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String Alignment + rigid axle + panhard

Motorsport Wheel Alignment Fundamentals

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Hi Guys,

How would you suggest squaring the strings on a rigid axle + panhard rod car?

The front side is quite straightforward, being a McPherson, but the rear suspension design would move the wheels laterally through the suspension travel.

Since the suspension is relatively soft (for gravel rallying), the (rear) ride height will never be exactly the same and therefore the panhard length cannot be perfectly optimized in length. In addition, the car is set at practically stock ride height, so the panhard length is more or less OK.

Assuming pure lateral movement of the axle, the toe readings should not be affected, as long as the strings are properly squared. In addition, the rear toe in this application is not adjustable, so it's more of a sanity check.

The thing is that, if the strings are not squared, the front readings will be affected and this in not welcome.

Currently we are using a simplified version of this principle, only for the front (ref: https://www.demon-tweeks.com/uk/longacre-toe-in-gauge-lon79620/), which is pretty quick and easy, but there is not way to measure the rear, except a shop machine. So we set the car up on the shop machine and use the ref. gauge only for on-site check and adjustments. The accuracy will never be perfect due to the nature of the events anyway.

My first thought is to use a fixed point on the body for the rear reference, like the special jacking points on the sills, but i am open to more suggestions and ideas or experiences.


Hey Armaki, welcome to the Forum! :)

Yes you're right here, in that, as the rear axle moves up and down, the panhard bar will also move the axle laterally (left or right) depending on the ride height. So when trying to square the strings, you would still measure from the front hubs and the rear hubs, but you want to set the strings whilst the car is resting at its set ride height. The problem you're going to have is, as the suspension moves through it's travel, the tracking of the car will constantly change and therefore you would need to constantly adjust your toe, which you're not going to be able to do. So set the toe to the normal set ride height, if you change the rear ride height at all, you will have to adjust your front toe to compensate.

I would not recommend setting the strings off of the chassis or car body as you bring in a lot of other variables here when trying to make them square.

Matt :)

I think you're over-thinking this, if you're concerned, think of the chassis moving on the axle, and how that will affect the vehicle - it should have minimal affect in practice, especially as the vehicle will be travellin some disatance as the suspension compresses and rebounds.

Set the panhard so it is flat at normal ride height, as that will minimise the arc the axle end will move through laterally - IF both ends of the axle move equally.

Check the axle is square to the chassis - this should be very close, unless the car has been crashed.

Set the front geometry as normal, with weights for driver, co-driver, half tank of fuel, etc.

What WILL cause you problems is the bump steer as, unless you're using something like a Jacob's Ladder, or Watt's Linkage, each end of the axle will move in an arc as seen from the side view. This will not be a problem if both sides are moving together, BUT if one moves more, or less, than the other the axle ends will move through different arcs - this means the axle will be turned one way or the other and this may be a problem as the rear will steer over bumps.

It is possible to change the linkage angles for different affects (over/understeer in roll), but having them horizontal will minimise the arc's fore-aft movment and steering affect.

You're probably thinking that's all quite simple, but there is another factor to consider - the torque reaction through the chassis and rear axle. This will load the left rear/unload the right rear as the tailshaft torque twists the diff' and load the right front/unload the left front as the chassis is twisted.

Thank you both for your input.

Matt, i see the effect of the panhard on the tracking, but besides having to "countersteer" it, i don't see why the front toe is affected. Front kinematics aside, obviously.

Gord, overthinking is the thing i try to avoid the most, especially considering the limitation during event adjustments. I just need to have a solid and repeatable reference.

I forgot to mention that the car is front driven, so no torque induced issues are present in the rear. At least not directly.

I totally agree about the rear bump steer, but my concern here is to have square strings to adjust the FRONT correctly. So, besides a sanity check in the rear, toe adjustments are possible. At least not yet.

Let's try to take it step by step...

Regardless of the suspension architecture and assuming different tracks (widths) front and rear, the strings should be set at a fix distance from every hub, every time, right?

There is no necessity to have the string at the same distance on the left and right side of the car, or at the same distance front and rear, as long as the car mid-line and the strings are parallel, right? For simplicity, if the rear is, for example 2 cm narrower than the front and the strings are chosen to be 10cm away from the hubs, then 10cm in the front and 11cm in the rear should bring the strings in the the right place.

[img="blob:https://www.racecrafthq.com/0015f2d1-36f0-4180-b01f-e7b1eb74440d" alt=""]

Now, with the panhard free to move, and keeping the 11cm from each side in the rear, the strings will not be parallel to the body any longer.

Even with the panhard parallel to the ground, for lateral move limitation, what dictates that the axle sit in the the middle of the car?

How is this lateral deflection dealt with in double wishbones or McPherson?

Ah, same book, different page :-)

Two parts to the answer, hopefully, depending on the aspect of the query.

For the initial string setup, are you asking what happens if the ride height is changed and the arc of the panhard rod moves the rear axle sideways, as that will move it relation to the chassis?As you point out, if the axle isn't centred to the vehicle's centre line, the sting setup will be slightly biased one way or the other - the affect should be negligible but...

As it is a motorsport vehicle I assume you have some adjustment in the panhard length - I'd suggest centering it between the chassis rails, or inner wheel arches, as they should be stable reference points. More than a recommendation, it is almost essential as otherwise you would need to correct any axle offset by a string offset. You can then do your front adjustments.

I don't get your second query, the only suspension there is a significant asymmetric movement is a panhard or, to a much lesser point, a Jacob's ladder, etc.

independent suspensions, or Watt's located beam, will move evenly side to side.

I think we are getting somewhere :))

Yes, this is my concern, especially considering that the ride height on gravel rally cars is almost always a +/-1 cm think, due to the softer springs. If i get you right, i just need to live with whatever error the resulting lateral movement brings on the squaring of the strings.

The panhard is length adjustable and adjusted close to the OEM length, but since the car is sitting more or less on the stock height, i don't expect the axle to be far out. A confirmation is always possible of course.

The very core of my initial question is exactly the string offsets you are referring to, given the (mostly rear) ride height tolerances i have to live with. This is what i am trying to quantify. Whether this +/-~1cm height in the rear will cause any real issue on squaring the strings (and therefore TOE settings) in the front of this 2.4m wheelbased car. Considering that the height has tolerance, the offset will also have, so this is not a constant thing.

As mentioned, so far we are setting the car on our workshop machine and confirming the set-up during the event but using a toe bar similar to the one on the link of my first post. And we are ok with this, since it's really a 2 minutes job and you can still do it, even if the car is crashed.

My second query is based on the assumption that, during the setup of the strings, the string-to-hub distance is a fixed value.

In my understanding, changing the ride height on virtually any design (except rigid), will lead to the track of each axle changing, due to the arc the hub center is moving on in space. So i assume that if you set the strings on the same distance from the hub at all ride heights, you are eventually adding an error in all cases, regardless of the suspension design.

In other words, the strings will need different distances to hubs to be squared on different ride height. Right? and the difference of those distances front to rear will be essentially the track difference, correct?

Ah, sorry, my mistake - you're correct that an independent will move the wheel in, or out, a little as ride height changes - well, except for full trailing arms like a classic Mini or CV2 - I was thinking of the change in relative centres.

Technically, the track is from the centre of the treads/tyres/wheels, so different wheel widths would alter that - but if you mean the outer sidewal, or rim edge, then yes - if I read the question correctly.

If you do the calculations, I would expect you'd find any changes in the front toe to be measured in seconds, rather than minutes - it isn't something I would worry about.

TBH, while I have used the string method, I consider it at best a fiddley pain in the bottom. I did make up what I considered a better, faster tool (opinions may vary, mind...) that consisted of a couple of 3 or 4 foot lengths of straight extruded alloy and bolted a couple of hooks towards one end of each that held them against the tyre, with the rest sticking out towards the front - use two hooks (I have seen versions with a 'string' instead) as that will always centre them on the wheel at the same place (think vertical). I had marked a 'zero' spot and 13, 14, 15, etc marks on each, directly in line with each other - that way I could just use a tape and directly measure the distance between them and hence work out the toe. If you prefer, you could make a note of the values for different angle values in a notebook, maybe engrave it on the beam(s) as you just need to know a single set of measurement points as it will be the same for any wheel size.

If need be, you could space it out for clearance.

Come to think on it, haven't seen them for years, wonder where they are now... 'borrowed' no doubt.

That is also my concern regarding the strings. Although i find it quite reliable and straightforward method to use for initial or base setup, in the comfort of a shop of paddock box, i don't really see how 2-3 guys can get this done in a 20-30mins midday rally sercive in which they also need to sort out the rest of the car.

What we are using is similar to yours. 3 welded extruded pieces, going around the front of the car, with pointy ends pointing on the rims edge. One end is fixed, the other sliding in and out in the extruded. Effectively we just measure the distance between and "front" and the "rear" side of the wheels, against the extruded pieces.

I came across this https://www.bg-racing.co.uk/B-G-Racing-Aluminium-Toe-Measuring-Plates-Magnetic-Tapes

besides the fact that i don't like referencing on the tire (not that the gravel rims have much straighter lips), i would give the concept a try.

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