Live rear axle longitudinal watts linkage

[veelen]

I have just joined the forum and require all possible advice on the longitudinal location of the rear live axle of a historic Ford Escort racing car I am currently building.

I am considering building a watt’s linkage system to locate the live rear axle both laterally and longitudinally. I believe the design has been used in some road going cars like the Alfa 33 and some Rover and Volvo models.

Surely this should be an excellent way of eliminating any roll steer common to the more conventional 4 or 3 trailing link systems.

I would be grateful on any opinions in this regard including obvious problems and disadvantages of such a system, compared to the 3 or 4 link systems

 

[Fabrico]

Have you considered the driveshaft's compatibility with longitudinal Watts linkage?

 

[patprimmer]

You will need at least 3 of them in the longitudinal direction if you want to control torque reaction in the axle. This will be a heavy and bulky solution for a problem that causes little concern.

Regards

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[GregLocock]

I suggest you dig up some photos of the old Australian Falcon's rear suspension, or that used on the V8 supercars (Ford or Holden). It is far less complex than your proposal, and can handle any amount of power whilst providing better handling than many IRSs, and is robust and relatively simple to tune.

Nascar use a Panhard rod rather than a Watt's link, the assymetry this introduces may be useful for oval circuits, but I doubt it'll do you many favours on twisty bits.

However I'm not really convinced that even that level of complexity is necessary, I'd be inclined to look at 2 longitudinal lower arms, about 2 foot long, and a pair of upper arms coming in to meet at the top of the diff, or possibly that system turned upside down to get the roll centre down a bit. Ah, Milliken Fig 17.37

Cheers

Greg Locock

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[Warpspeed]

A bit of designed in rear roll understeer is probably not a bad thing to have. But if you wish to reduce it, just lower the rear eyes of the leaf springs by lengthening the shackles.

A suitably located lateral Watts linkage will solve most of the other geometry related cornering problems.

 

[GregLocock]

Ah, leaf springs or coil spring?

If leaf springs, then consider a Crane-Simplex linkage, as used on Ford of Australia's RTV. This is primarily used to control diff attitude, but also provides some handling benefits as it effectively turns the Hotchkiss into a 4 arm suspension. It is simply a pair of longitudinal arms parallel to the front of the leaf spring.





Cheers

Greg Locock

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[Warpspeed]

As I remember ??? the original dog bone Escort used leaf springs (but I could be wrong).

The upper control arms Greg suggests are an excellent idea to control traction/braking geometry.

But if your main concern is rear roll steer, that depends on how the leaf springs are mounted. Usually the leaf spring rear mounting point is made higher than the front leaf spring mounting point. Each wheel will move forward in bump, and rearward in rebound. The result being that the whole rear axle then steers with body roll.

For zero rear steering effect with body roll, the leaf springs need (in theory) to be mounted horizontal in side view. But a bit of roll understeer will sick the back end and feel a lot more stable. Roll oversteer is absolutely diabolical, so you need just enough roll understeer to cancel any compliance in the leaf springs and rubber bushings. If it feels right, it probably is right. But how much rear roll understeer feels best, is entirely up to you.

 

[GregLocock]

Mk I Escort was one of the best cars I ever had. My tedious Corona is a sort of scaled up version with infinitely better reliability, and A/C

Cheers

Greg Locock

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[NormPeterson]

I think one of the SCCA Trans-Am teams running Chrysler Corp. vehicles back around 1970 experimented with longitudinal Watts linkages. Eventually, they gave up and went back to the production-based leaf springs.

Simple, reliable, and tuneable (if you build in some adjustabilities) for road course duty would be a symmetrical 3-link plus Panhard bar.


Norm

 

[GregLocock]

Has anyone ever thought about using hangers at the front of the leaf springs? It seems a neat way of introducing lateral compliance understeer. I imagine there is an obvious reason why you wouldn't....

Cheers

Greg Locock

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[veelen]

Thank you for the great response and input so far.

The later Mk1 Escorts (Zakspeed 1974) were running coil-overs. I will most probably go for them. It would just make the ride height and corner weight setup a lot easier.

Your comments have convinced me to scrap the longitudinal watts linkage idea and I will have to choose between a 3 or 4 link location system with a watts lateral location linkage.

My feeling is to go for the 3 link with coil-overs because the car will deliver around 270 bhp and not too much torque.

I am sure that with the right design I will be able to find a good compromise between achieving the best roll-steer and anti-squad requirements, although I understand that the anti-squad can induce brake hop.

Ideally I would like the car to roll oversteer lightly into the corners (to enhance turn-in) and roll understeer out of the corners (to get onto the throttle nice and early).

Your thoughts and input on this new direction would be most welcome.

 

[GregLocock]

OK, my first thought is not to worry about antisquat. Read many other threads here for the reasoning.

Dumping the leaf springs is probably a good idea.

270 in a Mk I Escort is fairly insane, but no worse than the Torana, so you aren't (quite) over egging the pudding.

I agree with your proposed balance out of the corner, have you read Mark Ortiz and Brian Beckham (? Physics of Racing)on the entrance to the corner? On road cars we tend to go for a very neutral rear end and dial in understeer at the front, because we are interested in transients and can't afford to wait for the rear to start to act. On a race car you have more freedom there.



Cheers

Greg Locock

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[patprimmer]

What Torana?

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

LX with a 308 in it (says Greg the instant Torana expert), although, wow, they were heavier than I'd thought - 1183 kg

Cheers

Greg Locock

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[patprimmer]

My LX Torana was truly insane.

Supercharged 350 Chev. About 700 HP. No it was not an everyday driver, but my brother had an LJ XU1 Torana with the original 3.3 litre 6 cylinder push rod engine heavily modified with twin port head, the biggest valves that could be fitted, 3 X 48mm Webber DCOEs 320 deg cam duration etc etc 270 HP and reved to 7000 rpm. It looked bog stock from the outside.

It was an everyday driver and doubled as a plumbers work truck.

He made a lot of money illegal street racing it, by walking into a bar with motor bikes parked outside, going to a group that appeared to own the bikes, proclaiming how nice the bikers where, and stating some looked almost as fast as his car. He never lost a race and regularly beat Kawasaki 900s, which in the era where considered "King" of the road. Finally every one in Sydney learnt not to race him.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[RickRacer]

My suggestion would be (similar to Norms) a 3-link with inclined lower links and a right offset upper link with a panhard bar as long and low you can make it. Volvo PV uses this layout.

The inclined lower links will minimize roll steer.
The upper offset link will minimize torque reaction on acceleration. (will have som brake torque reactions, which can be used for steer in, in right turns)
The panhard bar will minimize wheel scrub in roll+bump, and have a low roll center.
If you make alternative mounting points you can adjust the roll center.

I have competed in autocross, and the first year it was with a 4-link and a watts.
Upon contact (with other cars) the watts was the weak spot.
It was also very understeered.

So the second year I rebuild it to 3-link and a panhard. (mostly because of the understeer)
We do always drive clockwise laps so I mounted the panhard to the left on the axle.
It loads the inside wheel a little more on right turns.
( My thought was to take advantage of 360dgr more right turns)
The car was easier to steer and more stabile in turns.

I did not use inclined lower links but I think it is a better solution.

Gregs suggestion with a 4-link with inclined links is a good one, but the forces on the inclined links upon contact with other cars can be very high.

Best Regards
Richard Malmlöf Sweden

 

[GregLocock]

Hi Richard, well, that's certainly an aspect of the linkage design I don't routinely worry about. Fair enough, and our V8 Supercars also occasionally retire with broken Watts links. I can't really see why a Panhard rod would be less fragile, maybe it buckles elastically rather than snapping the bolt to the axle off.







Cheers

Greg Locock

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[RickRacer]

Greg I think your thoughts about the elasticall bar can be quite right, it once bent slightly upon heavy contact. But I was able to finish the race.
With a broken watts it was game over.

Maybe the watts need to be of big dimensions on the axle bolt to whitstand impact better?
But then the housing must be reinforced as well.

I used 16mm uniballs for the panhard.

Any thoughts about my other suggestions?

Please feel free to comment.

best regards
Richard Malmlöf Sweden

 

[NormPeterson]

Here's a link that might give you some help. The individual happens to be a Boeing engineer/stress analyst out in the Pacific Northwest region of the USA, but I don't think he's a member here.

http://www.jonaadland.com/Z28/Mods/PHB/AluminumPHB.html

Norm

 

[BillyShope]

I still consider the early Jaguar C-Type (pre IRS) rear suspension the zenith of live axle designs. Norm mentioned a 3link plus Panhard, but added the adjective, "symmetrical." I can't, for the life of me, understand why anyone would want to live with driveshaft torque effects when they can so easily be eliminated. Since the link loads are proportional to driveshaft torque, it is a relatively simple matter to go asymmetric with the 3link and dynamically cancel any value of driveshaft torque.