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PostPosted: January 27, 2012, 11:57 am 
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When setting up your cv axle angle...... is there an ideal angle for a CV axle? Meaning....would 0 degree angle be the angle at which the CV axle would be "technically" the strongest? Or would 15 degrees be the ideal angle where it would most likely be the strongest and not fail? Or is the strength of a CV axle basically the same at any angle through its spec'd travel?

Hope my question make sense?

JR

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PostPosted: January 27, 2012, 12:22 pm 
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Zero degrees is strongest, and strength will degrade by some geometric function the more it's angled... probably something like the cosine of the angle squared. Since the CVs only care about torque, it works in your favor since your car weighs so little, though if you have hot sticky slicks on it, that kind of goes out the window. Depends upon tires, engine torque, car weight, and of course CV angle. Sorry for the long non-answer.

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PostPosted: January 27, 2012, 12:29 pm 
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Kinda depends on the set up. .. a slight downwards angle at rest is nice when you have a car that squats when you apply power. Again another non-answer. :mrgreen:

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PostPosted: January 27, 2012, 12:44 pm 
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Unless there is somehow a dramatic difference in strength through its range of motion, even if there is some degree diminished strength (increased loading) on certain parts, I would think that any worries about strength at max deflection would still translate to a similar concern about strength even at 0 deflection.

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PostPosted: January 27, 2012, 1:04 pm 
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Use quality CV's from a German car, such as the 930 CV from the Porsche 911's and most BMW's. Those have been used in off roading since the invention of sand and have faired well. They are relatively inexpensive in stock form and can be ordered with race cages and or polished races.


If you must spend more and/or want bragging rights, there is the 934 CV or if you really want to spend money, there is the offroad machinery CV.

Personally, I think German Lofbro CV's from the old VW buses would be more than enough for a 1800 pound car with less than 200 hp.


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PostPosted: January 27, 2012, 1:05 pm 
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I'm just thinking outloud (OK, in print) here. Ideal transfer of forces would be at 0 degress. In this manner, the input axle axis is in line with the output hub axis and will not try to increase the angle as torque is applied. However that setup may provide a limited amount of increased wear at the balls and thier associated grooves. Since they don't move relative to the inside and outside as they rotate, there is a chance for more wear, adding some backlash. Similar to why you want some angle for u-joints I can imagine. However, the reason for the CV joint is to provide some freedom of motion. As the rear suspension travels, that angle continuously changes. So there is always some motion in the ball cage area. In reality, I don't think it makes much difference until you get to 1,000HP or so.

In my case, I have a few degrees of angle intentionally dialed in because my diff is a bit further rearward than my hubs. This was done due to some asthetics reasons and to increase room in the passenger compartment.

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PostPosted: January 27, 2012, 1:15 pm 
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It's only tangentially related but. ..
Here's a white paper with a graph at the bottom showing that the torque capacity of a (?) CV joint decreases fairly linearly according to angle. (Boy that's a tortured sentence)
http://www.ntn.co.jp/english/products/review/pdf/NTN_TR75_en_P016.pdf

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PostPosted: January 27, 2012, 1:17 pm 
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SInce you're running a dragster, jack up the rear end!

The joints would work harder with more inclination, but if you're not road racing they should last quite awhile...

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PostPosted: January 27, 2012, 2:06 pm 
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Oldejack, The problem with that curve is is it in a very limited range of angles and the vertical axis (torque) has no scale.

I was looking for more info and found this: http://www.gknservice.com/fileadmin/use ... hicles.pdf

Calculations and graphs start on page 30.

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PostPosted: January 27, 2012, 2:23 pm 
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Forgot a couple things, which were pointed out...

Agree that running truely zero degrees is a bad thing over time because the ball bearings will wear grooves in the housing. That's one reason to run a slight static offset.

The other issue noted is that zero degrees is most important when running full torque through the CVs. When that's no happening however, the angle is much less important. As noted, you can run them with an angle such as would happen with minimal torque - like running on the freeway for long periods - and have it move toward near-zero when you get on the gas. So running a static offset with the outboard hubs slighly lower than the inners should take care of both.

And finally, as was said, if you're using big-ass CVs none of this matters because they're such high ratings that they'll never wear out.

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PostPosted: January 27, 2012, 2:25 pm 
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horizenjob wrote:
SInce you're running a dragster, jack up the rear end!

The joints would work harder with more inclination, but if you're not road racing they should last quite awhile...


Surprisingly I am over 400 whp and my 1999 stock miata axles are doing great. I launch the car on drag slicks with 2-step engaged at 5200 rpm building 8 psi on the line.

I just can't believe they are holding up to that abuse. It has to be because the car is so light right? This is just insane

JR

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PostPosted: January 28, 2012, 3:10 am 
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You have traction control so the torque put out by your engine matches the torque available from your tires. Well it alwasy matches anyway, but the car just sits there and smokes the rubber.

Your doing it at a drag strip, so you have nice pavement. If you get some shock loads from bad pavement or potholes or whatever all bets are off. I think that's what happens a lot on the street.

I'm glad your having fun and everything is holding together. Miata diff too? People keep thinking they need really big Ford 8.8" or 9" rear ends and I mostly think it's not necessary. The torque at the tire and the engine have to balance out. If you have half the weight on the tires, you'll likely have about half the traction.

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PostPosted: January 29, 2012, 6:58 am 
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horizenjob wrote:

Your doing it at a drag strip, so you have nice pavement. If you get some shock loads from bad pavement or potholes or whatever all bets are off. I think that's what happens a lot on the street.


Exactly, not only designed to go through about 7" of wheel travel but to do so on full lock (FWD cars) while absorbing shocks from potholes and hitting kerbs.

Accelerating on smooth pavement in a straight line is pretty tame in comparison.


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PostPosted: January 29, 2012, 3:24 pm 
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This has pretty much been touched on but from a strength perspective you want the angle
To be a little high such that when your rear end squats down under full throttle the CV is straight.

Here's a crazy fact for you. . . A friend of mine with Some racecar building experience claims F1 teams paint a line on the axle before the race and it's not uncommon for the line to be twisted around the axle by 180 degrees or more at the end of the race. Everything else he says is true, he's not a tall tale kind of guy so I have no reason to doubt him.

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PostPosted: January 29, 2012, 5:11 pm 
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rust_bucket wrote:
This has pretty much been touched on but from a strength perspective you want the angle
To be a little high such that when your rear end squats down under full throttle the CV is straight.

Here's a crazy fact for you. . . A friend of mine with Some racecar building experience claims F1 teams paint a line on the axle before the race and it's not uncommon for the line to be twisted around the axle by 180 degrees or more at the end of the race. Everything else he says is true, he's not a tall tale kind of guy so I have no reason to doubt him.

I wonder how many degrees the tire rotates on the wheel, or do they screw the tire to the wheel like some of the dirt track cars do? Just wondering.


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