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Author Topic:   Trailing arm mounts
posted February 01, 2002 10:31 AM
The original question was to rear trailing arm mounts and pinion angle....and the timing of these on the rear end housing.

We all know I'm NOT the sharpest tool in the shed, BUT in my opinion the answer is based on the top arm location, and top arm's job...
....If you have a 3-link modified the angle on the pull bar has a lot to say under compared to the bottom arms.

If this question should be talking about a street stock with a stock metric rear mounting location rule.......then I would lower the mounts on the axle housing (making them longer)and have them ahead of the axle centerline when the pinion angle is set.......

The reason for torque is applied the rear axle housing rotates, if the trailing arms are behind the the housing rotates the rear of the trailing arm just goes downward....basicly having NO effect on tire load............with the mounts ahead of the axle centerline as the housing rotates it FORCES a upward torque on the arms pushing the tire into the ground.........

I may not have explained it very well....but it works like a cam over effect......if the mounts are forward of the axle centerline it has already cammed over center and the torque is used to lift the rear of the chassis...........if the mount is behind the axle centerline as the housing rotates it only has the camming effect happening...but never rotates far enough to get over center, and cause the disired effect.

Mongo and I were talking about this subject a few weeks ago, so anyone racing against MONGO is forbid from using this information on their chassis...........LOL

I hope this makes some sense.....and if NOT don't blame me..........I tried my best to keep it simple and understandable (IF THAT'S A WORD?).........


ps. the angles of the lower arms are also a VERY important part, think of it this way...take a piece of tubing and hold it straight up and down on the shop floor, you can apply all your body weight.......because you are directly on center.........

if you hold the tubing at a 45degree angle on the floor you can't apply much weight before it slips out ......that's because you are off center.........

in example 1 you are accelerating and going example 2 you are spinning your tires and getting passed.

posted February 01, 2002 12:40 PM
Azracer, You are absolutly correct in your way of thinking......
with every thing being equal and changing nothing else on the car EXCEPT the left rear trailing arm angle you will move the instant center.......

I prefer to build a rear end housing for a stock mounts on frame rule that the lower trailing arms are ahead of the centerline...
when jigging a housing and welding on mounts, I set the pinion at 0 then weld the mounts facing forward the same degree of pinion as the pinion angle is set in the car the mounts are perfectly vertical to the axle centerline........there is a reason to keep a bird cage square...or a trailing arm mount square (staight up and down)

If I run 8 degrees of pinion angle I will weld the mounts on the housing at 82 degrees down from the pinion.......because I ZERO out my jig to start with...........

I also make the mounts it's farther away from the axle centerline.....and increases the angle at the same time................

After doing all this work as the rear end rotates it is trying to lift the chassis off the ground.......thus planting the tires

If we use your example and leave the mounts at 90 degrees then set the pinion angle the rear housing rotates the angle does increase, BUT the center to center of the trailing arm mounts (from chassis to rear housing) also gets longer because you are still on the rise of the ramp if it were a the mounts getting farther away from each other you let the bushings rotate absorbing torque...instead of pushing against each other applying torque to the chassis.....

At least that's the way I see things.....

You are correct that the angle helps, when I jig a housing I increase the angle, move the mount ahead of axle centerline, and lengthen the mount .......every one of those changes HELP...........I see it as a WIN, WIN ,WIN deal.......the longer mount makes it work like a longer crow-bar pulling nails, the angle helps by moving the instant center, and ahead of the axle centerline helps by geomitry change that accures during rotation..........

Now we are ALL confussed........Mongo understood what I was saying...
I THINK ........hehehe

posted February 04, 2002 02:35 AM
Awkwardjeff, First,I would like you to know that I have alot of respect for you,your inteligent opinions,and the hard work it took to get where you are. The first post of yours that I read,You were showing genuine concern for the safety of a new racer who was going to build his first car....You're a real class act all the way around. There are several things I intend to consult you about in the future,such as your parts collection for the front end. Since we already have this 9 in. bag open,I thought I might explain myself a little better and get your input on the subject......I also intend to note this thread on the other so that everyone will know about optimizing thier pick-up points and the limitations of my original post.

Many streetstockers install thier rear ends by cutting off the lower brackets,and with the brackets attached to the trailing arms,try to position the 9 in. and tack it in place under the car
. I have seen this done several times myself,and judging from the questions I am sometimes asked, the practice seems to be wide spread. I might add,I've never seen it work out very well.......

This is not to insult anyone using that method.The people that I have seen using it were not fabricators,did not realize the consequences of doing it wrong,and had no idea how to make a jig.

It is no problem for you,experienced fabricator, to jig any pick-up point that you may choose.It's a little different for a guy with little experience,a hacksaw,and a arc welder to do the same. On the other hand,if this same guy has some solid point to jig to,he can do it pretty easily with just some strap or pipe..........In short, I was trying to describe the most simple and easy way for a guy to get the rear end installed and have that rear end; fit in the car with out binding,be square,fit the stock driveshaft,and have the pinion set without having to shorten the upper arms. Ok,I will readily conceede that I didn't do the best job on it.........When I mentioned that improvements could be made on it,I expected to be questioned on them,and was going to provide some different pick-up points.

OK,on the subject of trailing arm mounts located behind the center line of the axle,it seems to me that the trailing arm does go down,down and is attached to the housing,which is attached to the tire,which is sitting on the track......So,any downward motion of the arm is loading the tire......This is the way I'm looking at it,as the housing rotates upward,it is lifting the chassis through the upper arms,which is pushing down and away on the lower arms.thereby loading the tires.......So,help me out and tell me what am I missing????????

Hopefully,You will be able to beat some sense into my thick head about the above paragraph.Regaurdless,I actually Agree completely with You when you say to place the mount foward of the center line. Last season at the race shop where I used to work,we were experimenting with this on the house car.Now the latemodel birdcages have been redesigned to be adjustable foward of the centerline.

So,for my current streetstock 9 in.,I have the lowers 7in. down from centerline of the axle and 1in. foward. The uppers I have just as close to the housing as I can get them. I say current because I haven't yet tested it,and intend to keep experimenting untill I run into excessive rollsteer/brake hop. Where do You recomend the pick-up points to be????????

Sorry this is so long.As you well know,the subject gets very complicated very fast.Since I do value you opinions,I wanted to make sure you knew where I was coming from in my original post.For any help you care to offer,You have my thanks.........

posted February 04, 2002 04:49 PM
snowman, thank you for all the kind words.

The easy way for ANYONE to make a jig for a rear housing is take a piece of 3 inch angle iron about 4 1/2 feet long.......weld on two legs facing down....KEEP THE LEGS IN TIME WITH EACH OTHER. weld the legs on a piece of 3X3 square tube.(you should now have a square) you can cut the center of the angle iron out.......(that's where the punkin goes) you have a jig for re-tubing a housing.....the axle tubes are perfectly can plum the legs on the bench then zero out the pinion.......and weld the mounts on at what ever angle you desire..........

I don't know how to explain the forward of centerline any better right now......

I just thought I would through out the easy way to build a jig for about $30 or less.


posted February 05, 2002 06:19 PM
The really sad thing is that I was actually following all of this stuff until the last post describing how to make the jig, which shouldv'e been the easiest post for me to grasp. Would it be at all possible for you to post a pic of your jig, preferrably in use? If it's not handy to do then don't wory about it, I'll surely be able to figure out something halfway usable.

posted February 05, 2002 11:05 PM
Hey awkwardjeff, on this subject of rearend mounts I have seen some of the modified guys mounting their pullbar ahead of centerline on the rearend. Most all of the books you read say to mount it behind centerline. Seems these guys are doing pretty well with it that way but I'm not sure what to think about it. What do you think?

posted February 06, 2002 08:36 AM
blanep, let me see if I can word it a little better......

1. get a piece of 3 inch angle iron, lay it with the V facing upward. (this creates a craddle)

2. weld two pieces of 4 inch square tube facing down. Keep them perfectly in time with each other. these will be the legs on which the housing will stand when in the jig.

3. weld the bottom of these legs to a piece of 4 inch square tube (now you should have a square)

4. cut the center of the angle iron out. (this is where the center section of the housing will need to fit)

5. paint to you choice of color......

tls88mod, In my first post on this subject I stated that the top arm has more to say about forward bite than the bottom arms. And that the angles of the top arms will dictate the lower arms location.

If you took your current car and the only thing you moved was to move the pivot point from behind the centerline to ahead of centerline.......what you changed is, that as the housing rotates under acceration is now the pull bar is gaining angle. and producing more forward bite. But remember it changes the instant center also. A asphalt car mounts the pan hard bar opposite from a dirt car, the reason is a rising roll center or a falling roll center......I use this to help eveyone understand the dynamic change going on.

I have had this disuccion MANY times with different people......on the subject of rear end mounts.........most believe in welding the bottom mounts a 90 degrees from the as you set pinion angle the bottom mounts are behind the centerline. Then most agree to welding the pull bar ahead of the axle centerline far enough that under acceleration the top mount only gets to level......never going behind axle centerline.........

I have discussed this with people that are considered EXTREAMLY sharp in their knowledge of conclussion I believe it makes a differance, NOT A HUGE differance though........NO 1 thing will take a mid-pac car and make it a winner. I believe the angles and lengths of the arms are far more important then being ahead or behind of axle centerline. I prefer to mount the street stock different then a modified........because the rules allow different things. This only shows what is good for 1 car doesn't make it good for another.

It's all in what you know and understand, it's that knowledge that will help you win races.........I don't think I could take someones new Harris 6 link and go out and win.......I don't know that and understand that.........because I don't understand it doesn't make it makes me foolish for thinking it's the car not being fast. A car is a pile of tubing and a bucket of bolts, it's our job to understand how to best put it all together and make it go fast...........

posted February 06, 2002 10:32 AM
Harris is making a six link car?