240/260 Power Steering options - Subaru vs MX5
Posted 08 June 2010 - 03:47 PM
So the next decision is which rack to use. The suby option is reasonably well documented but I haven't seen the MX5 installation, all reports say its a better fit so only one way to find out....I went and bought one of each for comparison purposes. Hopefully this post will help other people with the decision making process but it also raises some other issues which I will ask for advice on latter.
Firstly the racks themselves:
Subaru - did a lot of research on the interweb and while it sounds like most suby racks look the same they come in many different ratios. Most shopping car subies are 3.2 - 3.5 turns lock to lock. The standard Z rack is 2.6 turns so I was looking for something closer and WRX's are 2.75 or STI's 2.5 and a thing called Spec C is 2.0 turns (but very hard to find). Found a WRX rack for $200.
MX5 - Have to use the first generation NA model rack as the NB went to a completely different mounting method. Good news is they are 2.7 turns lock to lock, bad news is I was told they are sought after as many NA's didn't have PS originally. but i paid $250 incl hoses and uni joints from All maz in Clayton.
I spent some time measuring and comparing dimensions.
Posted 08 June 2010 - 04:19 PM
do you know what model wrx yours was from? the older style (GC8) or the later 2002-2008 GDA/GDB/GDC etc. chassis?
Posted 08 June 2010 - 04:44 PM
My biggest concern is poor steering and feel so bump steer is going to be a factor. The centre to centre width of the LCA mount holes on the Z cross member are 600 mm apart and the standard Z rack measures 610mm between the pivot points of the inner tie rod ends IE the rack is 10mm wider than the LCA mount points. Both the Suby and MX 5 rack are significantly wider at 642mm and 650mm respectively at the pivot points. Surely I'm headed for bump steer city?
It looks like its possible to move the LCA points up and out quite some way to compensate but then I'm looking at too much neg camber. Anyone know what sort of camber you get from relocating the LCA points? The car will need to be engineered for rego so I'm hesitant to go with mods like camber plates and adjustable LCA's at the moment, once its rego'ed then yes.
The Suby rack really needs the power head rotated 180 deg so you get better clearance between the pipes and the X member. Ive done this but as per the photo below, once you rotate the rack back you're still going to have to modify the crossmember to provide clearance for the low pressure outlet on the back side, additionally you will need to make up new lines from the power head to the rack piston as well, all no big deal but not really neat and sure to attract the attention of the engineers at inspection time. Also the Suby rack has more total lock than the Z rack (69mm's each side from centre to full lock compared to 60mm for the Z) this may cause clearance issues at full lock for maxed out wheel/tyre packages.
The MX5 rack is a much tidier and neater installation. The lines to and from the pump are well out of the way and the lines from the power head to the rack only slightly interfere and will only need a slight tweak by hand to clear everything. both racks will require a notch and reinforcement of the engine mount, but one specific issue for the MX5 rack is that it sits 10mm off centre to the left, not too big a deal as it only needs the tie rod lengths to be 5mm shorter/longer on the respective sides to bring the rack back to centre and equidistant from the LCA points and don't think it will result in clearance issues on full lock.
Mounting the MX5 rack requires a small amount of modification to the original Z cross member mounts and use the Mazda rubber isolators.
Im going to wait for comments before going any further but Im pretty keen on the MX5 option so far.
Will also post details on outer tie rod ends and universal connection to the Z steering shaft latter.
Posted 08 June 2010 - 04:52 PM
Posted 08 June 2010 - 06:21 PM
Posted 08 June 2010 - 08:36 PM
Lurch, Im getting soft in my old age....not really. After years of driving late model cars that steer brilliantly and with good feel I really notice the heavy steering in my 260 and I would like to do some cone bashing with the 240 when its finished. Im putting a lot of effort into ensuring the car goes and stops well and want it to be nice to drive, not a pig.
Posted 09 June 2010 - 12:58 PM
good info im sure many will utilise.
Posted 28 June 2010 - 10:16 PM
Why does the mx rack have to be mounted off centre? That just makes your bump steer problems worse with unequal length tie rods. You could check the bump steer standard and with a shorter tie rod (longer rack) by pulling the spring off and jacking the suspension up and down with a plate on the hub and 2 dial indicators. To check with a shorter tie rod just wind the tie rod end onto the tie rod if it will go far enough and turn the steering so that the inner ball joint is where it would be with the steering centred and a longer rack.
The geometry may not degrade as much as you think.
Where are the suspension gurus?
Looking forward to more feedback
And what about the rear mount rack? How feasible is that?
Posted 29 June 2010 - 08:52 AM
Posted 29 June 2010 - 10:25 AM
- Clockwork likes this
Posted 30 June 2010 - 07:14 PM
Posted 02 July 2010 - 09:29 AM
ive done this to my 260z with rx7 hubs.
shortening the rack ends is failry straight forward on the subi. you unscrew them give it to machine shop and carton of beer they will shorten it and rethread it to lenght u want.
than N12 Pulsar tie rod ends (from memory which is fading a bit) finishes the dilema.
as for subi rack rotating the head around getting new pipes made up cost me about $30 from brake and suspension specialist.
TIP dont bother ABS and the likes they wouldn't have a clue. more of the oldschool places that rebuild racks will be better help.
or by a quality pipe bender and bend them up yourself.
Posted 02 July 2010 - 08:11 PM
The latest Street Machine magazine (July 2010) has a very interesting article on a power steering conversion in an LH Torana using a Suby rack where they have rotated the rotary valve by milling the original mounting "ears" off the rotary valve, repositioned the valve so it and pipework doesn't interfere with the crossmember and then welded on new "ears". Not a cheap process though. I spoke with Len Brown and he believes that apart from ratio's there were some subtle differences between the various model Sub racks, his preference is for late model outbacks but he has used WRX as well.
Before I committed to one rack or the other I spent a couple of hours at various wreckers looking at other racks and comparing the various Suby alternatives and to be honest while there are differences in the design of the Suby housing the rotary valve and the dimensions are all so similar its hard to say there is a difference (without pulling them apart and taking detailed measurements). Reading posts on Hybridz it does appear that the latter model ones used a larger M16 x 1.5 thread for the rack ends as opposed to the M14 x 1.5 on mine and most of the earlier ones but apart from that they all seem to be very similar. In summary it would be reasonably safe to say that most Suby racks are good candidates for this conversion, really comes down to ratios and that's the easiest thing to check!
I had read that someone had used an MR2 rack (second generation SW20 model) and did find one and thought it was a chance as the inner tie rod pivot to pivot distance was exactly the same as the Z rack...but fitment would be a problem as the piston inlets and outlets were in the wrong location, if you could move these the MR2 rack would be a good alternative.
Ive decided to go with WRX rack in preference to the MX5, primarily because I want to ensure I don't aggravate any bump steer issues. (My car will need to be engineered for rego so the closest it is to stock geometry and look the better, I can change LCA position and fit adjustable suspension once its regoed).
While the MX5 rack is a far easier and better looking installation the inner tie rod pivot to pivot distance is 40mm more than the standard Z rack and will definitely cause bump steer issues...and the rack cant be cut down without loosing a significant amount of your steering lock meaning you will be doing 3 point turns everwhere. Too many compromises in my book.
The Suby rack is 32mm longer (inner tie rod pivot to pivot distance) than the standard Z rack... but it can be cut down because it has 9mm more travel in each direction from centre than the Z rack (see my first post for "stop" measurements). Essentially you can cut 9mm off each end and still have the same turning circle as a standard Z. However taking the 9mm off each end will still leave the Suby rack longer than the Z rack so I'm going to take 13mm off each end which will sacrifice a small amount of turning circle but this combined with the shorter inner pivots (explained below) will end up with the rack being the same pivot to pivot length as the Z rack. All good.
Courtesy of Hybridz and others there are a few alternatives for tie rods:
1. Cut and re-thread the Suby inners and use N14 Pulsar outers.
2. Use Volvo S70, or 82-88 Tercel or 91-95 Camry inners and use standard Z outers but you will need to have two of the RHT Z outers as most modern stuff is all RHT not a combination with LHT like the original Z's.
3. Use the standard Suby outers (same tapper as the Z steerring knuckle so it bolts up) with 84 Celica (model code RA65 for the Aussies) inners.
Ive chosen to go with option 3. as I can use the Suby outers that came with the rack and one of my inners was worn and had to be replaced anyway so buying the Celica inners was easiest. The local powersteering shop I went to was very helpful (Supasteer in Bayswater), they had the Celica inners on the shelf and they were exactly what I needed. Reality is that there are hundreds of inners made and if you need something slightly shorter or longer it will be easy to find as there are many cars that have M14x1.5 thread at both the rack and outer end of the inners.
I took the rack to Robco Engineering this arvo and explained what I needed done and Rob C was very helpful, in fact he can cut the rack down without disassembling which saves on the cost of new seals etc which you would need to replace if you do disassemble it. Will post some pics when I get it back.
Pics below show how the distance from the base to the pivot point of the inner pivot is shorter on the Celica compared to the Suby inner...bonus as this effectively shortens the rack another 2 - 3mm each side. Also pics of the Z tie rod compared to the Suby/Celica version.
Posted 05 July 2010 - 12:54 PM
just a suggestion as they look alot like your one shown in pic above, with the vice marks on the flats...
Posted 20 July 2010 - 09:05 AM
I would be pretty keen to meet up for a chat on how you are going to do the rest. I will PM you.
I have done some home work but been waiting for some motivation to see the job through. This might just be it.
Posted 20 July 2010 - 12:56 PM
Posted 21 July 2010 - 11:12 AM
While I'm waiting on Rob to shorten the rack I had to sort out the steering universal to connect to the Z shaft.
The uni joint that comes with the WRX power steering rack is a weird double uni joint which we cant use. So I needed to find a single uni joint that had the same spline count and size. Once again Johnny RBZ 260 was very helpful. Went to the local wreckers and had a look at all the Subi's and most of the mid to late 80's L series Subaru (Leonie or Sports Wagon) with manual steering have a complex multi jointed shaft between the rack and the base of the steering column but critically this is can disassemble into components and you can unbolt the bottom single uni that attaches to the rack. This uni bolts straight into the power steering rack and the Z steering shaft fits neatly into the other end its the wrong spline count but a neat fit and then you can do up the pinch bolt and weld to the Z shaft.
Posted 26 July 2010 - 09:06 AM
Now getting back to installing the rack. It would appear the Subi rack is around 46mm in dia compared to the Zed one at around 35mm. So since I have a V8 installed I can't afford to increase the height of the steering rack because it will hit my balancer. So when Rob chops into my Cross member to seat the Subi rack we need to drop the rack by 11mm to ensure the overall height does not increase for the rack. Of course this will effect the bumpsteer but will be easily fixed to suit when installed. All I have to do is add 11mm to my spacers for the steering arm to keep the angle the same.
Now all I have to do is try and source an electric power steering pump. Trying to avoid a mechanical one. Dave is going belt driven. Now on this Subject Dave did you have any recommendations for power steering pressures to suit the Subi rack?
Hope I haven't hijacked the thread but I guess all info helps the listeners!
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