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Author Topic: Rear wheel steering on a murena!  (Read 15587 times)
gizmo
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« on: June 04, 2007, 12:33:17 am »


I sincerely hope the Prius is not the future, but just another example of Japanese dead-end engineering ( a la 4 wheel steer, rotary engines etc).

The murena already has rear wheel steering, if you don't belive me disconnect your rear spring and move the suspension through its full travel and watch the wheels turn in and out. This happens on almost all independant rear suspensions but the designers have kept quiet about it because it was considered an adverse side effect. Now however the clever engineers have learned to use it to advantage to increase dynamic stability.


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« Reply #1 on: June 04, 2007, 08:50:27 am »


I sincerely hope the Prius is not the future, but just another example of Japanese dead-end engineering ( a la 4 wheel steer, rotary engines etc).

The murena already has rear wheel steering, if you don't belive me disconnect your rear spring and move the suspension through its full travel and watch the wheels turn in and out. This happens on almost all independant rear suspensions but the designers have kept quiet about it because it was considered an adverse side effect. Now however the clever engineers have learned to use it to advantage to increase dynamic stability.

You are right, Gizmo.

But I think the dynamic stability increase on the Murena rear suspension is much more due to the camber control whereby the tyre contact can be maximised. The steering effect (toe-in) will be different on either wheel - during cornering, the outside wheel (admittedly carrying the most load) will be steering inwards thereby adding to an under steer effect, but the inside wheel will do the opposite.

So I don't think the effect is particularly useful.

That said, however, I think the Murena rear suspension is brilliant in all its simplicity. The Matra engineers really knew their tools there!

- Anders
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gizmo
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« Reply #2 on: June 04, 2007, 09:06:10 pm »

But I think the dynamic stability increase on the Murena rear suspension is much more due to the camber control whereby the tyre contact can be maximised. The steering effect (toe-in) will be different on either wheel - during cornering, the outside wheel (admittedly carrying the most load) will be steering inwards thereby adding to an under steer effect, but the inside wheel will do the opposite.

So I don't think the effect is particularly useful.

That said, however, I think the Murena rear suspension is brilliant in all its simplicity. The Matra engineers really knew their tools there!

- Anders

 Thanks for your reply Anders but I'm afraid I must disagree a little but yes the Matra engineers did do a good job.

The camber change on the rear of the murena is not ideal and the only reason the car has static negative camber is to prevent excesive positive camber during the suspension movement and to stabalise the rear suspension as it 'steers' in and out.

When the rear suspension is totally uncompessed (no weight on the wheels) the wheels have positive camber and they toe in, as the suspension is compressed the camber changes proportionally to negative but the wheels turn out (toe out) upto the mid point then they turn in again (toe in). This mid point however is not at the normal ride height of the car but when the suspension is raised by around 2cm.
So imagine we are taking a left hand bend at speed, the car leans to the right loading up the right hand rear wheel and compressing the suspension by say 2cm, the wheel will adopt increased negative camber (a little under 1░) and the wheel will toe in (ie it turns into the bend), the left hand rear wheel will be lightly loaded and the suspension will droop allowing the wheel to adopt a positive camber (1░) but the wheel will also 'toe out' (ie it turns into the bend). Assuming we have not exceeded the grip of the tyres all four wheels of the car are now turned in the same direction, into the bend. This is currently considered as the optimum statagy for a car which has a rear weight bias. WELL DONE MATRA. The downside ( there had to be one) is that lifting off the accelarator during a corner reverses all this and makes the rear wheels steer out of the bend causing at best a tail slide or at worst a spin. The morale of all this is that when you 'overcook it' you have more chance of getting round the corner by keeping the power on rather than 'lifting off'.

I would be interested in hearing from anyone that has lowered the suspension on a murena as this is a whole new ball game.

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« Reply #3 on: June 04, 2007, 09:14:33 pm »


 The downside ( there had to be one) is that lifting off the accelarator during a corner reverses all this and makes the rear wheels steer out of the bend causing at best a tail slide or at worst a spin. The morale of all this is that when you 'overcook it' you have more chance of getting round the corner by keeping the power on rather than 'lifting off'.


I don't think Anders needs reminding of this effect...
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gizmo
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« Reply #4 on: June 04, 2007, 09:47:46 pm »


I don't think Anders needs reminding of this effect...

Oh dear!  I didn't mean to offend anyone.
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« Reply #5 on: June 04, 2007, 10:15:32 pm »

Oh dear!  I didn't mean to offend anyone.

I don't think you have offended anybody - but you put your finger on a (for some owners) very well known effect, Lets put it that way :-)

I follow your line of thoughts, but refuse to believe Matra's engineers got it that wrong. The rear suspension is very ingenious, and has deliberately been setup to do exactly that - I think they were possibly after something else than steering, - maybe grip ? 

An interesting note is than many 1.6's are for some reason standing about those 2cm taller at the rear, - maybe that plays a role in the reports from owners of both models, that the 1.6 is more driveable at the limit ?

Just guessing.

- the normal argument has been something with weight balance.

/Lennart
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krede
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« Reply #6 on: June 04, 2007, 10:36:51 pm »

Quote
I think they were possibly after something else than steering, - maybe grip
Maybe for simplicity? 
I find that Matra has constructed the murena in a rater Un-French way, and many places has gone for some remarkable straight forward technical solutions.
The issue of how to make the electric window assembly fit, to name one.   
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« Reply #7 on: June 04, 2007, 11:07:08 pm »

I will NEVER forget that day when on a very wet track I lost my rear end because I was a bit too fast and lifted my foot from the throttle! It was NOT funny. Okay, maybe a little Cheesy

And no, I am not offended at all.

Roy was with me as instructor (so I can always blame him), and he actually spun the car just before I did. In addition to the water on the track, I'm afraid we had a somewhat serious oil leak from the rocker cover gasket, so we might have deposited a reasonable amount of oil on the track, and as you all know - oil is very slippery! I think it just had to happen.

Besides, it taught me a lesson, and I think I'm driving my Murena much more safely having actually tried that, than if I hadn't.

Back to the discussion:

I've got Warren J. Rowley "An Introduction to Race Car Engineering" here (despite the title it's very complete) Wink Camber as a way to control "lateral acceleration is pretty well established" and while the explanation is "still up to a considerable amount of debate, even amongst the tire engineers", Rowley goes on to explain how the tire's thread handles centrifugal forces as a function of camber.

This was known in the 70's so camber was a very desired effect, but I must agree with you that the toe-change can also assist balancing in a corner. Thank you for pointing it out. I don't agree, however, that that is the reason why I lost my rear end - don't forget that friction between the road depends linearly on load, so (in the above situation), the moment I lifted the throttle, friction decreased. I would have lost her even without the camber/toe-effects.

Interestingly, only the rear wheels have negative camber on the Murena, whereas the front wheels have zero camber (unloaded). I've been told that this is quite common on cars that are weight balanced towards the rear end, but I think it is designed that way to give added directional stability at the cost of some understeering tendency. Having much shorter A-arms on the top, the front wheels by the way has the same negative camber increase with load.

Michal has lowered suspension, I think. He loves it Smiley

I love my Yokohama tyres and plan to stay standard for exactly the reason you point out - it will be a different ball game changing ride height. That said, however - it may work?

- Anders
« Last Edit: June 04, 2007, 11:20:49 pm by Anders Dinsen » Logged

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gizmo
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« Reply #8 on: June 05, 2007, 12:00:06 am »


I follow your line of thoughts, but refuse to believe Matra's engineers got it that wrong. The rear suspension is very ingenious, and has deliberately been setup to do exactly that - I think they were possibly after something else than steering, - maybe grip ?

I don't think Matra got it wrong, I just think they only got it half right. After all the test drivers on the original murena program would not have been exploring what happens when the throttle is lifted but how it reacts on the limit (I don't really think test drivers know how to 'lift off').

This was known in the 70's so camber was a very desired effect, but I must agree with you that the toe-change can also assist balancing in a corner. Thank you for pointing it out. I don't agree, however, that that is the reason why I lost my rear end - don't forget that friction between the road depends linearly on load, so (in the above situation), the moment I lifted the throttle, friction decreased.

I don't cite this as the only reason for loss of rear end grip but it is a contributing factor. I assume by load and friction we are talking about the weight shift effect and not the tranmission of power. Also remember in the 70's high profile radial tyre were the norm and they needed the camber because of the side wall flexability. Now that we see lower profile tyres with stiff side walls camber change is less desireable. Of course you could say keep the high profile tyres but lower profile tyres have much greater control of slip angles, less tread distortion, less heat build up and lower energy absorbtion. That's a lot of plus points to ignore I think it will be better if we can find a way to use the new technology.

Quote
Michal has lowered suspension, I think. He loves it Smiley

If the suspension is lowered I think the least you need to do is fit stiffer shock absorbers or the handling could be worse. Better still to experiment with raising the roll center height back to its original position then you really would have an improvement.



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« Reply #9 on: June 05, 2007, 06:33:23 am »

Hi Gizmo

This is an interesting discussion Smiley

I don't think Matra got it wrong, I just think they only got it half right. After all the test drivers on the original murena program would not have been exploring what happens when the throttle is lifted but how it reacts on the limit (I don't really think test drivers know how to 'lift off').

I'm not sure I follow you here... you think the dynamic camber change of the rear suspension is "half right"?

Test drivers will certainly not get scared as easily as I did, but I don't see why the drivers testing the Murena wouldn't have experiemented with controlling the car on the throttle. Isn't that their job: To emulate the stupid things ordinary drivers like us might be doing with the car?

Quote
I don't cite this as the only reason for loss of rear end grip but it is a contributing factor.

Ok Smiley

Quote
I assume by load and friction we are talking about the weight shift effect and not the tranmission of power.

Exactly. Vertical load. 'Friction' is tire friction between the thread and the road. Frictional forces are proportional to the vertical load on the tyre thread. Power transmitted through the tyres add to the sum of forces excerted by the car to the wheels (the others being braking forces and centrifugal forces, and to a lesser extent gravity forces (on a banked road). You know this, of course, I'm just defining.

Quote
Also remember in the 70's high profile radial tyre were the norm and they needed the camber because of the side wall flexability.

The understanding among race car engineers is that camber is not depenent on side wall height. What matters is the construction of the side wall. There is a lot of difference in the way race tyres react to camber changes and the way normal road tyres do. This is, a result of different side wall construction more than flexibility.

Quote
Now that we see lower profile tyres with stiff side walls camber change is less desireable. Of course you could say keep the high profile tyres but lower profile tyres have much greater control of slip angles, less tread distortion, less heat build up and lower energy absorbtion. That's a lot of plus points to ignore I think it will be better if we can find a way to use the new technology.

You are touching an interesting point there, but we are kind of stuck with the Murena as it is as there are zero adjustment possibilities on the rear end. Some time ago there was a swedish guy on the forum who wanted to build a race car from a Murena, and one of the improvements he considered was an adjustable semi-trailing arm suspension. Unfortunately he had to abandon his project, but my point is that adjustability was by him considered a very important property.

For a road legal car, however, we are stuck with the original design and geometries. I agree with you that it would be fun if we had a fully adjustable suspension, but I'm afraid the Murena will just never have that - except adjustable dampers (and spring seats) like the Gaz dampers Krede has or the Spax ones Michal has.

I'm absolutly certain that you will get better handling on your Murena by going towards wider, lower profile tyres. Even without changing the suspension setup. The problem is that you will also be ruining drive comfort, which from my point of view is only to a lesser extent acceptible.

Years ago, Roy Gillard of the UK club realised that by going to a 55 profle on the front, he could improve handling significantly. I have accepted his experience and use it myself, even on my winter tyres. I find that my car handles exceptionally well, though different tyres work very differently. On the Murena you can really feel what is happening. I have chosen Yokohamas for their soft compound, which I feel that the Murena really likes, but unfortunately I had to select different patterns front and rear to do so. Had I run with the standard profile on the front, I think I would have been able to pick the same.

Quote
If the suspension is lowered I think the least you need to do is fit stiffer shock absorbers or the handling could be worse. Better still to experiment with raising the roll center height back to its original position then you really would have an improvement.

"Experiment" is exactly where it gets difficult unless you have lots of time and access to a race track. Do you? Cheesy

- Anders
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krede
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« Reply #10 on: June 05, 2007, 09:24:49 am »

Quote
If the suspension is lowered I think the least you need to do is fit stiffer shock absorbers or the handling could be worse.

Think you are onto something there Gizmo. Smiley
I have messed around a bit with the ride hight, and i have come to the conclusion that the murena is low enough as it is.
I started out lowering the rear a bit, while at the same time installing adjustable shocks.
At first I set the shocks to "half" stiffness , but couldnt tell much difference from the original setup.(then I set them to "kill" and that made a difference!!)
However.. I find that the camber of the rear quickly increases, witch might have some unexpected effects, and witch makes the car look rather .. well strange..
Also in my opinion the most important aspect of lowering the car, is to get the balance between the front and the rear just right.. the goal here being, to always have an adequate amount of weight on the front axle to get good grip for precise steering and good braking.       
Another important thing to remedy is the rear ends tendency to "sink" during acceleration, or hard cornering.Also the stiffer shocks I installed at the rear, made a huge improvement in my cars stabillity at high speeds (120km/h is high speed to me Wink )
 
I went to lower the front end as well.
But.. I find that you can only lower the front so much,before the wheels disappear into the wheel arches.. again looking silly..
So.. when i get around to it I will set up the ride hight exactly as matra prescribes, and then check the toe in/camber, and raise/lower the rear to get the latter to the original spec. If I'm satisfied with the result, Ill keep it there and never mess with it ever again.. Smiley   
« Last Edit: June 05, 2007, 09:37:57 am by krede » Logged
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« Reply #11 on: June 05, 2007, 09:53:39 am »

Also the stiffer shocks I installed at the rear, made a huge improvement in my cars stabillity at high speeds (120km/h is high speed to me Wink )

Interesting. What exactly do you mean by stability? I have a bit of shaking from the steering at 110 and up and am going back to have my Yoko's rebalanced tomorrow, but otherwise the car is directionally and in other ways stable even at 140 km/h. I think the problem is balancing, but it may also be a vibration in the chassis which can be damped.

- Anders Cool
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« Reply #12 on: June 05, 2007, 03:00:16 pm »

This is a very interesting discussion. To qualify the statement I made in a previous thread which sparked this lively discucssion off, the 4 wheel steer to which I refer is the electro-mechanical type with a separate rack for the rear wheels, like the Honda Prelude. I am all for passive rear steering like the Peugeot 306/Citroen ZX and Volvo 480 have, amongst others.

I used to always try and lower cars, but I realised one day, that one of the main reasons I like cars is handling. For me to then go and stick some cheap aftermarket shocks and short springs on was pointless.

Yes cars fell more planted etc, but the true test of a cars handling, for me, is the adjustability on the limit. The very best car in this regard is the Peugeot 306. Truly inspired handling and grip.
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krede
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« Reply #13 on: June 05, 2007, 05:43:46 pm »

Quote
What exactly do you mean by stability?
I used to get the feeling that the nose lifted when driving fast, or accelerating hard. Felt a bit like driving with a heavy trailer on the hook .., only of cause , my murena has none Smiley
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Anders Dinsen
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« Reply #14 on: June 05, 2007, 06:48:37 pm »

Quote
What exactly do you mean by stability?
I used to get the feeling that the nose lifted when driving fast, or accelerating hard. Felt a bit like driving with a heavy trailer on the hook .., only of cause , my murena has none Smiley

Okay, well that's not something I have experienced. What tyre profile are you running on the front? 60's or 55's?
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