Power steering - What systems are people using?
Discussion
If you have power steering on your kit or modified car, what systems are you using?
The "Corsa" electric column motor system seems popular but i don't think it's performance (power, inerta etc) is very good by modern standards, but it has the benefit of working with pretty much any steering rack!
Guess most people just use the std engine driven PAS pump and what ever rack they find that fits etc?
The "Corsa" electric column motor system seems popular but i don't think it's performance (power, inerta etc) is very good by modern standards, but it has the benefit of working with pretty much any steering rack!
Guess most people just use the std engine driven PAS pump and what ever rack they find that fits etc?
The saxo pump, which is actually just a standard "hydraulic power pack" from the Canadian firm HPI is ok for small light cars with low ratio racks, but it struggles for larger cars with "faster" racks that require a much higher oil flow rate. I found it had massive hydraulic lag when i tried one on my car (which to be fair, has a high volume ram and is just 1.5 turns LtL). Peak hydraulic power is only around 450W, which is low.
The corsa column driver has the advantage of being on the column (fairly obviously!) and so it's input rate is constant (pretty much how fast a human can turn a wheel) whereas systems that apply the assistance to the rack bar will have a variable velocity depending on the pinion ratio and steering arm effective length on the uprights etc! In this case, the low power of the system means that assistance falls with handwheel rate, but at least there are no hydraulic effects to cause additional resistance (for a hyraulic system, the ram on the rack turns into a PUMP if when the flow rate exceeds that of the pump...) On the "motor on column" systems the issue is the mechanical inertia of the motor is coupled to the column, which tends to damp the steering response,making the steering feel unresponsive, and unless you clutch that motor to the column,that inertia is there the entire time, even when no assistance is being delivered (ie driving in a straight line)
The corsa column driver has the advantage of being on the column (fairly obviously!) and so it's input rate is constant (pretty much how fast a human can turn a wheel) whereas systems that apply the assistance to the rack bar will have a variable velocity depending on the pinion ratio and steering arm effective length on the uprights etc! In this case, the low power of the system means that assistance falls with handwheel rate, but at least there are no hydraulic effects to cause additional resistance (for a hyraulic system, the ram on the rack turns into a PUMP if when the flow rate exceeds that of the pump...) On the "motor on column" systems the issue is the mechanical inertia of the motor is coupled to the column, which tends to damp the steering response,making the steering feel unresponsive, and unless you clutch that motor to the column,that inertia is there the entire time, even when no assistance is being delivered (ie driving in a straight line)
I'm looling to improve the linearity of the assistance on my rally car at all handwheel rates, expecially on big fast "oh s**t" moments when you need to use a lot of lock very quickly. At the moment the system weights up unacceptably.
I use a LHD GrpA/WRC Escort Cossie rack,which has a modified pinion/bar to achieve both a variable ratio and a faster one (compared to the std road cosworth rack).
As far as i can tell it uses the std sierra/escort housings and power ram:
I have no idea if the spool valve has been re-profiled, there is certainly a good chance that it has.
I power it with a TRW Gen2 EHPAS pump running with vehicle and handwheel rate based pump speed control:
but although this is one of the most powerful pumps available (1kW) it still struggles to keep up with the flow demands of the rack in-extremis ;-(
I use a LHD GrpA/WRC Escort Cossie rack,which has a modified pinion/bar to achieve both a variable ratio and a faster one (compared to the std road cosworth rack).
As far as i can tell it uses the std sierra/escort housings and power ram:
I have no idea if the spool valve has been re-profiled, there is certainly a good chance that it has.
I power it with a TRW Gen2 EHPAS pump running with vehicle and handwheel rate based pump speed control:
but although this is one of the most powerful pumps available (1kW) it still struggles to keep up with the flow demands of the rack in-extremis ;-(
Options seem to be:
1) beef up the current EHPAS pump (fit larger pump gears, modify current shunts etc)
2) Modify the rack to have a smaller dia power ram (not that easy) (less total assistance, but available over a wider range of rack velocities)
3) change to an aftermarket "race" style race and use an external power ram and spool assy
4) Move to completely electric steering, using the rack off something like the new M3 that has a beefy electric rack (also, an 'lecy RX8 rack looks like it might be modifyable, as the power electronics look to be separate)
1) beef up the current EHPAS pump (fit larger pump gears, modify current shunts etc)
2) Modify the rack to have a smaller dia power ram (not that easy) (less total assistance, but available over a wider range of rack velocities)
3) change to an aftermarket "race" style race and use an external power ram and spool assy
4) Move to completely electric steering, using the rack off something like the new M3 that has a beefy electric rack (also, an 'lecy RX8 rack looks like it might be modifyable, as the power electronics look to be separate)
shoehorn said:
How about a booster tank for the hydraulics they are used for lifting gear,etc.
A small tank inline that stores fluid,pressurised during normal pumping.
when the system back-pressure reaches a set amount the same valve releases that stored pressurised fluid back in to the system to assist.
Unfortunately PAS racks are "open spool" ie they have flow but no pressure when not servoing the rack, hence there is no pressure to store!A small tank inline that stores fluid,pressurised during normal pumping.
when the system back-pressure reaches a set amount the same valve releases that stored pressurised fluid back in to the system to assist.
I had a quick look at the calibration of the TRW EHPAS pump i'm using by pulling out the contents of the controller EEPROM and the characteristic tables for pump speed (and hence flow rate) are easy to spot!
Basically there are 6 different 5x5 tables for pump speed vs vehicle speed and handwheel rate, i imagine they are used for different driving "modes" in the donor car (eco, sport, etc).
However, all are quite similar, and all have the same maximal pump speed, currently 237 out of a possible max speed of 255. That suggests there might be a bit of pump speed headroom availible to get a little bit more flow but not a lot (assuming sufficient battery voltage is availible)
I'd also imagine there's a memory checksum in there somewhere so it won't be quite straight forward to change those characteristic tables!
Probably better to try to modify the gear pump for more flow at the same speed and modify the motor current shunts to "fool" the controller into not spotting the extra load current instead.
stevieturbo said:
Some PAS pumps these days are very very compact.
I'd find it impossible to believe one couldnt be squeezed in somewhere.
Job done.
Well, i agree, sort of!I'd find it impossible to believe one couldnt be squeezed in somewhere.
Job done.
The lhs of the engine is totally full of exhaust, turbo, wastegate and ALS valving etc, and it's pretty dam hot that side of the engine, so that would be EXTREMELY difficult to try.
The rhs of the engine is full of throttle, plenum, water pump, alternator and drysump pump.
Two options exist probably:
1) Move to a 'lecy water pump, fit PAS pump in space currently occupied by mech water pump. Unfortunately, as i'm sure you know, i'm really rather not keen on 'lecy water pumps ;-)
2) Fit pump "backwards" on front of engine somehow (space currently occupied by oil laminover and rad / IC exit shrouding etc, run off back of belt using std "front of belt" type pump. Possible, but will require the most complex mounting bracket in the history of such brackets, and i think belt wrap will be extremely marginal at high pump torque (back of belt is an issue for this obviously, as without the grooves it does not have the geometrical loading advantage of the V groove side!)
Edited by anonymous-user on Tuesday 9th September 13:33
Think i'm going to try a "quick and dirty" high flow mod on my spare TRW 'lecy pump. Luckily the gear pump driven by the motor is pretty modular (i'm sure they will do models with different volume flow) and it looks like by taking two of these parts, milling off the opposite ends of the centre housing and the gears, i can make a Frankenpump that has around 30% more volumetric flow rate per revolution than the std pump, but still fit back into the unit!
Currently not sure how i am going to re-join the gears, either i'll TIG em together or drill and pin/press fit them somehow. I imagine it'll whine a bit, but if it proves the point i'd then just get some proper new gears hobbed out!
I think also i can lower the pressure relief valve setting a bit (std is 120bar apparently, although i haven't measured it) to reduce the motor load in extremis, as i currently have plenty of actual assistance at low rack velocities. If i drop the pmax to 80bar, i think peak motor load will be very similar (need to go do some hydraulic calcs to check!)
Currently not sure how i am going to re-join the gears, either i'll TIG em together or drill and pin/press fit them somehow. I imagine it'll whine a bit, but if it proves the point i'd then just get some proper new gears hobbed out!
I think also i can lower the pressure relief valve setting a bit (std is 120bar apparently, although i haven't measured it) to reduce the motor load in extremis, as i currently have plenty of actual assistance at low rack velocities. If i drop the pmax to 80bar, i think peak motor load will be very similar (need to go do some hydraulic calcs to check!)
chuntington101 said:
Max i have the simplest and cheapest answer for you. GO TO THE GYM! 
er, have you seen the cost of gym membership these days!! WAY more than buying a new PAS system....... ;-)chuntington101 said:
Max, whats do the WRC teams use?
FEAD driven "std" mech pumps (std parts, modified pressure relief and/or delivery volume control valves)I'm not that keen on the corsa power column for my application tbh. it's relatively lower powered, puts the weight up high, adds bulk to the steering column area (where, due to my front mid engine position i don't have a lot of space anyway, and adds significant inertia to the system.
In your application, on low friction but bumpy surfaces i can see how it works, but for my car, pushing big slicks with lots of castor and camber on dry tarmac, i think it will struggle more than my current system tbh (it is less powerful than what i currently have)
In your application, on low friction but bumpy surfaces i can see how it works, but for my car, pushing big slicks with lots of castor and camber on dry tarmac, i think it will struggle more than my current system tbh (it is less powerful than what i currently have)
In an "off road" application, more system inertia may be a good thing! (as it will help prevent the wheel getting knocked out of your grasp on big impacts to the front wheels, and also damp out some of the "chatter" from broken surfaces at high speed ;-) Also, when off road, most rapid handwheel inputs are not occurring under a high lateral load (generally corrections on "slippy" surfaces etc, more like in gravel rallying etc)
stevieturbo said:
Build the PAS pump onto another stage of the dry sump pump lol
If i had any extra space around the oil pump i would do just that!stevieturbo said:
I wonder for the crappy electrohydraulic....could you somehow have an accumulator to store pressure so there would always be sufficient power for those fast times ?
As mentioned previously, the issue is that PAS is "open spool". When no steering assistance is required (ie most of the time) the fluid flows freely to and from the rack and hence there is no pressure built up to "store". The only time you have pressure is when you are requesting assistance, and at that point you might not want to loose any flow, depending on the rack velocity at the time. Hence a system to control a valve to let fluid flow into and out of a pressure accumulator would be quite complex and difficult to optimise, and suffer from being heavy, bulky and "finite" (ie it could only add an extra flow boost for a fixed max period, and would take time to recharge) Better to just fit a bigger (or second) pump!The RX8 'lecy power rack looks like it might be a possible solution:
Looks like effectively an "Linear" brushed DC motor on the end of the rack, so easy to drive. I imagine it's pretty heavy mind, but all the weight is about as low as it can go in the chassis.
But it's got a quite slow ratio at 16.4:1 (when installed in an Rx8, obviously different length steering arms on the uprights will change that in "non RX8" applications)
Looks like effectively an "Linear" brushed DC motor on the end of the rack, so easy to drive. I imagine it's pretty heavy mind, but all the weight is about as low as it can go in the chassis.
But it's got a quite slow ratio at 16.4:1 (when installed in an Rx8, obviously different length steering arms on the uprights will change that in "non RX8" applications)
annodomini2 said:
Column drive should be fine for that weight, unless you're running 3/4 turn lock to lock?
How about Fiesta Mk7 Column drive unit?
Oh i see, i was not thinking about the "Physical packaging" issues, but obviously there would have to be room to fit what ever solution i decided to use!How about Fiesta Mk7 Column drive unit?
Can't run with a column motor as i don't have any space around the column, and i hate how it puts lots of mass up high!
The latest ZF Servolectric Paraxial unit is ~1kW, with a brushless DC motor and belt drive into a ball screw directly on the rack bar. Looks petty chunky, but i'm wondering about taking it's internals (pinion, rack,motor,ballscrew) and putting them into a custom casing to fit:
Basically, anything mounted to the steering column is a no go. Because of the front mid engine location, the column threads it's way past the engine and ancillaries, missing by about 5mm max in most places!
I also don't really like the idea of having the motor on the column, much better to put the load where it's needed right into the rack bar, like we all did with conventional hydraulic PAS!
Those powered columns from that "electric power steering" company look, er, extremely weedy. Probably alright for a shonky "hot rod" that doesn't really need to go around corners, but not for a proper competition car. They also make no mention of their system fail safes / diagnostics etc, which is a tad worrying considering the safety critical application.......
I also don't really like the idea of having the motor on the column, much better to put the load where it's needed right into the rack bar, like we all did with conventional hydraulic PAS!
Those powered columns from that "electric power steering" company look, er, extremely weedy. Probably alright for a shonky "hot rod" that doesn't really need to go around corners, but not for a proper competition car. They also make no mention of their system fail safes / diagnostics etc, which is a tad worrying considering the safety critical application.......
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