De Dion geometry

[bigdog]

Had my 7 at the alignment shop and noted that the rear had a thrust angle of .17 Deg or a little over 10 arc minutes and showed as out of spec on the hunter machine. Of course the shop just scratched their head as a De Dion tube is a very rare thing in the USA. What should the thrust angle be? How does one go about adjusting the thrust angle? The net has precious little info.

 

Thanks in advance

[Mike Biddle]

Thrust angle explanation copied from the website of a provider that uses the hunter alignment system you mention.

Thrust angle (other than zero) occurs when the individual rear toe values are unequal.
The easiest example to imagine is for a car with a live rear axle (one piece) that has the axle fitted crocked in the body such that one rear wheel has toe-in and the other one has toe-out.
Thrust line is an angle of the resultant drive direction compared to the vehicle centreline. Cars driven with high thrust angles may appear to 'crab'.
On cars, which have independent suspension, some are individually adjustable for toe, which can then be used to correct both the toe-in and thrust line together.
Thrust angle should where possible always be adjusted to zero, to coincide with the vehicle centreline.

Sign definition
A positive thrust angle is one that points to the left.
A negative thrust line is one that points to the right.

Straight steering wheel
Errors in the steering wheel being straight on road test can be the rear toe being wrong and a high thrustline. (As well as the front track-rods being adjusted un-equally or steering wheel having been fitted incorrectly).
Computerised aligners measure and record the thrustline and at the adjustment stage will set the steering wheel level allowing for any thrust angle. It is for this reason that the adjustment step on all aligners takes you first through rear toe adjust, to give you the opportunity of reducing the thrust angle as far as possible, before moving on and adjusting the front toe and steering wheel.
 

So, thrust angle should be zero, and alignment of the ear axle is controlled by the radius arms, these run forward from the ends of the De Dion tube outside the body under the rear fenders to mounting points in the chassis. There are two mounting points available in the chassis, upper and lower. They should of course be in the same position each side. You can easily check just by taking a quick look under the front of the rear fenders, you will see the radius arms.

Maybe one of them is a bit bent? Maybe a worn bush?

If absolutely necessary, adjustable radius arms could easily and cheaply be fabricated, or adjustable sections installed in the existing ones.

Could also be a slight bend in the De Dion tube, or the plate on the end to which the "ear" mounts, which carries the rear hub, but unlikely because this should have been picked up by the alignment system.

 

[Englishmaninwales]

Or have the De dion ear(s) been incorrectly shimmed for rear toe?

[CharlesElliott]

You have to shim the De Dion ears in order to adjust the rear toe on a DeDion. 

[MADMALC]

Shims can be used to adjust the camber as well.

[aerobod - near CYYC]

Although shiming may be needed, the de-Dion tube may also not be perpendicular to the centreline of the car. 0.17 of a degree equates to about 4mm of displacement forwards or backwards at the ear relative to the perpendicular position. Best to confirm the measurement from the tube frame at the front of the wheel arch to a point on the de-Dion ear is even on each side, if it isn't I would check the bushings are not damaged and if you have a Watt's linkage it is evenly adjusted each side.

[7 wonders of the world]

If you have an individual thrust on one rear wheel then you can correct with a shim,

if the whole tube has thrust and is NOT sitting perpendicular to the centre line of the car then the easy way is to rose joint the radius arms so you can dial it out.

 

 

 

[CharlesElliott]

If you had it on a four wheel Hunter, did you get toe readings for each rear wheel?

[bigdog]

Thanks one and all for the detailed info.

The car was on a four wheel machine (hawkeye elite) and had loads of flashing led lights akin to a 70's disco. I was standing at the back of the bay and could only make out that the rear thrust angle was -.17 degrees. I will go back to the shop and try to pull up the files and look at all the numbers. We have a local community collage that teaches auto tech. So I might ask if they could let me put it on a lift to do a detailed inspection, measurements and take a few photos for this group. My front toe in was out of spec and the 7 was being really wondering all over and was what could be described as much more nervous than normal. This came to the fore after putting on the A048's for the summer. By the by, The A048's are pretty evenly worn across the tread 5MM inside, 6MM outside. The front toe is now set at 20" and feels much more settled.

The tech had never seen a De Dion before but had heard of them and I'm quite sure there are very few shops over here that would stock or even know how to get shims for them (if that is what's needed) That's why I wanted a bit of education from the group so that I wouldn't be operating in the dark.

[7 wonders of the world]

I assume all this data was gathered when the car was correctly ballasted with driver / passenger well weights? and the car was correctly sat on a level platform on slip plates?

Many is the workshop that spends thousands on kit and overlooks the basics !

You can buy shim kits form the likes of Westerman in Germany and a couple of the race teams offer them too - I'll get Googling for a linky for you now!

[7 wonders of the world]

Here you go

http://www.dpr-motorsport-parts.com/contents/en-uk/d12_DPR_De_Dion_Shims.html

 

[bigdog]

They had the proper bay and slip plates. But I was not sat in it. Of course the hunter computer had never heard of a Caterham.

[7 wonders of the world]

If the car was not ballasted your wasting your time.

On a light car such as a 7 the driver / passenger account for a significant percentage of the overall weight and this does affect ride height and all geometry readings

Take it back and get them to do it properly -

I use 25kg bags of sand as ballast placed in the seat and footwell both sides adjusted to match correct weights and always check mine with half a tank of fuel.

This way I can replicate any checks and data or corner weights. 

[Mike Biddle]

If the De Dion tube is out of line with the chassis, shimming won't fix it, best establish that first.

[7 wonders of the world]

Thats the point I made earlier with rose joint links as the stock set up has no adjusment facility. The shims are there to adjust or correct camber and toe as required.

 

[bigdog]

Took the 7 to it's Monday appointment with the alignment shop. All seems to be well. I printed out some of the informative posts from the thread for the techs perusal. The geometry shows as in spec. 

The tech (who is the owner as well) suggested that perhaps it could be the tires and their age, wear patterns.  These A048's were sold in the UK so I'm not privy to reading nor even if there are date codes on them, I sure didn't see any. So I have no idea of their age.

In all he had the car on the rack for an hour checking things out. He made note of how stout the frame looked. Even after checking the car out for an hour he sent me on my way with no fees due.

I dread having to replace the 048's as it's impossible to find any sticky 14" tires in the USA anymore. That's a bridge I'll have to cross later.

For now I'm just going to enjoy some twisty roads on Vancouver Island.

[martyn]

So it was out of spec one day, then in spec on another?  You seem happy with that, but did the tech offer any explanation for it?

I suppose with worn bushes, could account for a variance from day-to-day depending on how the car is rolled on to the ramp.

[bigdog]

Martyn. It could have been the fact that the first shop was not even attempting to align the rear suspension and they didn't seem to even want to mess with a De Dion set up. The only number I could see from the edge of the shop was the -.17 degrees thrust. One must also take into consideration that all the modern alignment machines in America are set up to use the VIN number (13-17 digits) as the basis for the spec. Mine being only 7-8 digits will stop their machines computer in it's tracks.

I suspect that you are correct in that worn bushes is most likely the culprit with the nervous handling.

[Paul Deslandes]

Another cause of nervous handling, especially on bumpy roads, is bump steer, caused by the rack height being set incorrectly.  From a straw pole some years ago it looked then as though most Caterham racks were set about 9mm too low.  In some cases it was a lot more if the lower rack clamp half was missing, as mine was.  An attempt was made in some later cars to correct this by using a lower clamp half 9mm thicker than the standard top half.  Otherwise it can be shimmed to achieve the same result.

More on measurement and correction here, although it would seem that most cars are the same and 9mm's worth of shims under each clamp will fix them, irrespective of what the front shock mount arrangement is.

The effect of getting it right is quite surprising and well worthwhile.

Paul