Mk 1 Cooper S steering arms and Ackerman angle

[Mike]

This may not be helpful to you Vegard as you are selling a pair of arms and I do apologise but I now have a couple of sets of the heavy duty early Mk1 arms spare as we removed them from my FIA type racer.

The reason, the Ackerman angle is wrong and was the cause of the handling problems we had suffered for two years since we started racing this car, firstly with a 970 and latterly a 1275.

Donington Park Masters was the first time we raced with the later Mk2 arms, also changing to a Mk2 steering rack, and the car was transformed, qualifying our Mini as fastest amongst the pre-66 up to 2 litre race and on pole by a second for the all Mini race.

I know when the arm geometry was changed BMC's publicity said it was to facilitate a change of turning circle but you don't need to change the Ackerman angle to achieve this.

Was it because the boys racing on the circuits discovered this at the time and the factory took action accordingly but offered a smoke screen reason? I'm sure someone will have the answer.

[sandman]

Mike,

I assume you guys were running a "Mk2" type rack / quick-rack with Mk1 arms ?

Could you describe how the car handled prior to you correcting the problem?
And what changed after you did?

[mini13]

I've been wondering if the Mk1 racks were a different length,

[Mike]

Hi Ed,

In complete ignorance we originally had a Mk1 steering rack with Mk1 Cooper S steering arms.

The characteristics of this were on initial turn in the car just hopped and would not go in the desired direction until the steering lock was reduced towards the exit of the corner. Aggressive or smooth, complete changes of damper settings to the extremes of their adjustment made no difference. You have to understand that after 30 years racing rear wheel drive this was my first real foray into front wheel drive so I had little data to analyse the causes.

Immediately the arms and rack were changed for Mk2 versions, the car was completely transformed. Although we were still running our only set of knackered CR65's (which were coming to the end of their second season) the car started to go in the direction we were steering although the set up was still far from perfect.

I have now constructed a correct ride height jig for the front subframe on which, with the cones removed, I can simulate roll and pitch and can measure the true Ackerman and set at the point I desire it to be in lock by shimming the steering arms accordingly.

Once set up it can have the cones re-fitted and re-fitted to the car.

Mike

[sandman]

Interesting stuff Mike.... although I fail to see how shimming can help alter the angle of the arms... ?
Got a photo or scetch that can help out?

[Mike]

Ed,

The angle can't be changed so to be technically correct it wont be altering the Ackerman but adjusting the steering arm height to eradicate the effect of bump in roll or pitch at an assumed amount of steering lock applied.

No sketches but I might photograph the jig in use in the future.

Mike

[JC T ONE]

Many lowered Race Mini,s suffer from this fact that when lowered, the angle change, and to help this, they use to move the steering rack up,
to keep the correct angle on the arms.
Dont know if its "legal" on a FIA homologated car?? but most teams did it here in the 60ies.
A side benefit in doing this, you get a better angle on the steering wheel

[Mike]

I suspect moving the rack has been done but probably contravenes the rules. My rack will be staying in the correct position.

We have worked hard on our 1460cc racer to get the geometry exactly correct in fact as on our Sebring Sprite neither cars have any bump steer throughout the full travel extent of their suspension.

It has proved a worthwhile exercise.

[dutchacme]

Mike,

I guess that you will find that the inside wheel will over steer, it will turn in too much compared to the outside wheel. (The BMC gods will have an explanation for this anti Ackerman effect). This over steering of the inside wheel gets worse if you add 5 to 7 degrees of typical slip angle for your outside wheel, where as the inside wheel is lightly loaded and can be assumed to have a minima slip angle. And of course the over steering gets worse for tighter corners.

You can use bump steer to prevent the inside wheel to over rotate. Theoretically this calls for reducing the “height” of your steering arm and straightening the arm (in a top view, bringing it closer to the tire) and you have to allow for somebody roll. This is a fundamentally different approach from trying to create the perfect bump steer free and perfect Ackermann set-up.

A subsequent contemplation is to make the inside wheel carry more load during cornering, this can be achieved by more castor angle. With significant castor the inside wheel will move up more (in a corner) and the inside wheel not move up so much. Making that the outside wheel will see less normal force compared to a lesser castor set-up and the outside wheel will have to carry more. Making that in the end you will increase the max lateral friction force.

This all is a paraphrase on a Vizard statement; “do not make the rear grip less but make the front grip better and more”.

970 racer

[Vegard]

This was covered in the last forum, however this might be of help:

http://www.shag.no/default.asp?itemID=2 ... =105&h=BMC steering arms&headline

[Vegard]

These look great

http://www.minitastic.co.uk/home1.htm

[sandman]

Well, I had a look at my own racecar yesterday and whaddya know.... Mk1 steering arms!

Funny thing is that I kinda like like how my car's working... but it is looooose in the back, tho...

Mike, have a look at some on my incar videos - does any of this look like what you experienced?

Lates from 2009 with current damper and spring set up: http://www.youtube.com/watch?v=NL1iy1TKtRI
(The Porsche infront was veeeery slow so I'm not on my regulare pace....)

From 2008 with "regulare" Gaz dampers: http://www.youtube.com/watch?v=kwu4gZ8RmYA

Any input welcomed.

[Vegard]

And you've got a Mk2-> rack?

[Mike]

Hi Ed,

It's hard to give an opinion from the videos as it is how the car feels from the driving seat.

We experienced severe hopping as the tyres were fighting against each other rather than a progressive slide. Immediate noticeable improvement once changed.

All I can suggest is - provided as Vegard says you have a Mk2 rack - fit Mk2 arms and see the difference.

Good luck,

Mike

[sandman]

I'm not sure what kind of rack I have.... I was under the impression it was a quick-rack... but late "discoveries" has me thinking it might be a Mk1-type.

Anyway, I've been doing some thinking and wonder if not Mk2 arms are the way to go regardless of what type rack.... the altered Ack-angle being a bonus for our usage.

Is there a difference in distance of travel between the racks too - or is it just the rack/pinion ratio ?

[Vegard]

I bought this beautiful red swan in England a few years ago. I always felt that it was hideous around the roundabouts in England. After some inspections I found that it indeed had Mk1 arms and a newer rack.



It's the only experience I've got on MK1 arms and mk2 rack, but it was indeed shite.