What causes heavy steering at racing speeds

Russ Noble

GT40s Supporter
Lifetime Supporter
After watching Ross' video at Phillip Island and his problem with heavy steering, I have been discussing this by email with another forum member in relation to my build.

Presumably Ross does not have any binding ball joints or any other resistance to free steering movement so would anyone care to comment on the biggest culprit to induce heavy steering at racing speeds?

Steering ratio....... Caster........ Scrub radius..... Ackermann ratio.....Something else.....??

Bear in mind that what might be true for a heavy front engined saloon, may not be relevant to a GT40.....

Cheers,
 
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I know the RF has a wide (very wide) scrub radius partially due to wheel offset.
I dont know how much effect at race speeds, more so in tighter turns one would think.
with any reasonable amount of caster it will make it worse.
In an earlyer post forum member Malcolm made the comment that once he rectifyied his ackerman it made steering at speed lighter.

I am interested.

Jim
 
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Ross Nicol

GT40s Supporter
Might have to change the title a bit Russ to " What causes heavy steering while cornering at racing speeds? " as the steering only loads up in the corners.My hands returned to the normal 10 & 2 o'clock position after the corners so on straight sections of track all is well.I've come through so many of these situations in the 4 years of racing with this car that I'm not phased by having to solve heavy steering.There was a time when I began to think the car was a dud and would possibly need a total engineering rethink. Now, as you can see with lap times at every track coming down I'm full of confidence and when I get the heavy steering sorted it'll be game on.I certainly won't be leaving the door open for a GT3 to pass me on the last lap. Of course there will always be further tweaks but the major issues will hopefully be behind me.
Now the course of action-
My reading is shared between Carroll Smith, Fred Puhn and Alan Staniforth and the general consensus is for racing cars, Caster should be set between
2 and 6 degrees I'm pretty sure mine is set to 3.5 degrees but I will be measuring it again today. It seems pretty widely accepted that 2 degrees is toward the light steering end with twitchiness on straights and less self centreing where 6 degrees heavy steering with good self centreing and no twitchiness on straight.If I confirm the caster to be 3.5 degrees I will knock it back to 2 degrees and try the car at the next meeting at Sandown. Of course having the video recorder in the car will be great as we will all see the results.
Here is an interesting paragraph from Staniforth's book tallking about F1-"Ground effect and late eighties cars used as little as 1 degree or on occasion nil and even 'positive' caster to try and help the driver turn extremely heavy steering.
Ross
 
Russ
I would concur with Ross in the respect of caster, and also Jims comment about scrub radius adding to the effort. Essentially the leverage the wheel has on the steering components and settings will make the steering feel heavy. Lots of caster, scrub radius, and KPI all add to the force applied to the upright, multiplied by the added force in the corners. Since positive caster essentially makes the chassis lift when cornering, the more you have the more lift and force created. I would think that is the main culprit. Although it gives great stability at speed the give back is the heavy stering. Also what ratio is in the steer rack? Maybe you need to change it over to give you more advantage. I would try setting the caster at the minimum amount that gives you stability and go from there, any other mechanical changes require more involved work.
Hope some of this helps
Cheers
Phil
 
I disagree, If you reduce castor the steering will be heavier in the turn as the KPI angle will negate the negative camber and load the outer tread portion even further than it does now- due to the large amount of positive offset( and resulting large scrub radius). Steering does not look to be too heavy in a straight line, so a bit more castor may make the car track straighter as well.
One other thing, I feel the LSD has a bit much preload, as car appears to be lazy on turn in given the amount of lock applied. Ross does not appear to have a problem applying the steering lock reqd, just maintaining it once the car is in the turn.

Jac Mac
 

Ross Nicol

GT40s Supporter
So Jac Mac are you saying that to lighten the steering in the turn I have to put on more caster ie move the top wishbone toward the rear of the car? You are correct the steering is not heavy on the straight.I've always thought the car tracks pretty straight but then I don't have a bench mark. If I make a plate to fit to wheel studs to attach a torque wrench I should be able to test the breakaway torque, can you confirm the process. Yes maintaining the steering in the long sweeping corners at Phillip Island is the main problem.I'm going to take a photo of myself in the driver's seat so you can check the steering wheel positioning and seat support.
Ross
 

Russ Noble

GT40s Supporter
Lifetime Supporter
That's great Pete. You've discovered free energy!:pepper: An entrepreneur like yourself should be able to make a fortune out of that!:lol:

Cheers
 

Pete McCluskey.

Lifetime Supporter
Russ correct, I should have said it uses a minute amount of energy compared to a hydraulic system.;)
It still may solve the problem for Ross though, which is what we are discussing.
 
Ross
Yes, to add more castor in your particular cars setup the easiest way would be to turn the inner rear rod end of each top wishbone say two turns further into the wishbone which due to the RF setup will pull the top ball joint to the rear and more castor without adding a lot more negative camber. If you go to your local track and simply do this you wont have to go thru the whole realignment process until you have tried it.

To check the breakaway torque of your diff you need preferably a 3/4" drive torque wrench like they use in large truck/tractor shops. Buy/beg/borrow/steal an old 3/4" drive socket thats not needed anymore and fab/weld it onto a piece of tube and on to a circle plate or old steel wheel center that matches your wheel stud pattern. Chock the opposite rear wheel while testing the breakaway torque- we are not really looking for the breakaway but the torque required to continue to turn the assy- It might require say 100 ft lb to start turning and only 70 ft lb to continue turning ( I dont know the ZF specs- these are only random figures ).

My concern re the driving position was that perhaps adding a spacer to bring the wheel say 2" closer may help with the steering effort in those turns and that maybe you need a little more support in the rib cage areas of your seat for lateral support.

As with all changes like this-one at a time- that way you know what works!

Russ- No wonder Pete gives us Kiwi's such a hard time- you dont hold back yourself Mate!!

Jac Mac
 

Russ Noble

GT40s Supporter
Lifetime Supporter
Sorry Pete, I wasn't really giving you a hard time. :D

It's just that I have an issue with the notion that because something is electric there is no horsepower drain. That idea is only true if you are running a total loss system, i.e. no alternator, but otherwise there is a constant ongoing power loss to recharge the battery.

Also, since you mentioned it, what we are discussing is the design causes of heavy steering, not the band aids to conceal it. However I am sure Ross will appreciate any solutions to his problem.

To expand further, there is an engineer on this forum who thinks that having nil scrub radius and true Ackermann are the prime targets to satisfy in suspension design. The result of this is that the KPI and consequently the caster are at extreme settings i.e. approx 15* I think. There have been favorable reports of driving impressions with this setup.

On the other hand I personally think that a minimum KPI and caster are preferable as the main design criteria (particularly because of its effect on the inside wheel) and that you live with the resulting scrub radius and Ackermann that follow on from those decisions. For several reasons I think that scrub radius and Ackermann are largely irrelevant to handling on the track.

I also accept that where one is using an existing front upright from a saloon car one is stuck with the KPI (and as a result the approx caster) provided by the manufacturer. These values may be OK for a car with a great mass of cast iron hung high between the front wheels, but may not be so optimum in a 40.

I may be totally wrong and in light of Ross' heavy steering, sought to open up a discussion on these matters. Particularly so, because whilst I can easily adjust my KPI and caster etc, I have a large scrub radius that I can't do much about.

With the comments about large scrub radius there was the thought that I have built myself a pig that will have heavy steering like Ross'. I personally have reason to think not, but was looking for some input. Time, of course, will tell!

Cheers
 
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Malcolm

Supporter
Have found out why my steering is too light and slow right now. Initially thought my steering rack was wrong. However it seems my steering arm on the upright is too long. We also tried castor changes and ran through from 4 degrees to 7.5 degrees to load up the steering. When I get the steering arm right will go back to 4 degrees castor and start that adjustment again. Shorten the arm and the speed of turn should increase as well as weight of feel. For Ross, how about consider this in reverse, ie lenghten arm? IMHO 2 degrees castor is way too little for a race car. 3.5 to 4 degrees is good.
 

Russ Noble

GT40s Supporter
Lifetime Supporter
No doubts at all Jim, I'm perfectly confident!:rolleyes:

Malcolm, you're right, steering ratio does have that effect, but from the video I didn't think Ross' steering appeared to be excessively direct.

BTW what KPI have you got? The caster on an existing setup is inextricably tied to that. I'm sure you are able to have less caster if you have less KPI. Particularly if there is a degree of self centering coming from the scrub distance.

Didn't someone say you also changed your Ackermann and got an improvement? Or was this at the same time you changed the length of the arms? In which case we'll never know the exact reason.....

What effect did almost doubling the caster have?

Cheers
 

Trevor Booth

Lifetime Supporter
Ross N,
Dont mean to be critical, however, the "steering effort" may well be perceived rather than actual. I might suggest that the right hand at the 9 o'clock position in a LH bend is imparting more load normal to the steering wheel as opposed to rotational effort. ie it is "pushing" against the steering wheel. This can create the impression that the steering is heavy.

In terms of Bio Mechanics--
The "hand of the bend" requires less input effort ie left hand on left hand bend, right hand on right hand bend. Try it in your car whilst stationary.

Just a thought
 
I'm watching this thread with interest because I've said since I've got the car that the steering is crazy heavy and stupidly uncommunicative (although it does liven up a bit at speed), especially given the fact that it's not exactly quick. I believe it to be a major flaw in the design of the RF. The rest of the car works well once setup.

As for perception - my shoulder (injured) perceives it to be heavy enough and painful enough to stop me driving the car, certainly at the track. I have no trouble in any other car I own. Driving position does make a difference but there are limits to how much that can be sorted and perception or not, if it feels too heavy then it's too heavy.

RF knew it was a problem (although there was occasional denial) as did Ross H. Indeed he told me not to bother trying to fix it as new uprights etc were on the way. Since then I have abandoned plans to use the car in any sort of competition so I've stopped development but any reasonably sensible "fixes" stated here will be tried.
I am looking forward to trying Jim C's setup when available.

Anyway Ross, you're not alone, I feel your pain... :)

Tim.
 
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Ross Nicol

GT40s Supporter
Hi Trevor- Thanks for coming in on the discussion.As Tim says the steering is heavy and the only reason my right hand is over on the left, with my left hand is to apply enough power to the steering wheel.I've just thought of something which may have a bearing on my situation.When I increased the negative camber on the front I had the upper wishbones altered as I'd run out of adjustment for the rod ends.I didn't check the caster after reassembly so this will be my first measurement to take. It's possible the caster is a long way from where I had set it previously at about 3.5 degrees.I think I will have to do the friday practice at Sandown and treat it like a test day trying a few different caster settings for steering feel.Thanks for the link to the electric power steering Pete it looks expensive, have you got any prices? Jac Mac I'm working on your suggestions, Russ good work starting this thread.Tim you will no doubt benefit from this and I promise to pass on any ideas that work.
Ross
 

Malcolm

Supporter
No doubts at all Jim, I'm perfectly confident!:rolleyes:

Malcolm, you're right, steering ratio does have that effect, but from the video I didn't think Ross' steering appeared to be excessively direct.

BTW what KPI have you got? The caster on an existing setup is inextricably tied to that. I'm sure you are able to have less caster if you have less KPI. Particularly if there is a degree of self centering coming from the scrub distance.

Didn't someone say you also changed your Ackermann and got an improvement? Or was this at the same time you changed the length of the arms? In which case we'll never know the exact reason.....

What effect did almost doubling the caster have?

Cheers

Russ

I use 5.4 degrees of KPI. I don't run a very high KPI like some others do as I don't go with their thinking on that. The ackerman is the same before and after my steering arm length change so is not involved in this discussion. There was a change in feel with the doubling of Castor however the arm length issue so took away feel generally that at this time I can't sensibly comment on it. However once I get the arm length reduced, do ask me again!
 

Trevor Booth

Lifetime Supporter
Ross N
I thought I may get jumped on by all and sundry. :))

Man that is heavy if you need to apply that much effort.

The car I built was set up as follows

KPI 5º34min
Camber -0º30min
Caster 6º
Toe In 0º30min
Scrub radius 110 mm (306mm Rol rad)
Steering arm 125mm
Ackerman was near zero (cant recall exact figure)
4º Roll gave 30' toe out
Standard Mk 2 escort rack
225/65-15 Avon front tyres
48% front mass
52% rear mass
Total mass 1250 kg c/w 68litres fuel and 2 occ

This was one handed steering at 140 mph. heavier at slower speeds but we drove all day in the Targa Tas and it was not tiring to drive.

Admittedly this was on road tyres. Certainly your car would be heavier steering with wider wheels and slicks but not that heavy.

I suggest that you get it on a wheel aligner and plot what the wheels are doing as it turns. also disconnect the shocks and jack the car up and down and plot camber change and toe in /out change. To simulate roll you can raise one side and lower the other. (this wont be exact as you wont get jacking from the rear but it will highlight major issues)

As others have suggested 3.5º of caster I would consider minimal

See if you can borrow a temp gauge with a needle probe and check tyre temp just under the surface of the tyre. Get your tyre supplier to have a look at the tyres just after coming off the track.

Enjoy Sandown, it is a fun track, especially the lefthander at the top of the back straight.
 
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