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> How flexible do you think a stock, trailing arm is
ChrisFoley
post Jan 1 2006, 09:45 AM
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QUOTE (Al Meredith @ Dec 31 2005, 10:03 PM)
Have you thought about using a lazer pointer

Thats a great idea Al.
I have a laser pointer so I will try it.
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ChrisFoley
post Jan 1 2006, 09:52 AM
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QUOTE (brant @ Dec 31 2005, 10:12 PM)
how much negative camber can you easily dial in?

I can make outer pickup brackets that change the starting camber - more positive or more negative.
QUOTE
how much does that one weigh, and how much total does your lovely new one weigh?
I will provide that data once I weigh them carefully.
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with the 2nd style of measurement, what is your measurement for an old style reinforced arm.
It was 0.5 deg. when I remeasured it.
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brant
post Jan 1 2006, 02:59 PM
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Chris,

I think this is an awesome product/service from CFR!

-50% stronger than stock and 20% stronger than the other "kit"

-I'm guessing 4lbs lighter (mostly unsprung weight)

-and available with an extra -1degree of camber (or other amounts)

sounds pretty hot to me!
brant
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ChrisFoley
post Jan 4 2006, 07:32 PM
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Here's some weight data.
Stock trailing arm: 15lb, 5oz.
Boxed trailing arm: 18lb, 2oz.
My reinforced trailing arm: 16lb, 5oz.
For reference, a pivot shaft with bushings weighs 1lb, 8oz which is included in the above weights.
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Eric_Shea
post Jan 4 2006, 08:12 PM
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When was the last time I read a 7 page thread? (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif)

Awesome!

Chris, thanks a lot (IMG:http://www.914world.com/bbs2/html/emoticons/beerchug.gif)

So, if I'm reading this right, a factory style stiffening kit is just under 3lbs. heavier (per side) and is 30% stiffer than stock?
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ChrisFoley
post Jan 4 2006, 08:23 PM
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QUOTE

So, if I'm reading this right, a factory style stiffening kit is just under 3lbs. heavier (per side) and is 30% stiffer than stock?

Yup. Actually more like 38%. (0.3/0.8)
More than half the weight of the boxed kit is unsprung.
No more than 4oz. of my design is unsprung, and is another 12% stiffer.

I got some info from a friend tonight. He modelled the trailing arm and did some FEA for me. I haven't had a chance to analyze the details but everything appears to agree with my empirical data.
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Eric_Shea
post Jan 4 2006, 08:37 PM
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QUOTE
More than half the weight of the boxed kit is unsprung.
No more than 4oz. of my design is unsprung, and is another 12% stiffer.


No, no... don't get me wrong. You're thing is the cat's ass, especially for guys in your arena that want to shed all the oz.'s they can.

I was just curious because I installed one years ago and was told that it was worthless, etc. Now I'm OK with the decision... it could be lighter and stiffer but frankly I'm just glad it helps and doesn't hurt. For me 6lbs isn't that bad. I saved that with my S-Calipers.

Thanks again. Killer thread. This needs to be book marked for a "Classic" for sure (IMG:http://www.914world.com/bbs2/html/emoticons/smilie_pokal.gif)
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TimT
post Jan 4 2006, 08:54 PM
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QUOTE
I got some info from a friend tonight. He modelled the trailing arm and did some FEA for me. I haven't had a chance to analyze the details but everything appears to agree with my empirical data.


hehehe

I have the same data... Im looking at it now... I dont see any hotspots which is a good thing when you do a wireframe FEA

One thing that I may offer is that there are mulitple forces happening at once on a trailing arm.. not just torsion .

In order to investigate this further we should agree on some nomenclature, call X the horizontal plane, Y the vertical plane. etc
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ChrisFoley
post Jan 4 2006, 09:11 PM
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QUOTE

No, no... don't get me wrong. You're thing is the cat's ass, especially for guys in your arena that want to shed all the oz.'s they can.

I didn't think you were giving me a hard time. I just thought it was a good intro for me to restate the facts. (IMG:style_emoticons/default/biggrin.gif)

QUOTE

One thing that I may offer is that there are mulitple forces happening at once on a trailing arm.. not just torsion .
I know, but I decided to ignore the lateral forces. I am guessing the lateral is only 20% of the torsional but I don't have any numbers to back that up. Can you do that calculation?
I also realize that my lever arm applies the force at 90 degrees to what would be required to measure the lateral effect but it was convenient to do it this way. Also the FEA says it doesn't matter whether the rear of the trailing arm is supported at the bottom as I did or at the shock mount, for the torsional measurements.
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Pinepig
post Jan 4 2006, 09:14 PM
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New guy here, I LOVE this forum, you guys rock.

If you want to watch it flex put a cable on the end of the pipe that you are using to load the trailing arm and hang your 220lbs on there, set them on a floor jack and just let it down while you watch.
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ChrisFoley
post Jan 4 2006, 09:14 PM
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QUOTE (TimT @ Jan 4 2006, 09:54 PM)
hehehe

Tim, guess who did the FEA for me. You know him from the PP racing forum. (IMG:http://www.914world.com/bbs2/html/emoticons/cool_shades.gif)
















john luetjen
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ChrisFoley
post Jan 4 2006, 09:20 PM
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QUOTE (Pinepig @ Jan 4 2006, 10:14 PM)
If you want to watch it flex put a cable on the end of the pipe that you are using to load the trailing arm and hang your 220lbs on there, set them on a floor jack and just let it down while you watch.

That would work great except my weight consists of 12 pieces of steel of varying shape and size (plates, sq bars, rd bars). If I was into freeweights I could do what you suggest.
What I really like doing is having a friend bounce up and down on the lever while I watch the trailing arm closely. (IMG:http://www.914world.com/bbs2/html/emoticons/wacko.gif)
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TimT
post Jan 4 2006, 09:20 PM
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Chris,

Im getting more concerned with the trailing arm mounts.... than the trailing arm itself... You obviously have by intuition, good ole common sense, and some inate engineering prowess cured a problem with the trailing arms in high load situations..

Now we bolt these arms back to the 914 tub.. I dont know the GCR but how much are you allowed to reinforce the pickups...

kudos to you and your development of these cars...

Ill play around and try and determine lateral forces
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TimT
post Jan 4 2006, 09:25 PM
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Chris I know John did the FEA since he cc'd me in an email

John sent me the FEA based on he is a salesman not an engineer.. well Im an engineer, and John continues to impress me with his insights, and grasp of problems that face us..
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ChrisFoley
post Jan 4 2006, 09:33 PM
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QUOTE (TimT @ Jan 4 2006, 10:20 PM)
some inate engineering prowess..

(IMG:http://www.914world.com/bbs2/html/emoticons/cool_shades.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/idea.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/happy11.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/mueba.gif)
QUOTE
how much are you allowed to reinforce the pickups...
Suspension pickup location and reinforcements are totally free in Production. That's why mine are already substantially altered, ie. moved and stiffened. The outer pickup bracket is a major concern so I designed a stronger unit. I still have to test it with my fixture so I can advertise just how much better than stock. (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif)
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ChrisFoley
post Jan 4 2006, 10:05 PM
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QUOTE

Chris I know John did the FEA since he cc'd me in an email

I shoulda known. (IMG:style_emoticons/default/rolleyes.gif) (IMG:style_emoticons/default/smile.gif)

One thing the FEA didn't pick up is the amount of improvement from adding the bulkhead. Is that due to the wire frame model being different from sheet material or some other shortcoming of the software?
One piece of information that I have is a damaged trailing arm from the wheel being hit by another car. It buckled not far from where I added the bulkhead. I am guessing that the box tube dimples in that area during torsional force application as well, and the bulkhead prevents the dimpling. Adding more bulkheads had no additional benefit.
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jluetjen
post Jan 5 2006, 03:54 PM
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OK. I'll stop lurking on this thread and post some pictures of the model -- without the bulkhead.

Click for bigger image...

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jluetjen
post Jan 5 2006, 03:55 PM
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And from the back side...

Click here for larger image...


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jluetjen
post Jan 5 2006, 03:57 PM
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And from the top... (Note that the displacement is exagerated)

Click for bigger image...

This post has been edited by McMark: Jan 5 2006, 09:41 PM


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jdogg
post Jan 5 2006, 04:43 PM
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QUOTE (Racer Chris @ Jan 4 2006, 11:20 PM)
What I really like doing is having a friend bounce up and down on the lever while I watch the trailing arm closely. (IMG:http://www.914world.com/bbs2/html/emoticons/wacko.gif)

Hey, I resemble that remark!

Chris, looks like you had brainstorm in another direction since I was there. Good stuff!!
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