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Replicar328
#1 Posted : 06 January 2019 11:24:31(UTC)
Replicar328

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Please forgive if this is not the right place to ask this question?

I've acquired a crashed 2005 MX5 to use as a donor car for building a replica 1936 BMW 238 which needs to have a track width of around 1200 mm to keep the wheels inside the wheel arches!  I'm hoping to use the existing uprights (front and back) with a combination of narrower wheels, shorter wishbones and moving wishbone pivot axes inwards to achieve this.

But I'm concerned that the wishbone axes will no longer be in a line with the CV joint at the differential.  So does anyone know whether the drive shaft/CV joints have some axial play in them, and if so, how much? And if not, is there a way to get around this?

Any thought most welcome!

NickD
#2 Posted : 06 January 2019 11:39:55(UTC)
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Originally Posted by: Replicar328 Go to Quoted Post

Please forgive if this is not the right place to ask this question?

I've acquired a crashed 2005 MX5 to use as a donor car for building a replica 1936 BMW 238 which needs to have a track width of around 1200 mm to keep the wheels inside the wheel arches!  I'm hoping to use the existing uprights (front and back) with a combination of narrower wheels, shorter wishbones and moving wishbone pivot axes inwards to achieve this.

But I'm concerned that the wishbone axes will no longer be in a line with the CV joint at the differential.  So does anyone know whether the drive shaft/CV joints have some axial play in them, and if so, how much? And if not, is there a way to get around this?

Any thought most welcome!

If you think about it, the existing wishbones do not pivot around the cv joints and so the arc is always going to produce some axial movement as will as the need to allow for heat, tolerance and simple compliance in bushes so there always "play" or movement in the splines of drive shafts or even in the design of the tulip style cv joints, if that however is a sufficient movement for you will be down to your design. 

Replicar328
#3 Posted : 06 January 2019 12:06:00(UTC)
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Many thanks for this quick response.  I'm sure you are right, I'll find out how much play when I dismantle...

Aldi
#4 Posted : 07 January 2019 12:52:25(UTC)
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Hi Replicar 328 - good choice of replica - they look a million dollars especially in silver like the originals.

One point to bear in mind is that the track reduction, if applied to front as well as rear axles, wil change the Ackermann angle and steering geometry, so might need the track rod end positions adjusting.
I don't mean just the shortening of the track rods from the rack, which will obviously need to be accomodated- and I don't know how much leeway you have in the existing track rod ends, but also the angle of the arms themselves to maintain correct steering geometry.
The inset of the arms carrying the rod ends away from the kingpin axis is a function of the track and wheelbase, and it is usually taken to be that the axes of the 2 arms intersect at or near the centre of the rear axle (in effect - centre of rotation of the vehicle) for true Ackermann steering.
(For thos unfamilair with the term - it means how much difference there is between the inner wheel's steering angle and the outer one when a steering input is applied)
If not set up correctly, it can generate tyre scrub and loss of adhesion, as well as increased steering input loads.

Depending on how much you are changing the track and/or wheelbase, it might need a 'good coat of looking-at' before ypou commit to building it.
Excuse me if I 'teach Granny to suck eggs', and this may have already been considered, but I know it can go wrong since I helped a mate build a Porsche Spider 'replica' from a VW beetle, and that was a nightmare until we corrected it.

Just my free advice - money back if not happy!

Aldi
Don't drive it like yer stole it...not all the time, anyway!
Replicar328
#5 Posted : 10 January 2019 11:28:31(UTC)
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Hi Aldi,

Thanks for your input, I appreciate it more than a Lidl Cool.  They not only look like a million dollars, you'd not get much change from a million if you bought one!

For the rear, I'm hoping I can get away with using the existing wishbones without the inner pivot points interfering with the diff. If not, it'll have to be fabricated wishbones.  The design won't be perfect, but I'm more concerned about handling than road holding. I think by just moving the inner pivot point of the upper rear wishbone up or down I can raise or lower the roll centre respectively.

I've yet to look at the front, but you are completely right about Ackerman.  Because the wheelbase of the replica is a little longer than the MX5 I get a bit of help with engine being further back (leaving the prop shaft unchanged).  If I can't get the front wishbones to fit, which is quite likely, I will have to re-design the whole front suspension. I've even thought about using a transverse leaf spring for the top 'wishbones' - an underrated concept in my view, also used on my old Triumph Roadster.  But if I can get away with using the same wishbones and uprights, apart from needing to shorten the steering rack, I think the positive ackerman can be reduced by moving the steering rack forward?  I believe the actual ackerman angles are never as positive as the theoretical geometry dictates because of slip angles on the outer tyre - see the negative ackerman on Formula 1 cars cornering - but most of all, I like your 'good coat of looking-at' comment!  I'm bound to get some things wrong, but I don't plan to cut metal until I've done the maths.

It would be nice to find drawings with dimension of all the four wishbones (2 front, 2 back) and their corresponding uprights? That would go a long way to allowing me to design on paper...

All thought gratefully received!  My Granny never knew how to suck eggs...

Adrian

 

Drumtochty
#6 Posted : 10 January 2019 17:35:53(UTC)
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