Bump Steer

[Spider]

Dutchacme, thanks for finding those Graphs, they are quite telling. I haven't yet considered a MKII Rack, MKI Arm configuration nor Bump Steer under Cornering - yet! One bite at a time for me!! Up to now, I've only been looking at the action in a straight line, but taking in to account road surface. There is some software available for which I'm likely to obtain only to speed up analysis and make it a little less teadious when looking at options.

The difference in lengths (per side) between a MKI and MKII racks is 0.230", but makes a big difference down on the road. The 'Ackermann Angles' are both a blessing and a curse! It gets considerable worse though when the rack is moved further forward or back from the line between the ends of the steering arms. What's not often stated in any litterature on this aspect is the Angle to consider is not one which intersects the centre line of the rear axle, but the angle between the Steering Arm on the Hub and that of the Arm from the Rack, though not an overiding aspect in a Mini as the option of moving the rack fore or aft is very limited indeed, however, without check 'on paper' I suspect the negative Ackermann you were seeing is due to the rack being in a raised location above the Steering Arms.

So, is it fair to say that the effect on a Mini is undesirable? As I have mentioned, I'm looking at road use, not track or competion use. It does appear that this has been purposely Engineered in to the original design and while it flys in the face of normal 'logic', my wanting to understand it is to see if it does have a useful purpose and is desirable to keep. I am of the impression that Jack Daniels and his guys knew exactly what they were doing.

While I see it and understand it - on paper - I'm still trying get get a handle on it in real life. When I drive these cars, I must say, I don't notice the effect when say going over rough surfaces in a straight line, if all the front end parts are 'matched' (ie, MKI Rack, MKI Arms), yet when there is a mis-match (MKI Rack, MKII Arms) the effect is quite noticable.

[JC T ONE]

A race mechanic have told me that some of the Danish Mini racers in the 60ies relocated the steering rack plate/bracket up on the shell,
in order to prevent the bump steer, but it was all kept "hush hush / in the dark" as they werent sure if it would pass the race officials .

[Spider]

A race mechanic have told me that some of the Danish Mini racers in the 60ies relocated the steering rack plate/bracket up on the shell,
in order to prevent the bump steer, but it was all kept "hush hush / in the dark" as they werent sure if it would pass the race officials .

Bhahaha,,,, wouldn't be the first time,,,,,

It seems it not a case of thou shall not cheat, but thou shall not get caught cheating,,,,,

850man (if he's about) has a bit to say on that, but for another time, another thread,,,,

<edit: http://forum.minidriver.com.au/index.ph ... d-classic/

Yes, those who make the most noise from suggestions of it are the ones who have the most to worry about. 850man, my offer still stands, if you'd like to lodge a protest, I'll happily stump up the cash. Be worth every cent to have their pants pulled down once again >

[Spider]

Dutchacme, was there any tweeks you did following your exersise here?

[850man]

A race mechanic have told me that some of the Danish Mini racers in the 60ies relocated the steering rack plate/bracket up on the shell,
in order to prevent the bump steer, but it was all kept "hush hush / in the dark" as they werent sure if it would pass the race officials .

Bhahaha,,,, wouldn't be the first time,,,,,

It seems it not a case of thou shall not cheat, but thou shall not get caught cheating,,,,,

850man (if he's about) has a bit to say on that, but for another time, another thread,,,,

I am always here, watching. mwah hahaha...

[dutchacme]

Spider,

I made a doughnut compressor to asses the spring stiffness of the doughnuts. In the end I ended up with front doughnuts softer than the standard ones (handfull of holes) and the so called competition doughnuts at the back with a 16 mm anti roll bar.



Tried straight steering arms with a reduced angle. The idea is not to worry about straight line bump steer but to use the bump steer effect in cornering to have the inside wheel rotate less due to the body roll. Effectively lengtening the rod between the rack and the arm.



Had ball joint extenders made not so much to effect the geometry of the supsension (camber) but to effect the side ways position of the roll centre line in order to have reduced weight transfer.



I also favoured lots of castor, kind of lifting the outside wheel in cornering and making the inside wheel not lift so much, reducing mass transfer.

Most of the gain was in tight corners. Went overboard with a data logger to make sure that the modifications did improve lateral acceleration.

[Spider]

Dutchacme, many thanks. There's a fair bit in all this and quite a lot to digest and properly take in to account, but none the less, your changes here have given me some places I could look if I find I need to adjust things.

Definitely food for thought.

I too have made measurements of the rates of the various cones that are available and even come up with some special trumpets to fine tune the rates and the 'shape' of the rate.

[dutchacme]

Spider,

have you given thought, played around with the metal cone inside the doughnut ?





It is as whether the rising rate is governed by the inside shape of the metal cone. The softer part of the spring curve beeing the shear zone in the doughnut, followed by the compression zone and in the third part of the curve the rubber is completely in contact with the metal cone.

Wim

[Spider]

On the back, that's the one on the Subframe itself? If it is, we cut them out



Again, I stress, I'm not looking at competition or track use!

We found by leaving them in, they do noting more than break Trailing Arm Pins. Here's one I caught, just befor eit went





We also fit Coil Over Assist units (though I have Air in mine)



I've found - for our use - the most 'tuning' can be gained from playing around with the size and shape of the flange on the trumpet, on the fronts we run 'specials' however after trying lots and lots of different profiles on the back, we found the stock Mini Van ones to be perfect.