10 replies to this topic

[SolarB]

When restoring my Mini years ago I was intending to take the VTEC transplant route and in preparation I moved the steering rack mount approximately an inch to reposition the rack to clear the larger Honda differential. In the end I never actually took the VTEC route and now with the car about to undergo a bare metal respray it could be the time to return the rack mount to its original position.

 

So, a couple of questions.

 

Would the moving of the rack approximately 1” diagonally affect the Ackermann angle enough to make a difference to the steering? The handling was never quite right and perhaps this was the reason. Having said that I never checked the geometry of the subframes or their alignment relative to each other so I can’t guarantee everything else was correct.

 

Could someone give me the dimension between the rack mount and the underside of the engine bay cross member? I can’t remember which direction I actually moved the mount all those years ago. The car is a Mk3 but I assume the position of the underside of the cross member is standard for all Minis.

 

 

 

[Ethel]

I'm not sure it is as the Mk4 subby mountings sit that little bit lower. It's one of those things you don't care about enough until it matters to check out properly, but back in the day a Mk3 front panel was cheaper option for a while & holes didn't line up, even though they didn't need to. Mind, it would still be well within the range of the front suspension travel.

 

The Ackermann angle is in the steering arms. As long as the axis of the rack is still parallel to the axis of the from axle I can't see it being affected. By moving the rack you'd just be altering the effective leverage - you're still creating a force perpendicular to the "kingpin" regardless of how the track rods articulate, it has to be a specific distance from the kingpin axis.

 

The thing you could alter is  bump steer, as you've got 2 suspension arms and a "steering arm" (aka the tie rod) all acting in the vertical plane. 

[Bobbins]

The horizontal position of the rack will mostly affect the Ackermann angle, the vertical position will affect bump steer ... as does changing the suspension height.

 

Is there no evidence on the toe board as to where the rack mounting panel was originally? Mounting bolts or the big hole for the splined section?

[stuart bowes]

I can't really measure mine easily now I have the front subframe and steering rack back in

 

but I can show you a picture

 

 

and a load of other pics here of pretty much the same thing https://www.theminif...ild-998/page-23

 

do you still have the pressed in 'lumps' where the spines go through (LHD or RHD) the bracket is fitted basically evenly in the middle between the two

Edited by stuart bowes, Yesterday, 04:58 PM.

[sonscar]

My understanding is the Ackerman is due to the angles of the hubs steering arms.Rack position affects bump steer and toe in turns.Steve..

[Spider]

The position of the rack is fairly critical. Changing it results in bump steer.

[nicklouse]

Oh the joys of suspension.

 

True Ackaman is where the angles made between the upper lower hub swivel points and the steering arm pivot intersect with the center of the rear axel. So you might think that the position of the rack has little affect. But it does more so with bump steer character but as you will need to change the length of the trackrods to accommodate the new position of the old rack the relation between the motion of the hubs will change.

 

there is software out there that will work all this out easily but pre 2000 we made scale models to see what happened.

 

as to putting it back in exactly the original position….. there might be a better place but where I don’t know.

[Ethel]

The simplest to get your head round is a live axle with a single, rigid, track rod connecting the steering arms as you'd find on an old school HGV, or Series Landrover. As wheels are steered, the steering arms follow a circumference around the "kingpin". Because the drag link ball joints are also slightly inboard, on a line  between between their respective kingpin axes and the centre of the rear axle, they behave with a similar characteristic to a piston conrod & the crank in your engine. On the side you're steering toward, the angle between the steering arm & drag link increases, in effect add more of the length of the steering arm to the drag link. For the outside wheel the rod to arm angle decreases (like a crank & conrod at bdc). So the inner wheel steers more & by the magic of geometry that Ackermann angle (pointing the steering arm at the middle of the of the rear axle) causes the road wheels to describe concentric circles as if your wagon was tethered the common centre point with a line right down the middle of the rear axle. A Mini isn't that different (in the horizontal): The steering rack replaces the track rod with it's own track rods compensating for the fixed rack not pivoting between the track rod ends as you steer.

[Spider]

Brilliant description Ethel

[SolarB]

Thanks for all the input guys.

 

I cut out a rectangular section of the toe board that included the bumps, column hole and rack mount and moved the whole thing. Still can't remember if I repositioned it higher or lower.

 

Looks like the consensus is that the bump steer is more of a worry for me. Shame I have taken it all apart as I could have measured what bump steer I have with the current setup. If I get a chance I might rebuild the suspension and put the subframe back in and measure it.

 

If all goes to plan the shell is off for sandblasting in a month so it should be clear afterwards how far I moved the rack.

[Ethel]

Here's an interesting topic, wouldn't like to say how directly relatable it is - regarding the rack location relative to the steering arms.

 

Sounds like you could use some measurements to  create a jig, that locates on the inside of the toe board, to bolt the rack to using the U bolts. Can't think of much that'd make for a precise datum on a MK3 (no rear subby mountings), but if somebody doesn't mind lifting their carpet it wouldn't be much effort to measure some perpendiculars from the ends of the U bolts to the top & bottom fold lines of the toe board.

 

There's also the heater mounting "hooks". A tad random & unrelated, but I expect Longbridge didn't take any less care in positioning them than the did any other bits of body shell.

 

I'd offer, but my toe board is around 1cm thick fibreglass.