Any engineers around need your opinion?

Shocks, Springs, Brakes, Frame, Body Work, etc

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davegt27
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Any engineers around need your opinion?

Post by davegt27 » Thu Jul 17, 2014 5:42 pm

I was wondering how does this test look


http://www.matrixgarage.com/content/tie-rod-test



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Re: Any engineers around need your opinion?

Post by Olefud » Fri Jul 18, 2014 5:43 pm

Might depend if we’re talking race or street. But keep in mind that a weak link isn’t all that bad if it’s strong enough for normal use. If you get into the wall and something’s going to give, I’d rather have it be the easily replaceable tie rod than an A-arm or ball joint. Of course the A-arm would have to be hardy enough for its normal duty but that’s a relatively light load.

Generally, I’ve seen bent tie rods but few with sheared threads

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Re: Any engineers around need your opinion?

Post by Greenlight » Sat Jul 19, 2014 1:01 am

I'm a mechanical engineer and will be happy to give you some help.

First, what exactly are you trying to accomplish? What is the application of your improved tie rod assembly (street, road racing, off road racing, .......)?

Do you know what the maximum loads are for the application? If not, are you willing to collect the necessary data (simple strain gage test) to determine the typical loads?

Here are a couple of SAE papers on tie rod ends that may also help you.

http://papers.sae.org/2014-01-9122/
http://papers.sae.org/2008-01-0177/
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Re: Any engineers around need your opinion?

Post by davegt27 » Sun Aug 03, 2014 6:57 am

I was talking road race

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Re: Any engineers around need your opinion?

Post by Greenlight » Tue Aug 05, 2014 1:15 am

Here are a couple of comments:

1) Your testing in the press has a generally good setup and the results are meaningful. The pressure measurement should be converted to a force measurement.
For example a pressure measurement of 80 kg/cm2 should be converted to either Newtons (N) or pounds (lbs). To convert it to Newtons (or kN), you need to measure the diameter of the hydraulic cylinder in the press. Convert the diameter to area by using the formula - Area = Dia. x Dia. x 0.7854. Multiply the area (cm2) by the pressure and you will have kilograms. Multiply kg by 9.81 and get Newtons.

2) For the compressive length measurement, a more meaningful measurement would be to measure the difference in length of the assembly as the load is applied. This is basically what you are currently doing except I think finding the load that caused permanent yielding is very important. To do this, you can apply and release the load in 5 kg/cm2 increments. Record the length change vs. applied load at each increment. At some point the tie rod assembly will not return to the original overall length. This is the actual point of failure, because the "toe" measurement will be changed (toe out). This will contribute to premature tire wear and possibly handling issues. Continue to perform the incremental apply and release of the load (while recording the length change) until either the assembly breaks or there is such a significant change in length that would render the vehicle unstable to drive (i.e. a 50 mm change in toe out).

3) Plot the overall length (center of ball to center of pivot) change vs. load at each of the incremental load points. Note on the graph at what load the overall length did not return to the original length and by how much the length changed.

When comparing your revised design to other designs, make sure and use the same center to center length and same overall end design (i.e. ball & socket on one end and pivot joint on the other).

The hex with threaded end style tie rod design that you have shown in your testing is a very poor design.

P.M. me if you want to discuss in more detail.
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Re: Any engineers around need your opinion?

Post by MadBill » Fri Aug 15, 2014 10:17 pm

Olefud's point is a good one. If there's a hard hit, well in excess of 'normal' operating forces, a tie rod that acts like a fusible link (or a vaulter's pole) may save far more pricey pieces.

Also, for said normal operation stiffness is far more critical than ultimate yield strength, so the left side of your graph is the important part.
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Re: Any engineers around need your opinion?

Post by rancherort » Mon Aug 18, 2014 1:11 pm

As you can see the engineers of the factory oem tie rod did the research for you. They need to make the factory rod twice as strong as needed. Just to simply avoid some lawsuit over a failed tie rod. The oem rod is a specific alloy also so the strength of the hollow tube design shows.

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Re: Any engineers around need your opinion?

Post by davegt27 » Wed Aug 27, 2014 9:48 pm

thanks everyone

this is what I have on my car

Image



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Re: Any engineers around need your opinion?

Post by MadBill » Wed Aug 27, 2014 10:54 pm

Musta' had a ton of bump steer before you spaced the joint down! #-o
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Re: Any engineers around need your opinion?

Post by clshore » Thu Aug 28, 2014 11:53 am

To stiffen a tube in compression, increase the diameter in the middle, taper it down at the ends.
Ideally the taper of the diameter will follow an elliptical shape, but practically you use the largest diameter that will clear, and then taper at the ends when within 10-15% of the overall length.

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