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 Post subject: Re: Rob7's 442e 5.0
PostPosted: February 10, 2014, 2:29 pm 
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cheapracer wrote:
nick47 wrote:
Cheap, where does the roll steer come from? Don't the arms on both sides shorten by the same amount if they start out parallel to the ground?


Explained by others but the worst is under brakes in turn in when the rear axle is "hanging" and the inside links shorten a lot more than the outside and pull the inside forward - I can show you plenty of 7'ish videos of the oversteer at that point but this one is a classic. This guy is fast and I know the car, he has spent a massive amount of time on suspension including everything adjustable such as roll bars, droop limiters and even a Mumford link on the rear but with standard'ish length trailing arms ...

Watch what happens for the first 1/3 of literally every corner on turn in while braking/off the throttle/transition, he has to correct the roll steer almost every single time especially the fast straights into slower corners where the rear just steers sideways, watch as he turns in, turns off then turns in again with the steering wheel - it's actually easier to count the corners when the rear doesn't walk out.

I can only think that the class is limited to having to use "The Book" suspension pickup points otherwise it is just stupidity.

http://www.youtube.com/watch?v=6DBbBR_kJtU

Sorry Rob, trying to point out it is something worth attending to.


um, oh wait, you meant the other Rob.

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 Post subject: Re: Rob7's 442e 5.0
PostPosted: February 10, 2014, 6:52 pm 
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Joined: August 11, 2012, 10:56 pm
Posts: 205
Location: Arizona
Had some time to work on the rear suspension this morning. Made a jig for 30" links and made two links: 1" x .120" wall DOM steel. Rod ends have 1/2" ID

Attachment:
IMG_4601_r.JPG

Attachment:
IMG_4602_r.JPG


Next will be the brackets for the rear end. Elbow clearance to the links doesn't look too bad. I'll make some kind of cover later on if it's too close. I don't think the lower link will interfere with the exhaust if the muffler is a few inches away from the frame.

Cheapracer: Definitely no need for any apology. I'm thrilled to get all this excellent [and free] advice from folks who know what's up. The short links thing was something I really should have noticed. :oops: I'm glad to be enlightened sooner rather than later.

Also, I'd say that the main reason people (like me) blindly copy what someone else did is to keep the design aspect from becoming too overwhelming and ruining the fun of the build. Building just the 442E frame from plans was easy and quite fun. But figuring out how to fit a 3-link in the frame was boring and tedious. I'll be very glad to move on to the front suspension after using up several months on the rear suspension.

Along those lines, I figure that a lot of my build time is spent just staring at the build guessing what will work well, fit in the frame, and won't interfere with other things later down the line. It's definitely challenging, but it's getting easier. Before starting the build, I figured things would just fall into place (spend 250£, build it, race it... profit). Now I understand that there's a pretty good learning curve for how to think and build in this way. After finishing the build, I wonder if kit builds would seem very easy

Anyway, It took me maybe 6 hours total to grind and cut the old brackets off the rear end. But cutting off the frameside brackets and a couple tubes didn't take very long at all, maybe an hour.


horchoha: I haven't thought much yet about the fenders, but yes I was thinking of cutting large-ish slots in the fenders. Then, perhaps stick-on or bolt-on rock guards with smaller slots could sit on top of the fenders and make the slots look better


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 Post subject: Re: Rob7's 442e 5.0
PostPosted: February 10, 2014, 9:20 pm 
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Joined: June 21, 2013, 4:11 pm
Posts: 96
Robovirus, Cr, Rob& All here; Thanks for the detailed explanation of "roll steer". Having lots of seat
time in 1/4 mile rocketships with license plates, I have experienced over and understeer in various
chassis, but even with a few "sporty' car friends (and some seat time) I have never quite been
able to "see" what caused it. Rob7, excuse my hijack- I'll go back to my pop corn! :lurking:
Chris


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 Post subject: Re: Rob7's 442e 5.0
PostPosted: March 3, 2014, 10:03 am 
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Location: Arizona
The last few weeks were quite busy with work-work. But I did make some small but good progress on the rear links. I also added the diagonal tube where the old links attached to the frame.

Attachment:
IMG_4671_r.JPG


I started with equal length links and tested for binding. I didn't find any when moving the rear end through a huge range of motion. Looks like there's plenty of clearance between the tires and links as well. So that's great news.

Attachment:
IMG_4673_r.JPG


Attachment:
IMG_4672_r.JPG


The frameside lower link pivot bolts interfere with the frame. I'm planning to notch the frame tube as shown below. I don't foresee any problems, but maybe I'm forgetting something..

Attachment:
notch_frame_tube.png


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 Post subject: Re: Rob7's 442e 5.0
PostPosted: March 3, 2014, 10:44 am 
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Joined: September 22, 2005, 8:12 am
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Location: 4AGE in S.E. Michigan
Rob
I would suggest that you not notch the main frame in that high stress area of the lower tube, but weld in a short cross thick wall thru tube [welded on both sides] that has the correct threads taped into the ID. Let the outboard tube end set out the thickness of your originial bracket on the outside so you weld around the OD. The inboard edge of the thru tube could be chamfer for welding. Dave W


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 Post subject: Re: Rob7's 442e 5.0
PostPosted: March 3, 2014, 10:58 am 
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Location: Arizona
Thanks Dave, your suggestion sounds better than what I was planning

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 Post subject: Re: Rob7's 442e 5.0
PostPosted: March 3, 2014, 11:56 am 
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Location: Pemberton, BC
Rob,
it looks like you are committed to your trailing arm length, as they looked to be finish welded. I would not notch the tube either. If you still can, shorten the trailing arms, so that you can weld the vertical tube into the frame. Don't know how that may also affect your lower mounting position, but I've installed bosses into the vertical tube. You also may want to think about where to attach your body panels. That's the trick, isn't it? Thinking 20 steps ahead of what you're doing now. Otherwise, looking good as always.

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 Post subject: Re: Rob7's 442e 5.0
PostPosted: March 3, 2014, 12:26 pm 
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Location: Arizona
Hi Martin - the lower links are only tacked in the middle. The ends are welded already, because they made from old links cut in half.

Are you suggesting welding the vertical tube a few inches further aft and doing what you're doing on your build. I considered that, but I wanted to get double shear for the bolts, even if it meant having the vertical tube outside the frame.

Also, the lower links are parallel to the bottom frame tube. I could probably angle them up to the front of the car to help clear to the tube.

I'll think about how to attach the body panels. So far, it doesn't look like it will be much more difficult than normal.

Anyway, I appreciate the comments ---

-Rob

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 Post subject: Re: Rob7's 442e 5.0
PostPosted: March 3, 2014, 12:33 pm 
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Location: Massachusetts
How about using a piece of 1"x2" rectangular tube for the trailing arm mount. From your picture it looks that if you shortened the bottom end of the smaller diagonal, the rectangular tube would weld right into your cockpit side truss. The diagonal would weld to the bottom of the rectangular tube maybe 1/4 - 1/2" up from the bottom.

It looks from the height of your trailing arms that the bulk of the forces( acceleration, deceleration and potholes etc. ) are going thru the upper trailing arm. My recollection was that 1"x1" is small for that and the 1"x2" eight times stiffer. The larger side will give you room to make a nice welded threaded bung for mounting the trailing arms single shear. Put in extra holes above and below to tune your squat and dive.

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 Post subject: Re: Rob7's 442e 5.0
PostPosted: March 3, 2014, 1:24 pm 
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Hi Marcus - the vertical tube is 1-1/4", 14 gauge tube. I could either reinforce it near the upper pivot or find some rectangular tube.

Welding the vertical tube into the frame has several benefits, but I thought single shear was a no-no. The Carroll Smith books said a few times not to do that. Hmm :?

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 Post subject: Re: Rob7's 442e 5.0
PostPosted: March 3, 2014, 4:19 pm 
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Rob,
standard disclaimer here; I'm not an engineer, nor do I stay at places that make me smarter than the average bear.
Double shear is always preferable, but sometimes not practical. I used 0.75 x 0.125 DOM for the bosses and welded them on both sides with re-enforcing plates. I figured that an AN 1/2 in bolt, in single shear, is good for about 14,000 lbs, which is actually close to the stated strength of the rod ends (16,000 lbs). I will use AN bolts throughout my build, for one I'm very familiar with them, and I do not like threads in the loaded area. I'm pretty comfortable that this is more than adequate for my trailing arms.

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 Post subject: Re: Rob7's 442e 5.0
PostPosted: March 4, 2014, 1:33 pm 
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horizenjob wrote:
. . . <SNIP> . . .

It looks from the height of your trailing arms that the bulk of the forces( acceleration, deceleration and potholes etc. ) are going thru the upper trailing arm. . . .


I'm curious as to why you said that. Is it because of the apparent shortness of the upper bracket on the rear axle (passenger side) in that one photo?

Cheers,

Lonnie

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 Post subject: Re: Rob7's 442e 5.0
PostPosted: March 4, 2014, 2:21 pm 
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I wonder about that too. I've never been able to figure out--or find online--what the forces are in those trailing links. I used to think they were both in equal compression. Then I learned that the links also help counteract the upward force of the pinion trying to raise the nose of the differential, which adds a tension component to the upper arm and more compression to the lower arm. I don't know if the upper tension component is greater or less than the upper compression component, so I don't know if the upper link is in tension or compression.

I can see where the relative distance of the links from the axle would shift some compression from the more distant link to the closer link. But what about the tension component of the upper link?


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 Post subject: Re: Rob7's 442e 5.0
PostPosted: March 4, 2014, 4:39 pm 
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Quote:
I wonder about that too. I've never been able to figure out--or find online--what the forces are in those trailing links.
Me too. I thought that the difference between the upper and the lower are the forces that move the car forward. So I went looking. I just found this calculator http://mysite.verizon.net/triaged/4linkcalcv15html/index.html You can input your suspension data and it will calculate the forces on the links, among other things. I have no idea of the accuracy. Give it a try and see what you get.

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 Post subject: Re: Rob7's 442e 5.0
PostPosted: March 4, 2014, 8:57 pm 
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Attachment:
IMG_4671_test.JPG

I'd like to try lengthening the lower links and moving the pivot higher so I won't have to drill into the lower frame tube. The links still wouldn't be far from parallel. I'll lengthen the test links and check for binding. I'm betting that the pinion angle would still be ok will suspension movement


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