Bump Steer

[Spider]

I'm just doing some detailed analysis of the Mini Front Subframe, Suspension and Steering.

One thing that I am curious about and hoping someone here maybe able to shed some light on.

The Mini does have designed in Bump Steer.

What it does is Toe IN on Compression and Toe OUT on Rebound.

This appears to be at odds to all (that I've read up on) Engineering Philosophies on the subject. It is often stated that Zero Bump Steer is the most desirable though some Toe OUT on Compression maybe desirable for stability.

So, in the true tradition of the Mini, it goes against most Engineering ideas. Clearly from the way it behaves, it works and they did get this very right.

I'm just trying to get a better understanding of it, whys and wherefores if anyone can shed any light.

Just as a post note, I'll add that the Design and contruction of the Front Subframes of these cars is nothing short of clever and supurb, fantasic bit of engineering.

[Dr S]

So as I understand it, bump steer is the change of steering angle through out the suspension travel.

You can minimise it my making sure the trackrod end to Hub is level (era turbos had spacers as they came lowered as standard).

If the initial state is that there is no toe, the if the rod is dropping down to the hub then as the linkage levels out it effectively gets longer. This has to push the back of the hub out. Causing the toe angle to change. This effect is reversed as it comes back to rest.

I don't understand how you can get toe on / out in different compression rebound strokes. The toe is increasing and decreasing but it's the same each way? It has to be as the axle path is the same

[Spider]

I don't understand how you can get toe on / out in different compression rebound strokes. The toe is increasing and decreasing but it's the same each way? It has to be as the axle path is the same

Thanks for reading and your reply.

The Mini Front End does it for a number of reasons;-

The Suspension Arms are unequal length.
The Pivot Point of the Steering Arm (on the rack) is above and inboard the intersecting lines between
the pivots of the Upper and Lower Arms
The Steering Arm is longer than theoretically 'desirable'
The Pivot Pont of the Steering Rod End is Inboard (only slightly) of it's Theoretical 'desired' location, however,
this is in line with just about every other car on the road.

While I have come to this conclusion here from working drawings, I have long ago noticed the effect when simply jacking the car up to do work on them and also after reset Hilos.

Next time you jack your Mini up, watch the front wheels.

[Dr S]

I fear I'm not getting it. Do you think the hub describes a different path in ascent to descent?

[Spider]

Excuse the crudity of these 'drawings', however, hopfully, these may show what's going on.

This is the Front End with all the Suspension and Steering Arm Pivot Points;-



and to achieve Zero Bump Steer, this is where the Steering Arm points need to be;-



Like I say, next time you Jack your Mini up, watch the front wheels.

[Dr S]

OK, I think we are talking about the same thing.

I'd like to amend your initial statement..
"What it does is Toe IN on Compression and Toe OUT on Rebound." quote mister spidey,

to "What it does is Toe increases on Compression and Toe decreases on Rebound."

If the rack was on the front as per a Cortina / Westfield or any number of other things then the steering arms are at the front of the hub and the converse is true.

I think with the mini its pure packing, there just isn't space to run the column to the front of the axle line.

[GraemeC]

I think Compression and Extension woul be better terms.

[Dr S]

To quote myself, Graeme and Spider then

"What it does is Toe increases on Compression and Toe decreases on Extension."

Does that cover it?

[Spider]

I think it depends on what country one is from and the general terminology used.

None the less, I think we are on the same page - now.

BTW, assuming Zero Toe at Static Standard Height (or what ever you guys want to call it), it goes to around 7.5 mm Toe IN on Compression to 11.5 mm toe OUT on Rebound / Extension. So while it does increase and decrease, it will go from Toe In to Out.

I agree that positioning the rack forward of the hubs would reverse this, but that's not what I am remotely looking at.

Just wanting some understanding or the theory of the set up on the Mini in this regards.

[Dr S]

I get it now, i think its pure packaging. It wasn't designed as a performance car. Better to put the rack somewhere handy and as long as it goes round corners under modest load its all good.

[graham in aus]

Just as a post note, I'll add that the Design and construction of the Front Subframes of these cars is nothing short of clever and superb, fantastic bit of engineering.

If I have the story correct, the original design had no subframe and Jack Daniels got the job of sorting it out when the monocoque could not take the stresses. He agonised over the design and could not get it to fit with the rubber cone in place, then he had the idea of the cut outs in the towers to allow the cone to go in sideways, then twist in the cut outs to allow fitment! Brilliant!

Well that's how Jack told the story anyway!

[dutchacme]

Spider,

have given thought to include the Ackerman performance of the Mini steering into your contemplations ?

Wim

[Spider]

Spider,

have given thought to include the Ackerman performance of the Mini steering into your contemplations ?

Wim

Yes, and in part that also contributes to the action of it in regards to Bump Steer. If you have a look at those sketches on the preceding page, you'll see in the as found one, the rod end joint centre for the steering is off set inboard from the 'ideal' location in the sketch under it. In this regards, the MKII fares better than the MKI.

Yet, when driving the cars, you don't feel or notice it one bit, however I have driven a Mini that had a MKI Rack and MKII Steering Arms, from the 'normal' designed in changes that occur, it only deviates the smallest amount (I think around 1 mm it was), yet the effect is quite noticeable. So, the Mini is very sensitive to the slightest change in steering angles (as we all know).

Every Engineering Text I've read up on and even my own understanding suggests that the action that occurs in a Mini is undesirable and would make the car feel unstable when the suspension goes in to compression, however, clearly this is not the case and it is this that I'm wanting to understand.

Thanks for your reply Wim.

[Dr S]

Here's a though, minis clearly do bump steer.

Is it the case that the minis short wheelbase wheel at each corner dynamic makes feel stable despite the bump steer? All the weight pretty much in between the axles and the driver in the middle. Perhaps you just don't feel the effect?

[ianh1968]

On a Mini-Marcos, the steering rack is mounted higher up in the body.
When mine was first screwed together, the steering was all over the
place and the cure was to make a set of "flat" track rod arms.

On another Mini-Marcos, I have seen standard arms used, but with 2"
tall spacers underneath them held together with extra-long screws.
It is clearly a recognised problem that if the arms point "downhill" too
much from the rack to the hub, the steering suffers noticeably.

Unless I have completely misunderstood, too much downward angle
would cause more toe-in as the suspension is compressed?

As regards sensitivity, my Mini has a coil-sprung rear and I had initially
built it parallel and vertical. The general idea was to make it this way,
then shim it as appropriate to get the geometry right. The trouble was,
it took me about 10 years before I actually got round to doing this.

Adding a very small amount of toe-in at the back has made a huge
difference, I am talking about a 20thou shim behind each of the
inboard pivot mounts. This gives some, but nowhere near as much
toe-in as the 1/8" specified in the manuals.

Small mods can sometimes make a lot of difference!

Ian

[dutchacme]

Spider,
it is a while since I dealt with the same issue for my racing mini. I did all the actual measurements with a bump gauge. Essentially what a bump steer gauge does is measure the toe-in as a function of the wheel lift. The rubber doughnut is removed and I used the jack to lift and lower the hub. I will try to find the actual toe-in / toe-out graph as a function of the lift.

What I remember is that a mini has clearly bump steer, however the actual up-down movement during ride (not cornering) is of such an order that the toe-in does not dramatically change and is not noticeable. I assume that radial tires are more forgiving (suppressing) in this effect compared to diagonals.

What the "built-in" bump steer seems to do is that under roll the inner wheel gets more toe-out due to the drop bump steer action.
The mk1's seem to have had an Ackerman setup, the angle of the steering arm in top view, which would aggravate this once more. Under roll ( 6 degrees and more) the inside wheel will toe-out due to the bump steer and it will toe-out due to the Ackermann. In tight corners (radius smaller than 15 meter) at high speeds (in the order of 1g lateral on a dry surface) you will definitely notice the not so beneficial effects. I do not remember the amount of steering wheel rotation, must have been about half a turn. Reducing the roll in this specific situation will help and using a flat steering arm (side view) in side view will help. It is as whether the engineers deliberately detuned the road holding at the front. In corners with a large radius the effects are hardly noticeable.

In rainy racing situations in 15 meter radius corners typically the car will roll less and only the negative effect of the Ackerman is felt.
I seem to remember that the negative Ackerman got worse with turned-in/turned out wheels.

What also seems to come into to play is the movement of the virtual roll line, in cornering it is as whether the roll line moves to the outside making that the outside wheel does not suppress so much and it seems as whether the car lifts the inside over proportionally (drop the wheel more). In racing the hand full of millimetres the outside wheel go in would not create any noticeable bump, however the drop of the inside would create loads of toe-out. Under 1 g cornering (Dunlop Cr65’s) maximum load transfer will occur and this makes that the inside wheel is lightly loaded and can get away with the too much toe-out (scrub) going unnoticeable.
The evolution of steering arm angles (top view) is interesting, from the well known mk1 to the mk2s to the mk5's the negative Ackerman gets reduced.


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[Spider]

ianh1968, Yes, spot on with that. That is 'on paper' what I'm seeing. Even in a stock Mini, lowering the rack could only be a good thing, though, not at all easy.

dutchame, many many thanks for your detailed reply. Your findings with your Bump Gauge are very much what I am finding on paper, though, at this stage, I've only made 'plots' at static height, full Bump and Full 'Droop' (Rebound / Extension). I must also add that what I said about the effect not being noticeable, was in a road car on road tyres, rubbered suspension (ie not rose jointed), so pretty much 'standard'. I did consider doing plots with a Bump Steer Gauge (yours is very nice it of kit indeed) and may still do just that.

I'll re-read your post several times yet, to get my head right around it. None the less, it does appear that we are on the same page in this regards.

I may have an 'opportunity' to design some Bump Steer out with something I'm 'toying' with at present, however I just wanted to try to get my head around what was going on in a stock Mini first, though from your experience, if we can engineer at least some of the type of Bump Steer that the Mini has, out, that can only be a good thing.

[dutchacme]

Found the two bump steer graphs, one for 1275 GT arms and one for mk1 arms, both for a mk2 rack. The bump travel is measured from the top of the arch to the centre of the hub.

Clearly the mk1 arms have a more severe bump steer compared to the 1275GT (eq. to Cooper S) arms.

[ivor badger 2]

I caught the end of a suspension talk at the Racing car show one year and roll centres were being discussed. So I waited till the talk was finished to have a word with the lecturer. Basically, with a lowered mini, the front roll centre must be close to the road or even below it, so how do you get any feel to drive the car?
Simple answer. A race mini is so stiff with so little suspension movement, the car rolls on the outside tyre and all the steering geometry and roll centre doesn't matter apart from the basic camber and caster and track alingment. How much does anyone notice of steering feedback on a mini with an LSD? Bump steer ?, that is totally lost inside all the other things coming back up the steering column.

[dutchacme]

Ivor,

I guess it comes down to the objective which is set for the suspension; a roll centre in the ground would be great if one wants no suspension angle changes. Most of the times this goes hand in hand with a tyre which is more than capable to handle weight transfer and to generate ample lateral forces (like slicks). CR65s are quickly overloaded so taking care of the bump steer and the movement of the roll line is quite beneficial.

With the roll line moving down more load transfer will happen, meaning the outside tyre will see more load and depending on the tyre it will at a certain moment stop generating more lateral resistance. Moving the roll centre up less load transfer will happen, meaning the outside tyre will not so quickly reach its maximum lateral force and the inside will contribute more.

For example in a 1 g turn the weight transfer is:
left front right front
147 kgf 284 kgf

left rear right rear
22 kgf 251 kgf

With the bump steer and the negative Ackermann the inside wheel is not truly contributing to the total lateral force at the front. Vizard in his first Mini book makes a cryptical comment, do not make the rear-end grip less make the front-end grip more.

Also the hardness of the springs/doughnuts at the front comes into play, softer springs are better for less weight transfer, so keeping the car flat is then down to the anti roll bar at the back.