Harv's traction bar thread

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Harv
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Harv's traction bar thread

Post by Harv »

I figured that the meth monster project would need some sort of assistance in the traction department. The HR plate-and-spring LSD diff centre was a good start, but when the engineer suggested traction bars I jumped at the chance. There is a fair amount of info out there about traction bars, but most of it is US-centric… lots of bolt-on advice for parts that will not fit an FB/EK. There is also quite a bit of outright crap. I figured I best learn what I wanted to know, and will share that below.

I am going to focus here on traction bars, as they are cheap, simple, and period-correct. Caltracs do a similar (but apparently better) job, but are not period correct (they are a ‘90’s invention). TractionMasters also do a similar job, but are more complex (needing welding back to the chassis). Ladder bars and other fancy setups equally increase traction, but are rather complex. I reckon that traction bars are about right for the typical 60’s-70’s local drag strip flavour I want with the meth monster.

Traction bars (also know as traction rods, tramp bars, tramp rods and slapper bars) are used to counter the effect of leaf spring wrap. The image below shows a standard leaf spring setup, with the spring eye to the left and the spring shackle to the right. I have borrowed the image from an internet site and will modify it to explain the story as I go. For my story, image the car moving to the left, as per the red arrow.

First image - red arrow.png
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When the vehicle is stationary and you mash the loud pedal, the axle pushes (twists) against the springs in order to turn the rear wheels. This is the force shown by the orange arrow in the image below. This force can make the rear spring deform. The spring is held at one end by the spring eye, and at the other end by the spring shackle. With nowhere for the spring to go, the twisting force tries to make the spring wrap around the axle, opening up one end of the spring pack and flattening the other end as per the image below. This is known as spring wrap or spring wind-up. As the spring deforms, it moves upwards and forwards, as per the blue arrow in the image below.

Second image - blue arrow.png
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Eventually, the wheel lifts enough that traction is lost, and the wheel starts to spin. With traction lost, there is nothing for the wheel to push against, and the orange-arrowed twisting force is lost. The spring relaxes, and the wheel comes back down again, chirping the tyre.

Scrt!

If you still have your right foot mashed on the loud pedal, the whole process starts again. The rear wheel repeatedly lifts and falls, losing and regaining traction. The back of the car shakes, and acceleration is poor. This is known as axle tramp, or axle hop.

Scrt… scrt… scrt… scrt… scrt… scrt…

Not only is axle tramp bad for quarter-mile times, it is also hard on rear end components. The poor diff gets loaded and unloaded mercilessly… and with the early Holden coarse-spline axles, can lead to failure.

Scrt… scrt… scrt… scrt… scrt… scrt…bang!....................@#%$!

To stop spring wrap, we can install traction bars. There are several different configurations (more on that later), but for now I have drawn below the universal type that clamps on under the spring. I’ve shown it painted yellow, as a nod to Lakewood (one of the largest traction bar manufacturers, and now owned by Holley).

Third image - green arrow.png
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The traction bar is a piece of rectangular pipe, with some brackets and u-bolts to clamp over the spring pack. One end of the traction bar has a rubber snubber, which is positioned just below the spring eye. As the loud pedal is mashed, our spring tries to wrap as per the orange arrow. The traction bar rotates (clockwise in the image above), and the snubber rises until it contacts the spring eye as per the green arrow. This gives the spring something to bear against, spring wrap stops and traction is retained. As the traction bar moves up, it slaps the spring eye… and hence the name slapper bar.

There is of course some down sides to running traction bars. They do reduce ground clearance. They also can effect the way the rear suspension works as it goes over a bump. The image below shows our vehicle going over a bump. The diff moves upwards, and as the spring pack flattens it elongates and moves backwards, taking the axle backwards too (notice the shackle moving backwards in the image below). The axle thus moves up and back as per the purple arrows.

Fourth image - purple arrows.png
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The traction bar also moves with the purple arrows, and can either contact the spring eye or the leading end of the spring. This can make the leading half of the spring pack tighten up, giving a harsh suspension ride. Just how much effect this has depends on the spring shape, it’s rate of response and how close the snubber is set to the spring eye – more on that later. So whilst traction bars stop axle tramp, they make the suspension ride harsher.

Another downside of traction bars is seen under braking. As the car brakes, the axle tends to roll forwards, as per the image below. The spring wraps the other direction, and can make the leading end of the traction bar drop down and hit the ground.

Fifth image - braking.png
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The traction bar leading ends are normally bevelled to reduce this risk. In some model traction bars, an additional set of u-bolts (often called the “square u-bolts”) is installed at the front of the traction bar, as shown by the pink arrow in the diagram below. The square u-bolts hold the traction bar to the spring pack and prevent the leading end of the traction bar from dropping down under braking.

Sixth image - pink arrow.png
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Traction bars are installed flat under the spring pack. This can be either by sitting under the factory u-bolt plate (as shown in the images above for the universal type traction bars), or by replacing the factory u-bolt plate. The latter option requires a specific traction bar to suit the vehicle rather than the universal type, as the factory u-bolts and plates differ with vehicle make and model. Regardless of the type of traction bar, the traction bar should be horizontal, or angling slightly upwards. If the traction bar is angling downwards, aluminium wedge shims are installed between the traction bar and spring pack to angle the traction bar back up, as per the pink wedge shown in the diagram below.

Seventh image - pink wedge.png
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Multiple wedges can be stacked to give more height as needs be. Under no circumstances should the traction bar be installed sloping downwards, as this greatly increases the risk of the traction bar dropping down (… and digging in to the road) when braking. Wedges are available from Lakewood as part number 20500 (2º) or 20510 (4º). Mr Gasket do the identical wedges as part numbers 1606 and 1607 and respectively. Both Lakewood and Mr Gasket are Holley brands and the parts look identical… you just pay a few dollars more for the Lakewood stickers 😊.

Note that traction bars are also of differing lengths, and that the snubber often has options for where it is bolted to the end of the rectangular bar ( a series of holes drilled in the rectangular tube). The snubber should be assembled so that as the traction bar rises the snubber comes into contact with the front spring eye, as per the green arrow in the diagram below.

Eigth image - green arrow.png
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The spring eye is a nice solid, reinforced location to bear the force required to stop spring wrap. The snubber should not be set so that it comes into contact with the spring itself, as per the red arrow in the image above. The ends of the spring are relatively weak, and the large spring wrap force is more than enough to permanently deform (and hence ruin) the spring leaf.

So how are traction bars tuned? A gap is normally left between the snubber on the front of the traction bar and the spring eye.
• A larger gap takes the traction bar longer to react, is more likely to allow axle tramp, but makes the rear suspension feel softer.
• A smaller gap makes the traction bar react quicker, is less likely to allow axle tramp, but makes the rear suspension feel harsher.

For dedicated race vehicles (where rear suspension harshness is not a consideration), the gap is often set to zero (referred to as preloading). For street/strip vehicles, a gap is normally used. Automatic transmission cars tend to load the rear axle more smoothly, and are less prone to axle tramp. As a starting point, use a ½” gap for automatic transmission cars. Manual transmissions apply power more harshly (as the clutch is dumped), and a smaller ¼” gap is a better starting point. The gap is then tuned by repeatedly launching the vehicle and watching for axle tramp, and keeping an eye on any quarter mile time changes. Note that the gap can be different on either side of the vehicle:
• If the car is pulling to the right (driver’s side), then the left-hand (passenger’s side) rear tyres are not gaining traction fast enough. Make the left-hand (passenger’s side) gap smaller than the right-hand (driver’s side) gap.
• If the car is pulling to the left (passenger’s side), then the right-hand (driver’s side) rear tyres are not gaining traction fast enough. Make the right-hand (driver’s side) gap smaller than the left-hand (passenger’s side) gap.
327 Chev EK wagon, original EK ute for Number 1 Daughter, an FB sedan meth monster project and a BB/MD grey motored FED.
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Harv
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Re: Harv's traction bar thread

Post by Harv »

The gap can be adjusted by several techniques. For small changes, the top of the snubber can be trimmed back with a hacksaw (as shown by the brown arrow in the image below) to make the gap larger. To make the gap smaller, washers can be added under the snubber as per the yellow arrow below… at least until you run out of thread on the snubber bolt.

Ninth image - brown arrow.png
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For larger adjustments, the aluminium wedges mentioned above can be used. Note that the square u-bolts (at the front of the traction bar) mentioned above must not be used to adjust the angle of the traction bar. They are there solely as a safety feature to prevent the traction bar hitting the ground under heavy braking. Using the square u-bolts can cause excessive pre-loading on the suspension, as the spring is squeezed and deformed as per the green arrows below.

Tenth image - green arrows.png
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The pre-loading can result in torque steer, where the vehicle pulls heavily to one side under acceleration (not a good thing with drag strip concrete barriers on one side and an opponent’s car on the other).

Some traction bars are also installed with “j-bolts”, as shown in blue in the diagram below. Note that the image shown is not the universal-type traction bar, but instead is specific to the vehicle and replaces the factory u-bolt plate

Eleventh image - J bolts.png
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The j-bolts were typically installed in competition vehicles, and are used to strengthen the traction bar. As per the Lakewood advertisement below, they are used to “prevent broken welds at high stress points”.

Thirteenth image - J bolt reasoning.png
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My guess is that these broken welds were on the leading and trailing edges of the traction bar mounting plate (the plate that bolts under the spring pack). As the traction bar loads and unloads, it acts as a considerable lever, pivoting under the axle as per the orange arrows in the diagram below.

Fourteenth image - orange arrows.png
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This twisting tries to break the welds holding the traction bar to it’s mounting plate. Note that the j-bolts are not used to adjust the traction bar angle, nor to set the snubber gap. The traction bar is firmly bolted under the spring pack – using the j-bolts to adjust angle would require the spring pack u-bolts to be slackened off, and wedges placed into the resultant gap. This is not better than simply doing that process without the j-bolts.

So now that we understand how a traction bar works, where can we buy one? The answer to that question is not so easy. There are a number of places which sell traction bars.

Lakewood is perhaps the best known. It was started in the mid-60’s by top fuel racer “Gentleman Joe" Schubeck”. I suspect Mr Gasket acquired Lakewood during an acquisition spree in the mid-80’s. Mr Gasket, including Lakewood, was sold to Echlin in 1993 and rebranded to Accel. Accel was purchased by MSD in early 2015, who in turn were purchased by Holley in late 2015, giving Holley the Lakewood brand.

Lakewood still make the following traction bar parts relavent to FB/EKs (https://www.holley.com/brands/lakewood/ ... tion_bars/):

Twelth image - table.PNG
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I've got some homework underway on the dimensions and limitations of the Lakewood bars, and the options for making a set from scratch. I've got most of the dimensions, and it looks like there is at least one option that will be close to bolt-up (the bars for X- and F-body GM vehicles). More info in the next update, which will start to become more FB/EK-centric.

Cheers,
Harv
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Re: Harv's traction bar thread

Post by Blacky »

I have a set on Morris Maximus in the shed if you need a pic or any dimensions Harv ?
When you're faced with an unpleasant task that you really don't want to do, sometimes you just have to dig deep down inside and somehow find the patience to wait for someone else to do it for you.


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Re: Harv's traction bar thread

Post by Harv »

Thanks Blacky. Are they the “Universal” ones that clamp under the spring pack, or the ones that replace the original factory shock absorber mounting plate?

The Lakewood X- and F- body ones are the latter, and I reckon they will nearly bolt up. Problem is, the are very pricey. I’ve got a set of the cheaper Universal ones coming, and reckon I will butcher them to make them like the bespoke ones. Probably easier to make some from scratch, but this will save hunting down the hardware.

I really need the dimensions of the Lakewood X- and F- body ones. Tried Holley tech via email, but they blew me off to the telephone help line. Not keen on a long international phone call. VPW stock them locally... will call them Monday and see if they will measure for me.

Cheers,
Harv
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Re: Harv's traction bar thread

Post by ardiesse »

Harv,

I'd like to propose a reverse-engineered 327/350 Monaro radius-rod setup.
Advantages: "cool factor". Invisible to the casual observer.
Disadvantages: lots of metal fab required. You might not have enough space between the inside of the rear tyres and the wheel arch for clearance. Possibility also of "what the . . . ?" reaction from the engineer.

Rob
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Re: Harv's traction bar thread

Post by EK283 »

ardiesse wrote: Sun Feb 28, 2021 12:51 pm Harv,

I'd like to propose a reverse-engineered 327/350 Monaro radius-rod setup.
Advantages: "cool factor". Invisible to the casual observer.
Disadvantages: lots of metal fab required. You might not have enough space between the inside of the rear tyres and the wheel arch for clearance. Possibility also of "what the . . . ?" reaction from the engineer.

Rob
Yes the X series Falchoons had radius rods above the diff and worked very well, again not seen by the average admirer !!

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Re: Harv's traction bar thread

Post by Harv »

I thought about radius rods, but figured traction bars would be simpler. They were tried on mid-60's Camaro's to fight axle tramp, but were only partially successful.

Aftermarket radius rods are period-correct - think TractionMasters: http://www.tractionmaster.com/.

The bit that intrigues me is what happens when a leaf-sprung radius-rodded vehicle goes over a bump. The rear spring elongates, and the diff tries to move upwards and back, as per the purple arrows below.

radius rod.png
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The upwards part is fine (the radius rods are bushed/pinned to allow movement), but the backwards part must be interesting - suspect the diff puts quite a pulling load on the radius rod (and hence the floor pan/mount).

Cheers,
Harv
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Re: Harv's traction bar thread

Post by EK283 »

Good point Harv, the fraud rods are actually set back behind the centre line of the axle from memory about 4 inches, the engineers may have considered your point exactly.
Anyway watching with interest as always !

Regards Greg
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Re: Harv's traction bar thread

Post by Errol62 »

Sorry Rob but the tramp rods get my vote but only if chrome plated harv

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Re: Harv's traction bar thread

Post by Errol62 »

This is all going through one of these harv?Image

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Re: Harv's traction bar thread

Post by Harv »

Lol... I’m thinking the mug lair yellow versions, with Traction... Action! decals 🤣

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Re: Harv's traction bar thread

Post by Harv »

Errol62 wrote: Sun Feb 28, 2021 7:07 pm This is all going through one of these harv?
Yep. 😀

I aim to learn again just what breaks in a 3-speed crashie. Got a pile to play with. Reckon if Spanner’s HAMBSTER survived 2 years of abuse, there may be a chance for me too.

If not, I will act like Steve Austin. We have the technology. We have the capability to make another FB drag car. The meth monster will be that car. Better than it was before. Better, stronger, faster.

Cheers,
Harv
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Re: Harv's traction bar thread

Post by ardiesse »

Harv,

Damn. Image didn't copy.

OK. Think parallelogram. In the Monaros the radius rod is the same length as the distance from spring eye to centre bolt, and is parallel with the main leaf of the rear spring. To a first approximation, the front part of the rear spring acts as a constant-length member. The spring and radius rod form a parallel-arm linkage, and when the rear axle goes into bump or rebound, the "caster" angle remains constant, and the loads on the diff are minimised.

Clay: I see lots of red motor internals.

Rob
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Re: Harv's traction bar thread

Post by Errol62 »

Yes rob the motor from the ute was guzzling oil


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Re: Harv's traction bar thread

Post by Mick »

i had a set of traction bars years ago simple to make, they used the FB/EK type bottom plate with a bolt/threaded rod etc welded out to the side with a round bar that bolted to it and ran forward to where the spring bolts on at the front and had what looked like the side of a shackle (the flat side with the holes) that bolted to the bolt that holds the spring and then down and bolted to the the rod, the rod had simple rubber bushs at either end
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