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Author Topic: Wheels and tyres  (Read 22115 times)
macaroni
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Murena and Multipla - I like it 3 abreast!


« Reply #15 on: June 28, 2007, 10:28:30 pm »

Anyone know the correct offset to use front and back?
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Anders Dinsen
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« Reply #16 on: June 28, 2007, 10:38:46 pm »

Anyone know the correct offset to use front and back?

Oh yes, that would be 29 mm
And there is plenty of space in the wheel arches to accomodate wider rims with the offset retained.
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Laurens
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« Reply #17 on: June 29, 2007, 01:10:39 am »

i have offset (ET) 20 with my 7j in front and 8j in the back. in the front you wont need a lower et. in the back it could be et 15 with no problem i think (with 8j).
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gizmo
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« Reply #18 on: July 01, 2007, 10:37:28 pm »


The subject of tyre pressures has arisen in several threads and recommendations of 1.6 front and 2.5 rear have been mentioned.  The way I see it, after we have inflated the tyres sufficiently to stop them being flat on the bottom any additional pressure is principally to adjust the tyres slip angle (ok. there are other factors but let's get the discussion started). Such a difference in pressures indicates to me a fundamental imbalance in the front/rear grip. Bearing in mind that the increase in rear pressure is increasing the grip at the rear, we also have wider rear tyres and a significantly wider rear track. If it is necessary to do all of this and still have a car which would like to oversteer what is wrong with the original set up? after all tyre pressure adjustments should be for fine tuning not basic set up. I have a few ideas but would welcome input from more experienced murena owners.

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krede
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« Reply #19 on: July 01, 2007, 11:27:07 pm »

My car is way more prone to "under" rather then oversteer.. especially in the wet.
As for the reason for any "patch up" technical solutions.. well.... oversteer is supposed to be a really hard to manage in midengined cars.... I for one am not brave enough to find out,  so I can only theorize as to how the murena behaves on the limit... the few times mine has had the tail out.. I've been sufficiently scared to make an effort to avoid it the best I can.
So maybe Matra went out of their way to build in some understeer to make the car safer for inexperienced drivers?

Another reason might be the weight distribution of the car.. witch... frankly is far from perfect.. 40-60 at best....and 35-65 is more likely for a 2.2 with a full tank of fuel and a case or two of (warm) beer in the trunk.... and thats rather bad..
I had my 2.2 on the brake tester a few weeks ago, and it actually said roughly 300/700kg!!.. of cause these figures are a very rough estimate as the tester is meant mostly for trucks.. but still... 
Now.. I know this will probably be a red cloth in front of some enthusiasts, but I suspect that as far as handling is concerned the mid engine is NOT an asset for this car.
Weight distribution aside, I still find the murena "heavy" rather then light at the controls... witch should have been another advantage of having the engine at the rear.
The only REAL advantage the murena got from the mid engine layout is the possibillity of having a very low nose and thus a low drag coefficient.

Of cause you could argue that the murena was intended for a lighter engine, that would have improved weight distribution, and that a front engine would have effected the aerodynamics, and that the propeller shaft tunnel would have made the 3 abrest seating impossible..but in my mind the mid engine in the murena is much more "show" then "go"...
Dont get me wrong... I love the mid engine configuration... the low nose and to some exte.... no!.. Id really rather just have had a 2 seater..
But as I was saying.. the things I have mentioned are what makes the murena so special, and I wouldnt have it any other way.. but as performance goes... well....

Also.. keep in mind that the murena is much more of an "upgraded" Bagheera then a new design... and (as far as i know) the Bag was an attempt at an affordable every mans "sportscar", rather then at a serious racer or gt.
Surely with a front suspension dating back to the first simca 1100's and an engine that can be traced just as far back a super car was never intended, But all its flaws aside, Id still say Matra did a very good job on the murena.. to quote a Danish  motoring magazine... "the murena is everyting the Bagheera should have been".. written, I guess, with mostly engine power, and rust problems in mind.
Only thing that went wrong, was the high price, that put it in a class of cars where it didnt belong...AT THE TIME!! (Today, I'd choose a 2.2 murena over a 924 ANY time)


     
« Last Edit: July 01, 2007, 11:34:40 pm by krede » Logged
Anders Dinsen
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« Reply #20 on: July 02, 2007, 06:49:55 am »

If it is necessary to do all of this and still have a car which would like to oversteer what is wrong with the original set up?

As Krede also mentions, Murena is tuned to understeer - it doesn't generally oversteer. And I don't think anything is wrong as such with setup, as a matter of fact I think it's quite perfect for a road car. The difference in tyre pressure is quite normal for a mid or rear engined car.

I've got no problem with the weight distribution, I think it's perfect. Lots of fun and excellent driver feedback on everything that's happening between the road and tyres.

Mid engined race cars generally also have 40/60 weight distribution by the way.

- Anders
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roy4matra
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« Reply #21 on: July 06, 2007, 08:11:19 pm »

Only thing that went wrong, was the high price, that put it in a class of cars where it didnt belong...AT THE TIME!! (Today, I'd choose a 2.2 murena over a 924 ANY time)

I'm sorry but you are definitely wrong here.  The Murena in 1983 was an incredibly cheap car!  Most people who saw my new Murena thought it cost twice what it actually did.

I bought mine new in 1983 and couldn't believe my luck.  A Lotus Esprit (also a GRP mid-engined 2.2 with galvanised chassis) was over twice the price.  My Murena 2.2 was £6900 all taxes paid and an Esprit was £15,000.  And that 924 was £8500, and if you consider the spec. it was no contest - steel wheels, 70 profile tyres, drum rear brakes, wind up windows...  to name just a few things.  I know since I had checked out all the competition before buying the Murena.  The Murena was only around the price of a plain Ford Cortina!  It was a bargain.

The Bagheera was over priced and being prone to corrosion, not as well made, and then the Murena went the other way, being more powerful, more modern, and galvanised and only just over £1000 more.  Whilst a Bagheera was more costly than even more powerful sportcars like an E-type, the Murena was cheaper than just about any other sportscar at the time.
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roy4matra
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« Reply #22 on: July 06, 2007, 08:16:18 pm »

Anyone know the correct offset to use front and back?

The correct offset is 28 mm.  This is the factory and original wheel spec.

Anything else means the wheel bearing is taking a side torque.  The vertical loads should go through the centre line of the wheel bearings, and that only happens with the correct offset.
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Anders Dinsen
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« Reply #23 on: July 06, 2007, 08:36:20 pm »

Only thing that went wrong, was the high price, that put it in a class of cars where it didnt belong...AT THE TIME!! (Today, I'd choose a 2.2 murena over a 924 ANY time)

I'm sorry but you are definitely wrong here.  The Murena in 1983 was an incredibly cheap car!  Most people who saw my new Murena thought it cost twice what it actually did.

And that is still true: The Murena is an incredibly cheap classic, extremely long lasting and one that can still compete in style and apperance with modern cars. We can keep wondering why, but it is a fact that it is and has apparantly always been underrated.

- Anders
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roy4matra
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« Reply #24 on: July 06, 2007, 08:40:00 pm »

The subject of tyre pressures has arisen in several threads and recommendations of 1.6 front and 2.5 rear have been mentioned...

Such a difference in pressures indicates to me a fundamental imbalance in the front/rear grip.

I have always run 1.8 bar front and 2.5 bar rear, which is the factory spec. for a 2.2 Murena running at motorway speeds.  Since the Murena can get up to 110 kph quite easily and quickly, I see no point in the low speed recommendations.

PLEASE NOTE: early handbooks had the wrong (low) pressures and a sticker was released with the correct ones.

From my experience over twenty five years of very fast driving of Murena 2.2, I know that anything less than 2.2 bar at the rear noticeably increases the oversteer tendancies of the car, so I always keep mine at 2.5 bar and have never had problems with grip or tyre wear in the centre (which would be the case if these pressures were too high).

As Anders has pointed out, I also found many years ago that 55 profile tyres at the front improve the car more than the small change would suggest.  I wish to keep the unique original wheels on my Murena so that limits the tyres available.  However, 185/55x14 front at 1.8 bar and 195/60x14 rear at 2.5 bar appear to give the optimum in ride handling and grip for a close to original spec.

Obviously if you don't mind changing to non-original wheels then you have a much wider choice, but bear in mind that the main reason why there is this disparity in front and rear tyre pressures is weight. The Murena 2.2 has a 41/59 front/rear weight distribution, and when loaded with a full tank and some luggage can even approach 35/65 I would guess.  So you need the higher rear pressures no matter what wheel/tyres you use.
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krede
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« Reply #25 on: July 06, 2007, 09:14:49 pm »

Well.. back in 81 the murena 2.2 was "fair" priced In Denmark (at aprox 180000dkr) ...
But the car was never "in stock" and had to be imported through special order.
And..just a couple of years later, in 84 the price of the car had risen to 300000dkr for the Murena "s".... and that was alot.
Maybe you got them cheaper in the UK because they were only made in left hand drive?   
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Anders Dinsen
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« Reply #26 on: July 06, 2007, 09:42:21 pm »

Obviously if you don't mind changing to non-original wheels then you have a much wider choice, but bear in mind that the main reason why there is this disparity in front and rear tyre pressures is weight. The Murena 2.2 has a 41/59 front/rear weight distribution, and when loaded with a full tank and some luggage can even approach 35/65 I would guess.  So you need the higher rear pressures no matter what wheel/tyres you use.

Tyre pressure and handling is a complex subject and as far as I understand even today still not fully scientifically explained.

But I am certain that the Matra engineers did a good deal of playing with the Murena on the test track to get it right, and personally I would only divert from the recommended numbers (even on non standard rims) after testing on a skid pan!

Roy, don't you think the recommedation for the low tyre pressures would be to increase comfort?

About the offset and only because I hate being corrected (joking!): My original workshop manual specifies 29 mm both for the 5 1/2 inch and 6 inch rims Wink

- Anders
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michaltalbot
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« Reply #27 on: July 06, 2007, 09:43:34 pm »


Maybe you got them cheaper in the UK because they were only made in left hand drive?   

I am not sure, but think that on International Simca Meeting in 2006, there was one British Murena with RHD, but maybe I am wrong...
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michaltalbot
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« Reply #28 on: July 06, 2007, 09:55:37 pm »

The Murena 2.2 has a 41/59 front/rear weight distribution...

Exactly! It's not bad. One day on race circuit can teach You how to feel where is the beginning of slide and how to respond. It's much better than cars with front wheel drive.

Yesterday I was sitting for couple of minutes and only watched my Murena. She looks so light, sporty with accurate portion of agresivity, every detail is on its place, all is in harmony... I love this car  Wink

Tomorrow at 4:00 AM we are starting our trip to Germany to visit the big Matra meeting, I am soooooooo looking forward to be there!!!!!!!!!!!!!!  Grin
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krede
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« Reply #29 on: July 06, 2007, 10:01:04 pm »

A few Murenas has been converted to right hand drive... but none of these were made by Matra themselves
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