How flexible do you think a stock, trailing arm is Options

[TimT]

great thread.... I love this stuff..

Im out also.. Ill check back on the rebound

[ChrisFoley]

QUOTE (URY914 @ Dec 16 2005, 11:38 PM)

I expect to see a full report on my monitor in the morning.

I won't have more until late afternoon. I have to do more tests in the morning, then watch an SCCA race at 1 o'clock. (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif)

[McMark]

I can't wait to see if my thoughts about stiffening the arms match with yours. (IMG:http://www.914world.com/bbs2/html/emoticons/boldblue.gif) Thanks for all the effort Chris. When are you going to make a similar jig for the chassis? (IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif)

[brant]

QUOTE (URY914 @ Dec 16 2005, 09:21 PM)

I spoke to a GT2 racer a Sebring once (actually more than once) and he said he cut a 1 3/4" hole thru the trailing arm and welded in a piece of tubing. The tubing stuck out of the arm about 3/16" on both sides, just enough to get a good weld around it. Said he could tell the differance next time out. Sounds easy to me. (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)

this is really a great thread!
(IMG:http://www.914world.com/bbs2/html/emoticons/smilie_pokal.gif)

I can't wait to see the test results of a stiffening kit equipted swing arm.
I'm dying to see if the commonly thrown around statement about it being "a waste of time" is actually hogwash.

I have it on good authority from AJRS, that it actually is important to stiffen them up. We have the braces in the chassis to the mounting points and I'm told that after stopping the console/ear flex it because necessary to stop the swing arm deflection.


Paul, regarding the guy with the tube welded into an arm...
ha... we put a tube into our Longitudinal for the same reason.
This is supposed to be a big reinforcement over a stock longitudinal for little or no weight... (not that I'm good enough to feel the difference!)

brant

[Mueller]

nice work Chris.....

QUOTE

If I had a strain gauge laying around the shop it would have been on the trailing arm.

come on Chris, you've got to have one of these sets to use to measure deflection (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif)

[ChrisFoley]

Mike,
I have a dial indicator and mag base but not all those tips. I'm not really a machinist after all. (IMG:http://www.914world.com/bbs2/html/emoticons/smash.gif)
Fine instruments aren't necessary to measure the trailing arm flex anyway. You can watch the movement and see exactly what is happening.
A stock trailing arm definitely isn't stiff enough for a competition car running on slicks. (IMG:http://www.914world.com/bbs2/html/emoticons/unsure.gif)

[IronHillRestorations]

I saw one of the 1997 GT1 chassis on a deflection dyno at the Wiessach facility, and they had around 40 dial indicators at different places on the chassis. Forget about any photos, strictly verbotten (sp), and they probably would have shot you on the spot.

Al Swanson has run this part on a computer modeling program where he works (can't tell you that either (IMG:http://www.914world.com/bbs2/html/emoticons/cool.gif) ), and found that welding the so called stiffeners actually weakend the the part, primarily due to annealing the steel.

The other consideration would be the bushings used. IMHO any stiffening would be mute with anything less than solid bushings, for obvious reasons.

I've always thought the tubing through the trailing arm would be the best way to go.

[Brett W]

Now you are seeing the reason why many professionally developed racers are using a different, custom trailing arm. You can bandaid the stock part or you can build a much stronger yet lighter custom part.

The only reason to keep the stock trailing arm is a class rule that says you can't change it.

For a race car I would completely eliminate the bushings. Run a monoball or rod end. Look at Blakewell's car and Sheridan's car.

Here is Blakewell's setup. This is a Preston built chassis. He has a custom rear trailing arm but I don't have a picture of the arm.

[Thorshammer]

I have'nt seen this thread until this morning, but wanted to add some comments.

Chris and I started talking about this because som eof the info we have found for 914's seems at times to be untrue. People have done things for different reasons and many are not known. Such as The outer (stock) pick up point piece is extremely flexible, Chris has a stronger piece, I used it it works well. Why some people use a trailing arm stiffening kit and others do not. Here are the things we want to know:

-How much does a standard trailing arm twist in degrees of deflection when we load it near max load that we might see with slick under racing conditions.
. Only flaw I can see in Chris's methodology is maybe we have under or over estimated the load at the wheels. It also has a lever that would act upon the trailing arm, the Wheel. 11.75inches from the centerline. So we really need to make sure we've thought that through.

-How much lateral deformation when a max cornering load is applied. Meaning doe the arm bend thowards the transmission when we load it and if so how much.

-Where does it bend or Twist

-How does a trailing arm that is stiffened using the typical weld on plates perform to the same tests.

-Where can the arm be properly strengthend while adding minimal weight. Keeping in mind that some bending can be advantageous in gaining car "feel" at the limit. Same with Chassis twist, if its too stiff it looks good on paper but does not give the driver any feel and therefore be difficult to drive at the limit.

These are the problems we (Chris) are trying to solve.


Erik Madsen

[groot]

QUOTE (Thorshammer @ Dec 17 2005, 08:49 AM)

-How much lateral deformation when a max cornering load is applied. Meaning doe the arm bend thowards the transmission when we load it and if so how much.

All very good stuff!!

Eric's question is very important. If you guys could apply to load at the tire contact patch you'd greatly enhance your investigation. Chris's current fixture measures the largest component and it's a great start.

Chris, if you could modify your fixture to apply the load at the contact patch so you get that additional force (bending) and then you could measure the trailing arm and then measure it again on the vehicle. Then you could determine what percentage of the compliance is associated with the trailing arm and how much compliance is associated with the chassis. That would be sweet.

I ran a crude version of this and decided to start over with a new trailing arm (and for some other reasons, too).

Keep it up, it is great when science is applied to making things work better!!!

[Thorshammer]

Something else I haven't thought about is the amount of spacers on the wheels possibly changing the criteria. I run almost 40 mm of spacers in the rear. This may change the information, but for the time being, we will at least have some relative data, and that will be useful.

Erik

[SpecialK]

Quick (IMG:http://www.914world.com/bbs2/html/emoticons/hijacked.gif)

Is Erik's and Chris' race E-Prod?

If so, it's scheduled for 4 am Monday morning. (IMG:http://www.914world.com/bbs2/html/emoticons/unsure.gif)


Now, back to your regularly scheduled 914 component testing...

[ChicagoChris]

This is great and even though he hasn’t yet done the experiment I have been learning a lot from the discussion. But it feels like people are making this mole hill into the mountain.

I don’t get the impression that it is important to know every force, in every direction, to the .001? If the goal of the experiment is to know all the forces and how each surface of trailing arm reacts then ok. But if the purpose of the test is to get a practical way to strengthen the trailing arm against twisting force then I say he is right on.

Personally I am anxious to see what he finds.
(IMG:http://www.914world.com/bbs2/html/emoticons/popcorn[1].gif)

[bondo]

Am I correct in assuming that 1 degree of deflection is no big deal for a street car?

As for race cars with big slicks, what kind of deflection are you shooting for? Is 1 degree 2 times too much? 10 times?

I love this stuff, I can't wait to see the proposed solutions and the results of testing them. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

[ChrisFoley]

OK
I did some more experiments this morning.
The first thing I did was to rotate the trailing arm vertical so I could easily apply a load to induce lateral deflection. I was all alone so I couldn't make accurate measurements but the deflection is similar to the twisting deflection under the same loads. If anything it was slightly less. Personally I don't think lateral deflection is a problem as I am pretty sure the force component in that direction is much smaller.
I thought about my setup for a long time before actually assembling anything. Even though it appears to make significant compromises compared to what happens in the real world I am pretty sure it would be tough to make a major qualitative improvement over what I have. However making quantitative improvements wouldn't be too hard. I don't know if the load I am using is correct, and a digital level isn't as accurate as a dial indicator. OTOH, that is the same digital level I use to set up the suspension on the car and I am really only after comparative data, not detailed design analysis.
Since I was working alone I decided to standardize my forces and make it possible to read the level while under load. That meant I replaced my body weight on the lever arm with a stack of steel pieces adding up to about 220 lbs. The only shortcoming to this method is I can't observe the deflection as it occurs. I can only read the level to see the final effect. Good enough for now.
So I repeated yesterday's test with my standardized load and measured a deflection of 1.1 degrees.
Next I replaced the trailing arm with a boxed trailing arm and the deflection was 0.8 degrees under load.
Finally tested my idea for a reinforced trailing arm. Using just on piece of sheet steel that weighed 4 ounces and measured approximately 3x4 inches I was able to make an equivalent improvement to just 0.8 degrees of deflection on the original trailing arm. That compares to about 2 lbs of sheet metal used for a boxed arm, and a corresponding smaller amount of welding which means less distortion of the trailing arm. (IMG:style_emoticons/default/smile.gif)

[ChrisFoley]

Am I correct in assuming that 1 degree of deflection is no big deal for a street car?

You will never load the suspension enough to reach one degree of deflection in a stock based street car. These values only apply to race cars on sticky tires.

what kind of deflection are you shooting for?

I would like to cut the deflection by 50%.

But if the purpose of the test is to get a practical way to strengthen the trailing arm against twisting force then I say he is right on.

That is precisely what I'm shooting for. (IMG:style_emoticons/default/smile.gif)

[jhadler]

Great project Chris!

I'm really interested in the results.

I'd really like to know where the most flex occurs, and in what axis. Does the arm bend more toward the transaxle? Or does it twist more? Where is the weakest link? The bushings? The mounts (inbaord and outborad)? The arm itself?

Keep the good info coming!

-Josh2

[ChrisFoley]

QUOTE (Thorshammer @ Dec 17 2005, 01:20 PM)

Something else I haven't thought about is the amount of spacers on the wheels possibly changing the criteria. I run almost 40 mm of spacers in the rear. This may change the information, but for the time being, we will at least have some relative data, and that will be useful. Erik

The amount of wheel spacers is immaterial. All that matters is the location of the contact patch. Not only that, within the range we are working on our cars small changes to the geometry will have correspondingly small effects on the forces applied.

[ChrisFoley]

I'd really like to know where the most flex occurs, and in what axis. Does the arm bend more toward the transaxle? Or does it twist more? Where is the weakest link? The bushings? The mounts (inbaord and outborad)? The arm itself?

Well, on a race car with hard bushings and a reinforced outer mounting bracket 95% of the flex occurs in the box section of the trailing arm. I am fairly certain that most of the flex is twist and only a minor component is lateral.
The stock outer bracket, unreinforced, flexes quite a bit. Stock rubber bushings also flex quite a bit.

[ChrisFoley]

Here's a pic of the trailing arm in my bandsaw. The kerf of the saw is about 60 thousandths of an inch.