66 replies to this topic

[Spider]

KPI is always going to be added to, or deducted from, the caster, so it will always be compromise on the Mini geometry - not that it can't also be an improvement. It's just nice to have another option to get roll, caster, camber and offset working for you.

 

Thinking aloud again, how much difference will longer steering arms actually make? If the rack is in the same place, its axis will still intersect the steering arm in the same place, giving the same lever rate. Of course, it's much more complex given there will be differences in the other 2 dimensions.

 

I'm 99% sure that KPI is measured with the vehicle on the road at normal ride height, so takes in to account Camber, though Caster will have a very small effect here.

 

Longer Steering Arms - whoa - my brain is doing flip-flops just tying to work that one out!  I'll just say that it's actually far from 2 dimensional, you can start with 3 dimensions - which will get you to a static ride height what's going on, cause and effect etc, but then start throwing in suspension movement and it really gets 'busy' to say the least.

 

Just considering the leverage aspect, longer arms will actually increase leverage, so apart from all other effects, the steering effort is reduced but more steering input will also be needed. Also, it will increase the over all turning circle, but if you also consider the actual Ackerman Angle it could also improve on this with some possible added turning scrub, if the Ackerman Angle is increased. But that will also increase Bump Steer.

 

It's the Cat chasing it's own tail at times!

[Ethel]

KPI is the the (fixed) angle the steering axis makes with the plane the wheel spins in, as I understand it. Think about an actual king pin on and old style rigid axle.

 

I meant the 2 dimensions on top of the one of the rack moves back and forth in. I find it hard enough to visualise this stuff, so apologies if I'm failing to put it into words too.   

 

The leverage thing is my point, you'll still be pulling the arm with the rack that's the same distance from the (steering) axis through the BJ's. Think about it in an instant in time and the arm and track rod just make up a tie with a bend in it.

[mini13]

LOL indeed it is a cat chasing its tail

!

 

its ween a while since i looked at all this stuff, but from memory the metro KPI once reset to give the correct camber cause the scrub radius to reduce by approx 10mm by moving the steering axis out toward the ctyre contact patch, so helps in this respect, also the added angle creates more self straightenuing affect to the steering,  but that can largly be ignored or adjusted witht the castor angle, from memory you wont need as much castor with metro hubs.I belive i ended up with 4 degrees. rather than the 5 deg I normally run

Edited by mini13, 28 September 2016 - 11:00 AM.

[tiger99]

Changing the arm length does increase "leverage". It is not counteracted by anything to do with the rack position. The effect of track rod angles is significant but last time I looked the outer ends were forwards of the rack, so fitting longer arms makes them more directly in line with the rack and results in a little bit more leverage on top if what the arm length increase is giving you.

[robbob1980]

Incase anyone is interested. I measured my bumpsteer yesterday with standard mini setup. I used a similar method with a laser on the disc pointing out onto some wood which was 1.5m away. From full droop to full bump the laser only moved 3mm total which wasnt too bad I thought.
Out of interest I spaced the steering arms up 10mm like someone said the ERA minis were. Laser moved about 100mm! Towing out under bump.
I then fitted metro arms for the extra leverage as I have a heavy vauxhall lump. Measured bump steer again and the laser moved about 10mm total. Quickly fitted a couple of 2mm washers under the arm and measured again and the laser was within a couple mm.

Rob

[racingbob]

just read thro all this got headache now glad my car drives perfect and no bump steer. need to get some panadol

[Bat]

Hi,

I've had metro hubs for years on my mini with a 1380 in it and never really given it a second thought.

I used my phone as a quick measure and look to be running 1deg pos camber!

I've got vented discs, 4 pots and flanges, all metro.

I'm looking to get adjustable bottom arms, already got the tie rods.

So do I go back to std mini hubs or stick with the metro ones and space the steering arms 10mm like ERA did?

Yes I've got 13" wheels and like them sticking out!

Cheers 

Edited by Bat, 03 November 2018 - 08:20 PM.