Subframe swap, tale of woe. Opinion needed.

[Pie-eyedpiper]

Over the Christmas break I replaced my original subframe for a PQ 25 one from an A1. Pressed steel wishbones and the chance to change bushes.
I changed the shocks, springs, ARB bushes and wishbones. I tried to use alignment pins but they didn't really fit in the PQ25 bolt holes, using the a marker pen dirty marks on the body work to "align". The subframe is a different design, so I'm not surprised alignment was way out. As shown -

What I didn't expect was the very heavy, notchy steering. After fitting a new battery and topping up the power steering fluid, I managed to get it into my local VAG specialist garage.
They had a look, ran some checks, and diagnosed a faulty steering rack. No warning lights on my dash, my VCDS didn't pick it up. Should it have done if faulty?
I obtained another steering rack from a club member. The garage has fitted that, and are saying that has an internal fault too.
I'm seriously doubting both racks are faulty, and the first approach would be to get the alignment as close as they could. They haven't seen the printout yet, but I have told them it's way out. I would have thought they would have an alignment jig that check it out though.
Any thoughts?

 

[Robin_Cox]

the >5 degrees of ?caster? on both sides of the front axle in both sets of measurements looks iffy to me - doesn't this generate excessive self-centering and therefore perceived steering weight (both to you and the steering power assistance)?

 

[L162LZ]

Does this mean that your subframe needs shuffling back a bit to reduce the 5deg caster angle. Did they try to align the subframe ?

 

[Robin_Cox]

or forwards? To lessen the angle between lower pivot and top of strut to 0.5-1.5 degrees?

 

[Howey]

I take it the steering was fine with the original factory set up and then went "faulty" with the pq25 subframe? Then a replacement rack did the same?
Your either unbelievably unlucky or something else is wrong.
Are those readings taken from before the pq25 was aligned and after or before with original set up and after with pq25?

 

[Pie-eyedpiper]

the >5 degrees of ?caster? on both sides of the front axle in both sets of measurements looks iffy to me - doesn't this generate excessive self-centering and therefore perceived steering weight (both to you and the steering power assistance)?

Thanks Robin, you might have it.
The steering is happy in the centre, and feels quite light there. When turned more that about 10° - 20° it is really heavy and notchy.
When I changed everything the wheel had turned to the left by about 30°. It was straightened back up when the tyre company attempted the tracking.
I'm hoping subframe alignment might help things.

 

[Pie-eyedpiper]

I take it the steering was fine with the original factory set up and then went "faulty" with the pq25 subframe? Then a replacement rack did the same?
Your either unbelievably unlucky or something else is wrong.
Are those readings taken from before the pq25 was aligned and after or before with original set up and after with pq25?

The steering was totally fine before I changed things.
The printout readings are both with the PQ25 subframe, the first one as it went in, with the suspension changes, then the second is the best they could get it, without touching the subframe at all.

 

[audifan]

There is something amiss in the subframe install then, or an incompatible part.

They should be capable of aligning the subframe on the rig if it is an alignment issue.

 

[Robin_Cox]

I wonder if having a more detailed look at the German A2 forum wiki where the PQ25 subframe swap is described - or if asking some of them with more experience in this change may give more insights.

From my recollection of what the caster/castor angle means - it all goes back to bicycle forks. With a typically just behind-vertical headtube angle in the axis of the frame, reasonably normal forks position the front axle centreline a few cm in front of the steering post centerline which should be unstable as it can tend to rotate in either direction as soon as it gets unbalanced. But our handlebar ends are wide and have lots of leverage, so we are able to maintain control of the bike, which steers easily. However, push the same bike unridden forwards without constraints on handlebar movement and it will fall over in a few meters. Reverse the forks so the axle centreline is behind the steering post centreline, and on a good flat surface you can propel the bike forwards riderless and it will keep rolling for many tens or hundreds of meters until it runs out of energy as the steering geometry now has a self-centering action. This is equivalent to cars having inbuilt steering stability. If this angle is excessive it will take a lot of energy to resolve, so clearly the angles need to be reduced - but how? Is the subframe in the wrong position (adjustable) in the fore-aft plane? Do the steering arms need lengthening? BTW - ignore my questions if you are not mechanically adept - I am all ears as well so seeking rather than providing information.

 

[Pie-eyedpiper]

I wonder if having a more detailed look at the German A2 forum wiki where the PQ25 subframe swap is described - or if asking some of them with more experience in this change may give more insights.

From my recollection of what the caster/castor angle means - it all goes back to bicycle forks. With a typically just behind-vertical headtube angle in the axis of the frame, reasonably normal forks position the front axle centreline a few cm in front of the steering post centerline which should be unstable as it can tend to rotate in either direction as soon as it gets unbalanced. But our handlebar ends are wide and have lots of leverage, so we are able to maintain control of the bike, which steers easily. However, push the same bike unridden forwards without constraints on handlebar movement and it will fall over in a few meters. Reverse the forks so the axle centreline is behind the steering post centreline, and on a good flat surface you can propel the bike forwards riderless and it will keep rolling for many tens or hundreds of meters until it runs out of energy as the steering geometry now has a self-centering action. This is equivalent to cars having inbuilt steering stability. If this angle is excessive it will take a lot of energy to resolve, so clearly the angles need to be reduced - but how? Is the subframe in the wrong position (adjustable) in the fore-aft plane? Do the steering arms need lengthening? BTW - ignore my questions if you are not mechanically adept - I am all ears as well so seeking rather than providing information.

No, I totally agree with you, with the bicycle analogy. To my understanding caster angle is all about self centering, just like any caster on a bed etc. The natural tendency is to be in the centre when being propelled forwards.
If my subframe is too far forwards then it will try to centre all the time and extra effort will be needed to turn, as in my case.
The subframe clearly needs adjusting, and to me getting the geometry right would have been the starting point.
A give away that it isn't the rack is when I had the front of the car up on axle stands to see if the fluid reservoir had air in, the steering was nice and light, with no ground resistance.

 

[Howey]

I recently wheel aligned Mr Silver following tie rods and track rod end replacement the week before. Car was driving fine prior and after replacement with good tyre wear. Below are my readings and my caster is out of spec so either my cars not right by a long way or there is a decent tolerance meaning you wouldn't have to get your caster to the optimum to at least rule it out, would be good to see what others people's readings are to compare

 

[steveb]

Castor angle is like a shopping trolley wheel that always ends up the same way round, it gives you self centering, if that makes sence

 

[Pie-eyedpiper]

There is something amiss in the subframe install then, or an incompatible part.

They should be capable of aligning the subframe on the rig if it is an alignment issue.

Definitely something amiss.
I couldn't use locating pins on the PQ25, they would fit in the bolt holes. And it's clearly a slight different shape from the original so lining my pen marks up hasn't helped much either.

I'm hoping they can get the alignment sorted before progressing any further.

 

[Robin_Cox]

No, I totally agree with you, with the bicycle analogy. To my understanding caster angle is all about self centering, just like any caster on a bed etc. The natural tendency is to be in the centre when being propelled forwards.
If my subframe is too far forwards then it will try to centre all the time and extra effort will be needed to turn, as in my case.
The subframe clearly needs adjusting, and to me getting the geometry right would have been the starting point.
A give away that it isn't the rack is when I had the front of the car up on axle stands to see if the fluid reservoir had air in, the steering was nice and light, with no ground resistance.

I have a suspicion we are talking in opposite directions here.

If the wheel centre of fore-aft axial rotation is ahead of the strut centre of rotation, it will willingly depart from vehicle direction - ie, need to be pulled back into line by the steering mechanism. If it is behind the strut centre of rotation, it will trail, and therefore need to be pushed to steer outwards (ie, self centring is inherent with vehicle movement forwards). So - if the subframe control point is rearwards of where it needs to be, so will be the wheel centre compared to the strut top bearing, and everything else we've been talking about above.

 

[Pie-eyedpiper]

There is something amiss in the subframe install then, or an incompatible part.

They should be capable of aligning the subframe on the rig if it is an alignment issue.

Definitely something amiss.
I couldn't use locating pins on the PQ25, they would fit in the bolt holes. And it's clearly a slight different shape from the original so lining my pen marks up hasn't helped much either.

I'm hoping they can get the alignment sorted before progressing any further

 

[Pie-eyedpiper]

I have a suspicion we are talking in opposite directions here.

If the wheel centre of fore-aft axial rotation is ahead of the strut centre of rotation, it will willingly depart from vehicle direction - ie, need to be pulled back into line by the steering mechanism. If it is behind the strut centre of rotation, it will trail, and therefore need to be pushed to steer outwards (ie, self centring is inherent with vehicle movement forwards). So - if the subframe control point is rearwards of where it needs to be, so will be the wheel centre compared to the strut top bearing, and everything else we've been talking about above.

My thinking is if the castered wheel is too far forwards then it will try to hold the centre line strongly, hence heavy steering to pull it off line.
If the castered wheel (ie subframe) is too far back then it will want to wander and the centre position would not be strong.

 

[Robin_Cox]

If we take the position of the upper bearing here as the strut top, and the wheel centre as the wheel centre, we've got a trolley wheel that is inherently self-centring when the trolley is going forwards. This would be the equivalent of the subframe and other components securing the wheel being rear-wards compared to the fixed point of the strut, and require more effort from the (steering) arm to move, so it is inherently stable but heavy. Lengthen the rearwards offset and it gets heavier. Moving the centre of rotation of the wheel forwards compared to the strut will lessen this effect until once past the neutral point it will become dynamically unstable and actively want to go one way or the other.

(I think - does this make sense?)

 

[Pie-eyedpiper]

You're probably right Robin. Whatever the effect, the caster on my subframe is too great and would need to be aligned better before other components are considered, in my mind.
I find it hard to believe that 2 steering racks are faulty. Mine was totally fine before I made the changes, and the seller of the other rack tells me that the car drove fine also.
BTW my car is an '02 and the doner car is a '52. I'm not sure what year the G250 sensor was deleted.

 

[Robin_Cox]

Another thought, although this is less likely. Re-reading the PQ25 german wiki, they note that the PQ25 subframe / control arm combinations need to be from the production variants with "long" arms - fitted to PQ25 models with the larger / higher performance engines. If the shorter arms (from smaller-engined vehicles) are fitted this would also result in the track being reduced and affect camber - and I note that the camber is also negative on both sides and outside specified limits. Do you know which A1 model this one came off?

 

[Pie-eyedpiper]

The subframe is a 6R0199315. Its actually from an A1 1.2TFSI, but fits a whole multitude of cars from Polo, through to SEAT Ibiza and Skoda Fabia. It's pretty generic, and came bare.
And after reading the Wiki article I ensured I got the longer arms from the part numbers mentioned.