Front Upper A-arm Design Change

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HydroJim
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Front Upper A-arm Design Change

Post by HydroJim » 06 Nov 2018, 14:44

Originally, it looks like the front upper A-arm used a Mazda tie-rod end as the upper ball joint. Then, the design ended up switching to a bearing cup using a pressed in spherical bearing.

What was the rational for this change? What was wrong with using the tie-rod end?

The negatives I see to the spherical bearing are the use of a straight bolt in the upper spindle attachment point instead of the tapered mating surface of the tie-rod end. How is the tolerance handled when using a straight bolt?

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Re: Front Upper A-arm Design Change

Post by Midlana1 » 06 Nov 2018, 17:25

What's the book say?...

A tie-rod is not designed to be used in bending, and it was being used in bending. Was it a hazard? Probably not, due to its sheer size, but I chose to go with a spherical bearing and not have to worry about it.

Regarding the bolt tolerance, again, the book discusses this; since the hole is reamed, the tolerance is ~0.001".

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rennkafer
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Re: Front Upper A-arm Design Change

Post by rennkafer » 07 Nov 2018, 07:52

Midlana1 wrote:What's the book say?...

A tie-rod is not designed to be used in bending, and it was being used in bending. Was it a hazard? Probably not, due to its sheer size, but I chose to go with a spherical bearing and not have to worry about it.

Regarding the bolt tolerance, again, the book discusses this; since the hole is reamed, the tolerance is ~0.001".
If you use proper AN bolts (and you should be on something this critical), your fit is quite good. A spherical bearing with a bolt through to the upright is pretty much standard procedure on race cars.
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Re: Front Upper A-arm Design Change

Post by mainlandboy » 05 Feb 2019, 21:05

Midlana1 wrote:What's the book say?...

A tie-rod is not designed to be used in bending, and it was being used in bending. Was it a hazard? Probably not, due to its sheer size, but I chose to go with a spherical bearing and not have to worry about it.

Regarding the bolt tolerance, again, the book discusses this; since the hole is reamed, the tolerance is ~0.001".
Because the front springs that support the vehicle weight are connected to the lower control arms, I can see that the lower ball joint that connects the lower control arm to the spindle will have a bending load applied to it, but I don't see how a tie-rod used as an upper ball joint would have a bending load applied to it, since the upper control arm is free to rotate without a spring trying to counter the rotation when the spindle pushes up on it. The upper control arm is just controlling the camber, not supporting the weight of the vehicle, so my understanding is that the upper control arm (and a tie-rod used as an upper ball joint) would only be loaded in compression and tension and not bending. Am I missing something?

EDIT: I was only considering the up/down bending plane, but realize now that there would be some forward bending load during braking.

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Re: Front Upper A-arm Design Change

Post by Midlana1 » 06 Feb 2019, 06:02

Correct, it tries bending forward under braking.

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