What causes heavy steering at racing speeds

[Ross Nicol]

I'm trying to retain my 18° of steering angle for the reasons you describe Kalun.

Ross

[Kalun D]

Increased outer offset could be more room for bigger calipers/brakes, suspension points further outboard for increased footbox room and scrub...

[Pete.]

Ross "Bloody Hell" what tyre pressures do you run in the dry?

Jim,
°j°u°s°t°°f°o°r°°u°°p°e°t°e°

How nice thank you.

[Ross Nicol]

Pete - 23 lb cold pressure in the slicks all round.
Russ - I know I seem to have hijacked your thread but we are still on topic. The reading I have done today on 'scrub radius' seems to confirm that too much can lead to heavy steering and too little or zero can leave you with no feel in the steering as well as heavy steering while parking.
Ross

[Ross Nicol]

Some pics - Cortina upright, Original Rotor/hub, Slick tyres after PI race meeting.

Ross

[Frank Catt]

Ross, why such high tyre pressures ? Have you checked cross tyre temps after a race ?

[Pete.]

Just beat me to it Frank, Ross grab a temp gauge if you don't already have one and measure the temps across the tire, after a couple of hot laps. Ideally no greater variation than 10 degree's F. from inside, middle, outside is aimed for.

[Trevor Booth]

Russ N,
My comment was not necessarily directed to Iains setup, it was a general comment as to what can be achieved by mounting shocks/spring on a "linkage' system rather than direct to wishbone.

I am not in agreement with zero scrub radius, perhaps Kalun Likes finger tip steering, it is but one of the many things that go to steering load.
There is no "rule Book" that states X amount of scrub radius is excessive, it all depends on the package and what the designer wants to achieve.
Looking at a front suspension in isolation does not give you the total picture.


Ackerman
here is a series of plots
Vehicle configuration
Wheelbase 2159 Track 1346 KPI Static 9.5º castor 4º Toe in 1mm Max shock travel in bump 50mm in droop 30mm. Steering rack ahead of the front axle

The only change in the plots is the location of the tie rod at the steering arm
First plot #94
The ackerman is behind the rear axle by 70% of the wheel base

second plot #85 the tie rod location has moved outboard 9mm
The ackerman is behind the rear axle by 23.5% of the wheelbase.

Third plot # 80 the tie rod location has moved outboard a further 5mm
The ackerman 1.97% behind the rear axle Basically 100% ackerman.

Fourth plot #70 the tie rod location had moved outboard a further 10mm
The ackerman is 13.23% ahead of the rear axle approx 86% ackerman.

The tie rod location has moved a total of 24mm. Toe change ( out) on bump steer has increased dramatically (% wise) whilst the toe out in turn has gone from 0.58º to 3.10º @ 30º turn.

The golden rule " you should only change one thing at a time,but you cant"

Who would have thought that moving the tie rod location to change the ackerman characteristics would also have changed the bump steer. The changes to bump steer in the plots may well change a car from good to bad handling.

[Trevor Booth]

Ross N,
Have you got your tyre rotation correct ??
This clearer set of pictures than previously posted, me thinks outside of tyre working very hard. That crap on the tyre looks more like your own rubber than pickup after the event. Have a chat to your tyre supplier

[Jim c]

Trevor

So the swing radius of the control arms and tie rod (all travelling in the same radius or as close as possible) is obviously more important than the Ackerman if you had to choose.

Jim

°n°o°°p°r°b°l°e°m°°p°e°t°e

[Ross Nicol]

Frank and Pete - Tyre temps have been taken post the major camber adjustments and have confirmed tyre pressures pretty right and camber under control. In other words the temp at tyre centre matches the inner and outer and all 3 are within a good band. Not to mention the car feels really hooked up.
Ross

[Ross Nicol]

Trevor - Yes the outsides do appear to be working hard and I think the inner edge appear like they could do with more camber. Over all though they look a lot better than they used to.
Ross

[Trevor Booth]

JimC,
Thats my philosophy, varying rack up down back forward plus raise and lower the tie rod at the steering arm to give the desired toe in toe out in turns and or bump steer. FWIW I find ackerman in excess of 100% easier to work with as I am not stuck with the values given by 100% or less ackerman and I dont push cars around the pits.

[Trevor Booth]

Ross ,
With what did you measure your tyre temp, if you used one of those heat seeking non contact devices they will not give you a true picture. The tyre appearance belies the even temp situation

[Ross Nicol]

Ah yes I have been using the heat seeking missile infra red type Trevor. I will lash out before Sandown race meeting and buy the proper temp probe type. I'll rotate the tyres prior as well. Everything I'm reading seems to confirm steering will be heavy with a large gap between steering pivot and tyre centre (scrub radius by name I believe).
Ross

[Trevor Booth]

Check with the avon yoko guy at the track, he had a very small unit that was reasonably priced.

Swap left front for right front dont have to change on the rim, check also the back ones

Scrub does contribute but it is not the be all and end all of handling

[Richard O'Clery]

Ross, please forgive me if you've already given the data that covers this, but how much further outboard is your tyre centreline from the hub face compared with the original torana? I'm asking because a large change in wheel offset over the original spec will wear the bearings prematurely.

The Federal Office of Road Safety sets the maximum permissable change in offset as 25mm per side, but this is of course going to be conservative on a road car. With slicks even this much may be a bit of a worry if you're using standard bearings.

Sorry if this thread is turning into "suggestions that will cost Ross a lot of money", but I support Jac Mac's suggestion of reducing the wheel offset and lengthening your wishbones to keep the track the same (ensuring all the bolt on bits will still fit) particularly if the current load is going to hurt the bearings. The resultant lighter steering would be a bonus.