Rack and Pinion - that old chestnut!

[ChrisDixon]

This topic has been discussed on and off. My 3.8S has a "Wilkinsons" system which comprises an XJ-S or XJ S3 hydraulic rack with the control arms shortened and a remote electric pump. So we have the fundamental problem of the rack being in the wrong place geometrically. But is this such a big deal? Some owners report no issues, others complain of bump steer and excessive tyre wear. My own experience is of responsive handling, but also lifeless steering, problems getting the tracking right and inner edge tyre wear.

No-one locally I have found seems to have the equipment or the appetite to sort it. My feeling is that camber and caster should be looked at, as well as the tracking again.

I have considered reverting to the steering box. I have all the bits to return to an unassisted setup, would this be the wrong thing to do? Other options of course being to buy a 420 subframe and associated box. Which begs another question, would my Wilkinsons electric pump work with a 420 steering box?

And finally, the issue of geometry might well be solved by fitting a rack with a shorter body. But will any rack always be in the "wrong" place geometrically?

[piman]

Hello Chris,

the geometry of rack and pinion is well known and I do understand that the various conversions for the MK2 have been wrong in some aspects.

I have not seen such a conversion but if they have errors it suggests that the correct placement conflicts somewhat with the subrame, and the best of a bad job has been done? The length of the rack is determined by the inner pivots of the track rods coinciding with an imaginary line drawn between the inner wishbone pivot points. As I say it is basic suspension geometry and is not a black art.

Alec

[Martec]

As you say this topic has been kicked up and down the road a few times.

When you consider that every new car has rack and pinion steering, why can't it be set up correctly for the MkII? The answer is in the length and angle of the steering tie rods, look at a modern car and the tie rod is usually the same length as the average length of the wishbone at that height, so as the hub goes up and down the toe in doesn't change. If it does appreciably then you have 'bump steer'. Assuming that the tie rods are of the correct length then the height of the rack becomes important, I have seen racks shimmed up or down to improve the situation.

Then we come onto the steering arm length (the one on the hub/upright), the rack is secured to the modern cars body or subframe at the correct distance from from the axle line so as to keep the tie rods in a straight line, not at an angle.

So for the MkII just bolting on a steering rack from another wider car isn't going to work unless all these angles and lengths are taken into account. Don't get me wrong it can be done and well, but the cheapest isn't necessarily the best it would need a system that has been properly thought out and have the rack mounted on purpose built brackets to move the rack aft of the subframe and/or shorten the steering arms.

About 15 years ago after struggling for two years with my own worn and knackered manual steering box I asked Ken Jenkins which rack I should use and he directed me to a 420 front axle which went straight in and hooked it up to an XJ6 steering pump (hydraulic driven from another crank pully). With the rightly so interest in the 420, I doubt you would find a subframe now.

Moving onto the subject of caster and camber, most cars of this age had positive camber to lighten the steering so it seems daft to still use this when we have power steering fitted. When I changed front springs continously to find comfort the steering also changed to massive understeer (loading on the tyre in a corner) and on advise I dialed in 1 degree of negative camber and a bit more caster to weight up the power steering, toe in just the same. This created a radical improvement of the handling (the negative camber).

Hope that helps, but it means you need to get out and get under, think, look and measure if you really want to improve it!

Brian

[ChrisDixon]

I have not seen such a conversion but if they have errors it suggests that the correct placement conflicts somewhat with the subrame, and the best of a bad job has been done? The length of the rack is determined by the inner pivots of the track rods coinciding with an imaginary line drawn between the inner wishbone pivot points. As I say it is basic suspension geometry and is not a black art.
Alec

Yes Alec there isn't any space behind the subframe to mount a rack further back. A far cry from some modern FWD cars that have the rack mounted on the bulkhead! It seems that the problem with using Jag racks is they are from later models with wider track. So the shortcut is literally that, cutting a bit off the control arms. Would a rack with a narrower body work better, so the control arms pivot further in and more in line with the wishbone pivots?

[ChrisDixon]

About 15 years ago after struggling for two years with my own worn and knackered manual steering box I asked Ken Jenkins which rack I should use and he directed me to a 420 front axle which went straight in and hooked it up to an XJ6 steering pump (hydraulic driven from another crank pully). With the rightly so interest in the 420, I doubt you would find a subframe now.

Moving onto the subject of caster and camber, most cars of this age had positive camber to lighten the steering so it seems daft to still use this when we have power steering fitted. When I changed front springs continously to find comfort the steering also changed to massive understeer (loading on the tyre in a corner) and on advise I dialed in 1 degree of negative camber and a bit more caster to weight up the power steering, toe in just the same. This created a radical improvement of the handling (the negative camber).

Hope that helps, but it means you need to get out and get under, think, look and measure if you really want to improve it!

Brian

Hello Brian, the 420 is often suggested as a subframe and PAS donor, however it still ran a steering box as opposed to a rack. I did idly wonder if an XJS front subframe could be made to fit, or even an XJ40. Yes the mounts are different but maybe possible if the wheelarches were flared, and you'd need to do the back as well. Alternative solution here is to buy an XJ6 Series 1 .......... But I will play with camber and caster, after all my car has done over 100,000 miles on rack and pinion so unless the previous owner was happy to put new tyres on every 10,000 miles it can't be that bad.

[piman]

Hello Chris,

"Would a rack with a narrower body work better, so the control arms pivot further in and more in line with the wishbone pivots?"

It is a critical dimension and unless the track rod pivot points coincide with the inner wishbone pivot points there will be track variation with suspension movement, also known as bump steer.
I don't entirely agree with Brian that the reason for positive camber was to reduce steering effort, it also ensures understeer which is the most stable state for safe handling. Imagine how a MK 2, especially the more powerful ones would handle with negative camber as they were originally on relatively narrow cross ply tyres. I'm old enough to remember the Hillman Imp when it first came out, rear engine and they had noticeble positive front camber and tyre pressures of 15 psi front and 30 psi rear, all to give understeer. It wasn't for steering effort a sthere was little weight on the front.

Alec

[ChrisDixon]

Ah yes the Hillman Imp! Great little car much more refined than a Mini. I can recall a number of cars that appeared to have unfeasibly high camber one way of the other. Triumph IRS fully laden, rear engined Renaults, and so on. Aside from the handling I always wondered how long the tyres lasted.

The car is going to a local guy who knows these cars next week to have all the geometry checked, so I look forward to what he has to say.

[Tim.T]

Regarding castor and camber... castor ( inclination of the king pin or suspension upright) is what helps centre the steering, and more castor adds 'feel' ( which is lacking in the Mk2 and its derivatives.. probably to keep the steering as 'light' as could be achieved back in the day with those crap bias ply tyres) so one can re-position the shims on top ball joints to adjust this as required.. normally ex factory, most of the shims are at the back to give close to zero castor..
Camber (angle of wheel in vertical plane as viewed from front) :- slightly negative camber helps with traction..hence on the race track you will see cars with enhanced negative camber.. IF the original camber settings have not been messed with ..leave it alone..

in conclusion :- when adding a power steering rack you can transfer all the castor shims to front of the upper balljoint and gain more 'feel and stronger centering effect.. not wise to mess with camber settings on a road car.

Various rack and pinion conversions :- I had major issues with kit sourced form XKs unltd here in California.... firstly the 'reconditioned rack' supplied in the kits was junk.. it leaked like a sieve from the pinion seal before the cars engine was even started..then the hoses have crap fittings.. the one for pressure side that fits into pump bottomed out before it cinched up.. so it leaked all the fluid out AGAIN.. secondly the length of the pressure hose is WAY too long..( needs to be at least 20" shorter) I had to fabricate a bracket to secure it so it did not chafe on the engine block or chassis.. Thirdly, the 'shortened' tie rods were still too long by minimally 3/8 " each... 4th, the suggested 'shortening;' of the original steering shaft is complete BS..they say in instruction sheet it needs to be shortened 'about an inch'..in actuality it needs to be well over 2" shorter to fit.. fortunately I had some parts from a 'vicarage' conversion, which has an altogether different design for upper steering linkage, supplied as a 'bolt in' part of their kit..this saved major headaches after messing with the original bits several times..
Frankly the kit as sold by XKs unltd is not a well thought out conversion at all.. as i understand it there are 'other' conversions that utilize the XJ 40 rack..which is, I believe, slightly narrower then the XJ6/XJS rack, and hence should produce better angles for the tie rods, thereby reducing the issue of possible bump steer.. Id love to hear from anyone with updates on this. I'm about to convert another car..and I'd prefer something that is better thought out and is tried and tested and known to be 'good' ..before diving in yet again..

[davidcmaggs217]

A very interesting thread.

I was so unhappy with the rack and pinion I had fitted to my 3.4 Auto about 14 years ago, I have been working to return the car to the original system. I swapped to a rack, as I could not stop the hydraulic oil leaks and back then (in Holland) could not seem to find replacement steering boxes or other solutions to fix the leaks. The rack and pinion never felt right, and the turning circle was greatly reduced - maybe it was badly done?

I rebuilt the PAS box using a cheap set of seals, and it was not that tough once I had worked out that there are two types of Burman PAS box with different numbers of balls. I had to buy a vernier caliper to measure the balls, but a man can never have enough tools. I am not surprised it was leaking given how hard and flat the old seals were. I used the old tie rod and track rods with new bushings on the end of the tie rod. I mounted the pump on the end of a Dynamator - this is a C42 look-alike rather than the C-48 dynamo I had originally, but it looks good in there.

I just put the car back on the road with the new old setup and took it for a mini MOT.
I was quite happy with the steering, I got my steering lock back and while things were a little vague around straight ahead, it is a 3.4 Auto and I will use as intended for A road cruising rather than robbing banks. The assistance is pretty good and the car tracked straight. The toe in was perfect as I had not changed any of the pieces since they came off.

There are a couple of challenges
1. The steering feel is different left to right - which means I have not quite got the nut centralised - still scratching my head on that one.
2. The new bushings on the end of the tie rod (e-bay), were a devil to push in and did not last until I got to the MOT station

3. The track rods were pretty lose, so I have gone for new ones, unfortunately I cannot find original solid one, so have had to go for adjustable one which just don't look right.

A rather long intro, but what is so wrong with the original setup, maybe the focus should be on ensuring a good supply of refurbished PAS boxes, and components and a reasonable price. I have been quoted over 2000 GBP for a refurbished Burman box, when the seal set cost 30 GBP?

Regards
David