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modifications to custom irs rear trailing arms to lower car

28405 Views 67 Replies 19 Participants Last post by  Richard D
just got in from the garage and figured i would put something up and see if anyones come across this while working on there rear suspension on there drag cars .i realise this may be difficult to visualise without some pictures .but can add some later if needed.
I'm swapping over to a bus box and of course at the same time doing the IRS conversion from swing .
i have made a trailing arm jig for final arm construction as well as a test trailing arm prenarrowed 1" that allows me to start doing some on car fitting .the test trailing arm is also adjustable to allow the bearing housing to float and can be moved in or out so there is a total of 3" of adjustability for wheel position .
got the test arm in and the wheel mounted and noticed it actual works , nice .
BUT what I'm after is the ability to actually lower the cars rear stance 1-1.5 " .i figured since i have irs and a flatter wheel contact i will jack it up and see how high i can get it to go .now i know why people raise the rear torsion housings .what happens is the spring plate hits the top stop of the torsion housing and stops upward travel .this is also the position i would like the rear wheel to sit / start from not end at
options are
- mount the bearing Assembly 2 " up and refab the arm assembles to accommodate and leave basic geometry stock
- leave spring plates and stop alone and mount new trailing arms at a angle onto the springplates say with a 15 deg dogleg idea
- notch the spring plate to allow more space before hitting the torsion housing stop
- trim the torsion housing stop to allow more springplate upward travel
if anyone has some advise or has come across this i would love to hear from you
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Are you using shock towers from an IRS car or from a swing axle car? The difference in design is for that reason
sorry .
the pan is a 68 swing axle type 1
The reason I asked is because the shock towers on a IRS chassis have more control arm clearance built in, and the body mount right there is located higher up.

Steve and Adele from the UK and Mike Herbert did a narrowed and raised IRS deal. Maybe they can elaborate on what they did. Both have posted photos here, I think in the project race cars section
thanks mike ,
just went out and took another look ,always good to walk away for awhile .one thing i have noticed is the irs springplate is alot narrower than my modified swing axle spring plate i cut down ,that gives me some more up .will trim the spring plate a tad and see what that does .also notice i will need to extend the arm 3/4 " to push the wheel back to compinsate for the fwd pull when swung up .
Jim, I don't think I'd worry too much about where the wheel ends up in the extreme travel positions unless clearance is a problem. When I made my control arms, I used a straight "mandrel" to position the wheel to be perfectly in line with the axle centerline of the trans at ride height, so that the IRS joint isn't "working" any more than it needs to. The right way? I don't know for sure.
i'm with you mike on keeping it all lined up .if mine does great if not i will not have to worry about it to much .also thanks for posting your trailing arm photos ,huge source of information .

you can see in these images what i want to try to accomplish

first photo shows how nice the rear wheel fits , and a nice ride height split frt to rear .
this is the height i'm after but without the negative camber .
[img= width=600 height=450][/img]

when i go to the track i swap on the dot's and turn my spring plate adjusters in 1 turn to get the wheels flat .
you can now see the difference in stance /rake as well as the big difference in tire vs fender height .
[img= width=600 height=345][/img]

so that what i'm trying to accomplish , flat tire contact , wheel pulled in and tucked up .
not sure if its all possible but will see if i can

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Mike's thread has me motivated to switch my setup over to IRS, and a Bus box. Going to look at a car today to see what kind of room there is with the Bus box before I jump off on the deep end.

Looking forward to following your progress too Jim.
Jim, your bottom photo illustrates one of the prime reasons I wanted to switch over from swing. Now, get to work so we can see!
you've got that one right mike!!!

my test arm is proved to work very well and have narrowed from a stock IRS trailing arm just over 2.5 " .positioned the arm onto the spring plate and set the bearing housing to 0 deg from the position as in the top photo . had to weld/fill in the originall metal stop on the torsion housing spring plate stop and reshape it to allow the spring plate to come up even with the shape of the torsion housing spring plate/shock/body mount .this now allows me just about 2 3/4" wheel travel , better than before .plus the wheel clears everthing . will get some photos up by tomorrow .also noted that when the wheel is at full lift and on the stop i have only 3 deg neg camber ,which i can still adjust and actually get it to around 1 deg .quite surprised with the adjustability in this IRS setup .
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Hi Jim, good choice going IRS! :D Our '59 bug is IRS'd and I can run the car really low and the wheel sits straight all the time. I believe it's safer too.

I did encounter a few problems on the way though. I decided to run the rose joint conversion which negates the need for torsion bars and then run a coilover shock instead. The first problem I found was the stock IRS set-up is wider than the swing axle set-up which meant I needed to narrow the A Arms 1" but then I found it still wasn't enough as the car sits low! I then decided to use some 4cm wider fenders from CSP which work great and virtually no-one notices there wider!

The next problem we found was the coilover interfered with the stock shock tower which meant new shock towers were needed! We then had the A Arms flipped upside down and side to side and re-located the shock mounting position which then sorted it out! The pics below were took before the A Arms were flipped.

[img= width=600 height=450][/img]

[img= width=600 height=450][/img]

[img= width=600 height=450][/img]

One thing though, I wish I had done was raise the transmission up as as much as I can now run the car low and everything's ok, the axles and CV joints are on an angle which will cause premature wear. But it does work well!

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Mike Herbert's set-up is awesome! Ron Lummus carried out the narrowed A Arms and transmission raise and is definately the way i'd go if I did it again! It does however mean cutting the luggage area.
Good work Jim, I look forward to the pictures. Question to everybody, how much camber should we be running on our irs cars for the strip? 1-2 degrees?

Guys -- I think the way that Mike did his swing arms is easier in the long run than trying to modify/use VW IRS swing arms -- -- I played with stock Fiat swing arms, Stock VW IRS arms, then built my own. When you build your own you can choose the exact location (Height/width) and it just works out better IMO. And on a car, having the exact alignment of CV's/axles is highly over rated -- -- Buzz has his arms on his ProMod buggy 2-3 inches longer and he hasn't had any problems with his 930 CV's and on my Fiat, I was 1.5-2" over ideal length, had only 12" long axles and never had a problem with my Bus CV's and I drove the Fiat a lot around town and on the Freeway


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yes i agree . i started out by attempting to fit stock trailing arms and working on locating the stock inner pivot .the stock arms and the inner pivot would take more modifying than it was worth .trust me i tried to make them work before going a new route .but also wanted to work on the geometry and how the raising and lowering of the inner pivot changes what .
i started with my basic jig which i have modified a dozen times from fitting stock arms to the point its at now which allows me to put the pivot in and location as well as the bearing housing at any depth or height .
[img= width=600 height=450][/img]
since i will use it for the test arm and the left and right arms ,i needed to yet again retool and locate the centre and mark it out so both arms will be the same .
[img= width=450 height=600][/img]
tossed a stock arm to give you a ruff idea .you can see i have dropped the pivot down 1" also this was my first mistake trying to make a jig to fit a stock arm and modifying it .it just did not allow the same arms to be made for left and right sides .
[img= width=600 height=450][/img]
here is the finall version of the jig .still allows adjustability for testing but allows the arms to be made for left and right sides.
i did used the stk arm to give the the bearing position which was marked on the green tape .with the new bearing housing shimmed away with a 1/4" shim from the angle iron it locates the bearing housing dead centre , nice .
then its just a matter of using shim blocks to raise it up and down for depth and clamping in place for welding .
the test arm has been cut up and welded as it has been in and out a quite a few times while trying different lengths ,pivot points and depths .as well as the bracket has been move inboard of the spring plate
[img= width=600 height=450][/img]
the poor test arm after taking a beating ,the bearing slides in and out nicely on the angle iron support to allow for depth testing .this has been a big help as the tires are tight up there .

after bolting the arm on for the last time i realised yet another overlooked item which is making sure the bearing housing is actually square to the torsion tube .i decided to use another piece of angle iron bolted to the spring plate retainer .this will now give me something to measure against to make sure i have a 90 deg angle from the bearing housing to the torsion tube.
[img= width=600 height=450][/img]
then slide the bearing housing in with a 1/4 " plate wedged between the end plate and the bearing housing , yep its square , thank god
[img= width=600 height=450][/img]
i will most likely add a touch of inner toe when i make the final arms but the inner pivot will allow for that adjustment .
new stock ride height at 0 deg
[img= width=600 height=450][/img]
and when fully raised i have 2 deg neg camber and a hair over 2.5" travel
[img= width=600 height=450][/img]
i'm very pleased it actual works and better yet fits in there .and with only a 2 deg change in camber .
finally get to start on the final rear arms ,just got to figure out the rear brakes
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finally got a few spare hours the other day and with too many other things on my mind i need to make sure i'm on the correct path with squaring the rear of the car and tire clearance , let me know what you think just incase i'm on the wrong track .
armed with info on tire growth from john at M and H ( very knowledgable , thanks again for the time on the phone ) i went out to make a few adjustments to the test arm to see what changes i could make .
adjustments and mods were needed over the last test arm mostly to allow a better shock position and to better my camber change , both have worked out very well .more on those items later .

just for kicks i raised the bearing housing 3/4" in relation to the centre line of the springplate as i wanted to see how far up the wheel would go before i got actual tire contact to wheel well or fender .
i was about 1/2" away from the rim being in line with the fender lip when the tire would rub a tad on the wheel well inner section just above the spring plate , see bellow .to bad looks great
[img= width=600 height=450][/img]
reinstalled my 1/2 " water hose which has a o.d of 3/4" onto the tread surface of the tire and spun the tire back and forth at the same time lifting the wheel up through its range of motion until it just scraped, measured from the floor to the centre of the axle at 24 1/4"
[img= width=600 height=450][/img]
and dropped the wheel to the new wanted ride height and new measrement taken from floor to centre of axle, 21".
[img= width=600 height=450][/img]
very happy as the mods to the test arm now allow tire clearance at the sidewall buldge of 3/8" at the spring plate and 1/2" at the fender lip at what i consider the wheel to be in a straight ahead position , as well there is total of 4 " of travel starting at 3/4" below my intended ride height .at that point the camber is +0.4 deg and at full tuck is - 1.2 deg and the mid range area that the car would be operating in is from 0 deg to -1.0 range .
[img= width=600 height=450][/img]
adjusting the inner heim joint does adjust the toe and seams to be a good source of toe adjustment , 1.5 turns netted a 1/8th toe in .and the tire to spring plate clearance stays the same at 3/8" and the fender lip to tire clearance is changed to 3/8" .the clearance is only at certain sections of the fender depending on the tires raised or lowered position and also confined to 1-2 " section at a time .

now for the question ,anyone who has made or modified there own rear arms or modified there rear torsion may be able to answer this .as at some point i would have to assume they would have to square the rear torsion housing to rear wheels during fabrication.

what i did was attached a 14" long 3/8 " piece of angle iron squared to my bearing housing and measured at points A and B with a caliper up to the 2" angle iron and noted the measurments .lucky off the bat the frt point B was a toed in a hair and with turning the heim joint 1.5 turns out toed point B in 1/8" in relation ship to point A .

so , am i correct in assuming that if i square the rear wheel /bearing housing to the 2" angle iron that is bolted flat against the spring plate retainer that is bolted to the rear torsion that my wheels are at 0 toe / 90 deg in relation to the rear torsion housing .or am i missing something ??
if not i'm ready to make the final arms

[img= width=600 height=450][/img]
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JIm -- I believe that the way you set that up you are correct that you are square with the torsion bar -- maybe one final check would be to do x-measurements up to the front of the car or to the torsion and make sure that you are square that way. But I think you will be OK even if your not square as a lot of race/street cars run some stagger in the wheel base from side to side (I did on the Fiat). As long as you feel that you have the ability to make adjustments in the rear of the car to correct issues as they arise, your there.
Nice work Jim. I wonder how much those camber #'s will change once the car is on the ground under its own weight.

I think you're on the right track Jim.

As long as you start out square, it'll stay that way unless your trailing arm bends. And if you leave a few threads on the rod ends for adjustment, you'll be good.

Billyisgr8 makes a good point. You will want about 1 degree positive camber built into the arm to allow for flex, maybe just a smigin more so that the wheels are at zero camber at ride height.

You'll still end up with a couple degrees camber change throughout the travel because the arc is not at a perfect right angle to the torsion housing. So if you run it lower that where you do the initial set-up, the camber numbers will vary slightly.
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