Setting rear toe with Dunlop optical toe gauge

[BDA]

I have a Dunlop optical alignment gauge for setting the toe in (see attached picture). This works well for the front I don't know how to really do the rear since it does not take the center line of the car into account. When I first got my car on the road, I sent it to an alignment shop to make sure it was right.

Now that I'm resetting the alignment for the rear after taking it apart. I want to reduce the toe I had to the lower limit (1/16") because I drove through a set of rear tires in less than 10,000 miles. I think I can add the same height stack of washers on both sides but I'd like to be able to confirm that myself. I remember seeing a picture of a Lotus crew setting the alignment (on the rear, IIRC) with the same type of optical system I have so I figure it must be possible to set the toe on the rear reliably. I think a race car is different from our situation. For example, I think it was EuropaTC who posted once that his stack of washers at the head of his radius arms were different so I can't count on that. On a race car, the toe is set a bit differently but I think you can trust the frame is "square."

I can get it as close as I can and take it to be checked to make sure again, but if there is anybody out there who can help me do it myself, I'd be grateful.

[EuropaTC]

Yep, it was (and still is) me who has different spacers on either side of the car and I think it's probably correct even though I'd like to have identical stacks.

As you say, doing the front is easy because the steering "self aligns" so even if you added/subtracted everything on one side all you'd be doing is affecting the lock and the overall tracking would be ok.

But the rear, well I can't see how it can be done easily on a DIY basis other than the old string method or possibly the method used by Arno (?) on the Elise forum which is basically a long straight edge which you align using the hub centres and difference in front/rear track so that you know the straight edge is parallel to the car centre line.

In your case though, if you know that it's right or at least the same toe-in on both sides, then why not just add identical washers to whatever is there now and use your dunlop gauge to check ? I'd need to go back to my notes but I think from memory the formula was something like 1mm of washer gives 0.75mm of toe change. So if you want to go from say 1/8" toe to 1/16" then roughly 2mm of washers should get you somewhere near ?  (assuming I've remembered the relationships correctly that is....     )

Brian

[jbcollier]

It's looks complicated but it is actually easy in practice.  All you need is a long piece of alloy extrusion.  This assumes your tires are the same side to side, and are in good condition.  And, that your suspension is to factory specs: no worn or bent parts.  This is the LAST adjustment you do.  Front alignment and rear camber should be set first.  If you are using adjustable lower and upper rear arms, just adjust one set to be EXACTLY equal before using the other set adjust the rear camber.

Before making any adjustments, see where your steering wheel points in the straight ahead position.  Place the vehicle on a level section of hard surfacing.  Mark the extrusion so you are always placing the same side and end against the tire.  Place the extrusion across the rear tires, roughly aligned with the hub -- it's not critical, just be consistent from side to side.  Compare where it points to on the front tires.

Same each side?  You're good.  Off-set to one side or the other.  Adjust your shim packs to suit.  With each adjustment, re-check the steering wheel straight ahead position first.  It doesn't move much but it's best to be sure.

[BDA]

Thanks Brian and John for replying. I was in the process (a long process since I was on an iPad at the time) of replying to Brian's post when I found out John had posted and your method is what I have questions about

John, I have good tires I'm pretty sure my front is in good shape (I've only tested the caster on the driver's side). I do have staggered tires (195/50-15 on the front and 205/50-15 on the rear) the fronts are in good shape and the rears are new. I have an old version of Richard's twin link rear suspension. The lower arms have left and right handed rod ends but the upper arms only have one "rod end." This should work with your method, I think.

I've done the front alignment already so rear camber was next on my list. I have adjustable upper and lower rear suspension arms so I'm assuming you're saying that the upper left and the upper right arms should be the same length (or the lower left and lower right should be the same length) and then set the camber. I'll do the upper arms to maximize the track and set the camber by adjusting the lower.

Since I've just finished the front alignment and I haven't taken it for a drive, my steering wheel may be off a spline or two at most (actually the steering wheel isn't even on it yet!) so I assume "as straight as I can tell" is good enough for steering position.

Here's where I'm a bit confused. Do I put an extrusion against a rear tire sidewall and see where it points on the front tires? The body is in the way. I cant put an extrusion across both rear tires... I must be missing something obvious!

Then you warn me to recheck the steering. I'm not sure why it would move. I don't see where I've moved the car...

Sorry for all the questions. I've heard of a couple of methods of setting the toe but this one seems unlike others I've heard of and I'm missing something.

Can you help me out?

[EuropaTC]

Here's where I'm a bit confused. Do I put an extrusion against a rear tire sidewall and see where it points on the front tires? The body is in the way. I cant put an extrusion across both rear tires... I must be missing something obvious !

I think you've got the method right but it depends where you put the extrusion (and it needs to be a long one !) If you try for the hub centers then as you say, the body is in the way. The only way I can see of that working on a Europa is for the straight edge to be below door level but with some care I can't see why that won't work on many cars, especially if you are trying to set the wheels parallel. And of course you need the front alignment and steering spot on to start with.

There are things to be aware of though, e.g. what your front & rear track & overall width measurements are. If F/R tracks and tyres are the same, you're good to go. Our TCs have a wider front than rear track and different width tyres. I don't know what affect that has on this method, I would suspect nothing significant if the rear ends up wider than the front, but I don't know.  I'm struggling to see how to set a specific rear toe-in value if the straight edge is hard against the rear tyres but on the plus side it will tell you if one side is "out" & the other "in".

The rig I quoted from the Elise forum also uses a straight edge but relies on the front & rear tracking dimensions taken at hub centres.  Once you've got these dialed in then it's very simple to use and set up, it's basically replacing the old string with a steel bar and can be set very accurately.

The photo here shows the copy I made to use on the Elise and I've also used it out of interest on the Europa & Elan to cross-check the string method I used on them.

It's a steel section with two threaded bars, A & B, set for the wheelbase of the car you're doing. Then all you do is work out the difference between front/rear track at hub level. You can measure it at hub centre (and I did) but from what I've found the tech data in the manuals has been all you need.

You get the difference and if there's a 12mm overall difference F/R (53.5" F, 53" R) then that's 6mm/side. Set one screw 6mm longer than the other (A/B) and the steel bar is parallel to the axis of the car measured off the suspension. The steel just replaces the old string and you can measure toe at rim or tyre as you wish, but it's much quicker to set up !

The other thing I'd mention is that I now use a couple of layers of thin plastic rubbish bags/bin liners underneath the tyres. It's much easier to slide/adjust them because the wheel slides on the multiple layers of plastic. Not as good as a ball bearing platform, but quite effective.

Brian

[JR73]

As JB has said, it looks complicated but once you get your head around it all it really isn't!

It is possible to use Laser Spirit levels (can get them from DIY stores or here in the UK they come up on offer at the discount supermarkets like Lidl/Aldi every now and again!!) - they can be used where JB is describing the lengths of extrusion. Ultimately what you are trying to check is that the rear wheels are pointing straight down the centreline of the car (with whatever toe applied!) in relation to the fronts. It doesn't really matter what the difference in track is front to rear as long as the difference is the same on both sides.....

Back to the Laser levels....
I have done a VERY quick mock up in CAD to try to show what you are doing with the laser level - the laser is projected onto a 'flag' that you hang off the front wheel - this can be made from wood, steel or whatever you have to hand as long as it is easy to fit against the tyre/wheel and you can swap it from one side to the other - the laser dot will basically allow you to see the thrust axis of the rear wheels relative to the fronts. - you could mark vertical lines for reference every 10mm across the flag to make things easier to see.

A piece of wood cut to fit across the flat faces of the rim can be used to space the laser level out enough to get you clear of the bodywork (hopefully this shows clearly enough below?!) and you hold the laser level against this whilst looking at the dot being shone onto the flag hanging on the front wheel....

Take a measurement/get someone to put a marker dot on the front flag and then transfer it all to the opposite side and compare (the front wheel flag will use the reverse face when moved to the opposite side!!)

I hope this makes sense, if you want to stick with long lengths of extrusion then you can space them out from the wheels using the wood across the rim in order to miss the body work but i find that the laser level is easier to handle generally.  - All of this is based on a 4 wheel alignment tool that i used to have for setting up the geometry, the laser level bit had tracking gauges included to make it all a little easier (!!) but for a basic set up where you want things pointing somewhere close to the right way then this does work!!



*Edited to add an example showing the toe to be technically correct across the Front and Rear wheels BUT with the Thrust axis off centre from the rear wheels - laser/extrusions can be seen at different distances from front wheels*

[jbcollier]

BDA:

- Yes, either both uppers or both lowers should be the same length.  You then use the others to adjust the camber.

- Set toe front and rear, then take it for a short drive to determine straight-ahead, steering wheel position.  Don't wing it, it's important.  You can't be precise if you are basing it on a "guesstimate".

- If the body is in the way, go lower on the tire.  Just be sure to be consistent from side to side.

-  If you change the rear thrust angle, you change where the car points when going straight.  You have to check in-between settings so you know your chosen reference point (the front wheels) are accurately placed.

There are all sorts of ways of measuring the thrust angle and nothing wrong with any of them.  The use of an alloy extrusion is just the simplest.  It is very accurate.  Take a 2m (6 ft) bar and it's an 3cm (1.25 inches) per degree.

[BDA]

Brian, JB, & JR, Thanks for your comments. I see how it works now. I guess I got "wrapped around the axle" with "the body in the way." It was pretty obvious once I got over that! I really appreciate all the detailed information!