Improved Front Geometery with Sierra Uprights

[Maverick440]

After having my IRS Striker for some time (one of the first IRS chassis) I've never been happy how she handled on bumpy country roads, and I remember having a conversation with Jeremy who basically said that nothing had changed regarding the front setup when moving to a single Sierra as the donor vehicle.

From what I've seen there are two problems with the standard setup:
1, The steering rack is mounted to low in relation the front upright steering arms.....
2, The castor angle does not take into account the fact that the Sierra stub axle is mounted forward on the upright centreline......

Before my car was SVA tested some seven years ago I had 18mm removed from the rear of the rocking arm and a single 18mm spacer made, this was done to give me a degree of self centring for the SVA and once passed the spacer could be moved to put the rocker back into its original position (on the road it was awful to drive with so much added caster) however with the top rocker in its original position my car was very nervous on country roads at speed and felt like you were always correcting the steering even after I had the track setup correctly, so I went about trying to get things sorted.....

After drawing out full size the front suspension and upright I found the offset stub axle on the Sierra upright significantly reduced what's called
the "trail" and one of the by-products of trail is stability / self centring.

I calculated that moving the top rocker back by 11mm would give me the same amount of "trail" as Capri / Cortina upright....

The second issue is that when the rack is in its original position, it’s too low and gives you "bump steer" this coupled with little trail is what causes the nervous feel. If you look at the front of your Striker (Sierra uprights) you will see the track rods rise upwards and do not run parallel with the lower wishbones......

When you move the rocker back you exaggerate this problem and make the bump steer worse, so you have to raise the steering rack by 25mm......

To do this you will need to steel plates as per the drawing along with four M8 countersunk machine screws, ones the plates are made, put the two
countersunk screws in place and bolt the plates onto the original rack mounts. You should now have to "studs" sticking up which will act as the
lower rack mounting bolts.

You will also have to enlarge the holes in the side panels so that the rack can move freely from lock to lock before refitting the rack.

Refit the steering rack, the mounting bracket lower holes go over the studs, and the upper holes use normal M8 bolts Once everything is tight, reset the front tracking, go for a blast and enjoy.
Mark

Front spacer fitted to move rocker arm back


Rear spacers


Rack riser plates fitted


Modified rack openings in side pannels


Front shot showing parralell track control arms


Rocker arm spacer drawing


Rack riser plate drawing

[JohnEM]

I can confirm that this works, having modified my own Striker.
To be fair the front suspension on the Raw Striker was sorted by them soon after taking on the production of the car.
So not sure how many it effects (my own chassis was from 2000) and the modification has increased my confidence in the car no end.
Thanks again Maverick.
JohnEM

[Maverick440]

Thanks John............

[waddock]

Hi

I have a IRS striker and suffer with very poor self centre and also darting around the road when it gets a bit bumpy. As a result of this I don't really have much confidence in the handling. To be honest this is not so much of problem given the little amount of use of the car.... However that’s about to change.... Time to use this thing like it was meant to be used!

Before I go ahead and perform the above modification I wanted to try and get a bit more detail on the process and its effects.

Looking at the pictures I presume that I need to cut out 11mm from the rear of the top arms and then provide 11mm of spacing at the front? Is 11mm the correct distance after testing? Has anyone found 10 or 12mm works better?

Once the modifications have been made do I then set the car up with standard geometry?

Front camber angle = 1.5 – 1.75 degrees, negative.
Castor angle 5.5 degrees positive.
Toe out 1mm

With regards to the steering rack modification, it seems simple enough. Just wondered if there anyone out there who has tried this and possibly made some variations on the design that they could share?

Any information / pictures that people can share regarding this is greatly appreciated!

Regards
Ben

[sullayton]

Excellent post, I suffer the same issues with my Fury. It's a very fast car but I can't drive it with confidence at speeds over 50 - 60 mph on anything less than perfect flat roads and to drive fast is why I have a Fury.

I did spend some time in the spring meddling with the front set up, the Fury has escort uprights and is an ex RGB car. Basically I reduced camber to 1.5 degrees and toe to 0.0mm from 1.75mm toe out, using a stringline to measure and adjust at home as this was easiest to try first and the result was a definate improvement although not the leap forward that I need. I have measured the castor to be around 0.2 - 1.2 deg negative so it is interesting to compare this with what you say although I was hoping to not have to make changes to the rack mounting. Another thing I think has an influence is the lack of suspension travel my car has, probably 1.5".

I have noticed that with a passenger in the car that the extra weight makes it less darty. It would be interesting to hear how you feel your car drives after the re-engineering.

Regards Sully.

[JohnEM]

Sullayton
Not been any response as I guess no Fury owner is going to read a post about Sierra Uprights and Strikers!
I would have thought that a small amount of toe in might be beneficial? however early Strikers did have Escort geometry.
It is all a case of making adjustments and seeing what works.
John

[Darkspeed]

With that amount of caster its no wonder your car is not confidence inspiring - I am not sure what you mean by negative - But the car needs 5-7 degrees of positive caster trail to make it driveable - 5.5 is generally a good start - Your Toe out would not improve things especially if the car is still running the donor upright Ackerman angle in the steering arms.

Andrew

[dopdog]

i like this post but am lost dwith the last comments

[Darkspeed]

Sorry did not make clear the response was to Sully with his ill handling Fury and the base settings he has indicated.

Andrew

[RobMsport]

I had a Stylus that I was working on a few years back with Seirra uprights and really struggled to dial out the lift off oversteer plus it was very nervous at high speed . Tow out plus too little castor is bad news stability wise at high speed . I sorted that but could never get it quite as good as my own Stylus. I slotted the engine side rocker bolt hole then mounted a 30mm x 80mm plate drilled for the bolt near the top then hinged via a bolt at the bottom . You can then move the rocker inwards to increase castor then bolt the plate top and bottom to clamp it . This has been on my car for ten years without issues of coming loose etc .
Bump steer wise --- my uprights have steering arms bolted to not welded to the upright so I removed them and cranked the angle downwards about 15mm's , an easy cut bend and weld job . Steering arms and bottom wishbones were then parallel.
Ackerman angle on Escort steering arms/uprights are perfect for Stylus live axle wheel base but I've actually adjusted that on mine to meet in front of the diff , not on it and have no issues caused by it.

The solutions in the above thread are very ' neat ' , well done that man Having a full bodied car without flip front at the time put me off trying to relocate the rack