Another "Learn me" Thread! CV joints this time!

Over in my Midlana build thread (http://grassrootsmotorsports.com/forum/build-projects-and-project-cars/midlana-first-book-build/74633/page1/) I am having a small issue with the rear suspension subframe hitting my differential. This conflict could be solved by moving the engine forward 1 inch or so. This will have the effect of making the axle ports on the transmission 1-inch in front of the axle ports on the uprights...should be no problem since the axles use FWD CV joints and can take quite a bit of flex...right?

How much angle can your typical CV joint take while still effectively transmitting power, and what if any effects on service life will be seen from running them with a permanent "bend" in them? I will still need to allow for 2-inches in either direction for suspension travel too.

Impart your wisdom!

They can take a lot of angle. An extra inch shouldn't bother them. I'd say just go for it and don't worry about it.

Agreed!

Yep, they'll be fine. To even the wear out, you could swap them R/L from time to time, but that's within tolerances for the stock SRT4 drivetrain as it moves around on the crap stock motor mounts.

What's an inch amongst friends?

turboswede wrote: Yep, they'll be fine. To even the wear out, you could swap them R/L from time to time, but that's within tolerances for the stock SRT4 drivetrain as it moves around on the crap stock motor mounts.

Every time that I rebuild cv joints I do this.

I thought about this some more over the weekend. Obviously I want the engine as low as possible to help my CG. I currently have the drivetrain about 3/4" or so off the table (bottom of the car). I do need to raise it to fit some bracing under the engine, so say it will end up 1.25" off the table. This puts the center of my axles at about 6.5" off of the table. With a 25" diameter rear tire and 4" of ground clearance the axle centers should be 8.5" off the table to make them level at ride height (25/2-4). With 2.5" of compression the axles will be 4.5-inches lower at the transmission than at the wheel at full bump. This seems like quite a bit to me, especially with the 1-inch of forward offset that I need to make the engine clear the rear sub frame. Thoughts?

Thank you

Can you rotate the drivetrain forward to correct the angle a bit?

That's not a huge angle, look at it this way, if this were a car with front driven wheels the CVs would be spending a lot of time at huge angles from steering. The normal angle is only 2" of offset from one end of the axle to the other, probably in the same ballpark as most FWD cars. I think any 4WD IFS truck runs their axles with way more tilt than that at normal ride height.

I don't know this, but the answer is math.

Find a real source, mine was found after a few seconds.

http://www.autoserviceprofessional.com/article/91844/CV-joint-tech-A-primer-on-constant-velocity-drive-joints-and-diagnostic-tips

"47 to 50 degrees"

Then do the math.

Thanks, math is always the answer. Since the wheels won't steer in this application the inner joints will be the limiting factor. I called a local axle shop and they said 20 degrees is a good rule of thumb for inner joints. To the calculator!

When I used to race our vw buggy, we had a good 12" of travel. I don't know how the Porsche 930 cv's compare to what you are using but I would think a inch would not hurt a bit. Maybe rig it up and you could check it for bind and measure your angles at the spot it binds

Axle length plays into the available travel quite a bit. I am dealing with a FWD drivetrain with stubby 11is inch long axles. You probably had much longer ones.

I did the math using an assumed 11.5" axle and get 4.2" of travel at the wheel for 20 degrees of motion. If 2.5 inches of the allowable 4.2 inches of travel is taken up by bump I have 1.7 inches left over to have the engine sit lower than the center of the wheel....All in all it looks like I should raise and/or rotate the engine forward a bit so that my axle port is no less than 6.8 inches off of the table. This calculation neglects the 1" forward offset so 7" plus would be better.

If you ever take a cv apart, you can see that they are more built for large angles than u joints are. It's big ball bearings in grooves and they don't really care where in the groove they are.

Looking at the pics in the build thread, assuming a solid mounted powertrain you should be able to alter the offending part to clear the diff.