frustrating. However, now that the milk had been spilled, we could only hope to learn from the situation by analyzing what went wrong.
We could immediately see that our primary drive chain from the bike gearbox to the custom differential had stretched, but the real trouble seemed to be inside the aluminum differential housing. We could hear some pretty terrible noises from that area when rolling the car on the ground, so we were sure that something inside had likely come apart. Once we had returned to GRM headquarters, we were able to remove the differential carrier and inspect it.
Once we’d removed the carrier from the chassis, we removed the screws holding the top access plate and looked inside. Our fears were confirmed as we saw that not one, not two, but three of the gears in our special gearbox had failed. This sucked on an epic level, since it wrecked the gears completely and made it hard to determine what broke first. After looking at the damage closely, we are facing a few hypotheses:
a) The aluminum carrier flexed enough to spoil the precise gear mesh, destroying the teeth.
b) The gearset at the upper end of the carrier—the one that switches between reverse and forward—wasn’t lubricated sufficiently.
c) The gearbox had somehow gotten halfway between forward and reverse, which created too much strain on one side of the gears and sheared them.
Regardless of the root cause, the failure made us rethink our need for a reverse gear. The additional complexity and expense simply aren’t worth the trouble at this stage of the project. We could add these features down the road if necessary. By simplifying the differential assembly, we should lose a little weight and a lot of complexity. Plus, we might even recover some of our budget. We’ve also considered leaving out the Torsen differential and replacing it with a solid spool for some of the same reasons.
The simplest solution would be to weld a sprocket on a solid intermediate shaft; this shaft would run to the two drive axles. Unfortunately, the reversed mounting orientation of our GSX-R 750 engine prevents that simple solution. We might be forced to rebuild or recreate the existing differential carrier after determining the true root cause of this failure. We’re also going to keep our eyes open for a small, strong and inexpensive gearset stout enough to handle the load and reverse rotation of the drive force.
I would bet on the housing flexing. Look at any differential and even with cast iron they have reinforcing ribs.
A simple answer could be an ATV differential. I considered it for the Toyzilla but i'm going to see how long the flipped ring and pinion last first. Some of the Honda ones have a torque sensing LSD front diff. Not sure how it would work out as far as needing a front or rear depending on engine rotation... Good news is that they are CHEAP!
Talk to Scott at Taylor Racing. They make several different chain drive diffs for DSR's and Formula SAE cars. I wouldn't be surprised if you had some flex in there. Also, that short chain run with the two small sprockets was gonna generate some heat, chains need some free length to cool off.
If you are really hung up on a reverse gear, another system to check is the one found in MC engined Gloria race cars. I think they have a reverse incorporated somehow.
The Gloria MC cars use an electric reverse. It's a separate gear on the rear that is driven by a starter motor.
Since you need to change directions anyway, sounds like the perfect time to fab up something with a pair of quick-change gears and stub shafts with sprockets welded on them. Since the covers hold support bearings already, you could cut the intermediate shafts (or use two-speed ones) and maybe even get by with a couple of covers bolted together with a simple plate in the middle. Sprockets on one side (with the chains entering and exiting through a couple holes on each end) and gears on the other. Would also make for some killer options when it comes to gearing.
Wonder if one of those starter/generator thingies they used to use on old riding lawn mowers could be made to charge a battery one way and run the other way as a motor? Then you could just belt drive it off the driveshaft and use it to back-up the tiny bike alternator as well as give you reverse?
We had a similar diff on our SAE mini baja car. Case flex was an issue and produced similar results.
What about dumping the idea of gears altogether and use a CVT like the A Mod guys do?
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