What causes heavy steering at racing speeds

[Howard Jones]

I have a question for the group. Would increasing neg. camber help with the heavy steering effect caused by excessive scrub radius?

Lets say we went from -.25 degree to -1.5 degree. This seams like it would reduce effective scrub by moving the top of the wheel rim inward a little bit. I know the actual geometery has not really been changed and the bottom of the wheel will also move outward a little too but can the effect be mitagated to any extent this way?

Kind of like changing wheel offset a bit.

 

[Russ Noble]

Interesting replies so far...... Could it be that this is mainly an RF problem? Why?

Some have suggested scrub radius but Trevor's car has 110mm. What is the RF figure?

Malcolm was worried about his steering being too light! It would be nice to be able to compare some basic figures like effective length of steering arm and linear rack movement for one turn of steering wheel.

Phil suggested it was a (cumultive) combination of Caster, KPI and scrub, but Trevors car has figures for all of those that I would think are towards the high end of the range used in most cars.......

Not sure about Howards suggestion, but I think Ross is already running in the region of -3* camber. This seems a heck of a lot for a car without much body roll. Is there some really basic cockup with RF suspension that causes the heavy steering and necessitates -3* camber to maintain tyre contact? If so, what is it?

Trevor, your comments are always appreciated.

Cheers

 

[Pete McCluskey.]

Ross, sorry I didn't get prices but if you follow the link and email them I'm sure they will oblige. You can also download a PDF file that shows installation and measurements.
I would be interested to know if anyone on the forum has fitted electronic power assist.
I also have an injured shoulder like Tim. But unlike you Ross it is the heaviness at low speed that is my problem. (parking). At higher speeds the steering lightens right off. I haven't tried it at racing speeds though.

 

[Ross Nicol]

Yes I've driven the car at Sandown with 7" rims and road tyres, different ball game with 9.5" rims and 265 slicks on the front,great to have all you guys helping me solve the problem.I blame the video for stirring this positive action.

Ross

 

[Ross Nicol]

Russ - I'm sure I've measuresed 10mm of linear rack travel for 12 oclock to 3 oclock turning of the steering wheel. I'll measure again to confirm and also measure the arm length on the upright.I think like you these are more meaningful measurements on a race car's steering,w'ere interested in the ratio not so much the turns lock to lock.With rack limited for 18-20 degrees of wheel travel left or right,multi point turns are the norm in and around the pits.
Ross

 

[Jac Mac]

Ross,
I was just looking at a plan view of an RF upper w/bone, to creat the amount of castor for your test you will also have to wind out the front inner rod end to make the change- so wind rear rod end into the w/bone two turns, front rod end out three turns. This is due to the assymetric design of the w/bone in plan view ( front leg of w/bone is approx 1.5 times longer than rear).

Jac Mac

 

[Ross Nicol]

Ah Jac Mac does that plan view show the rod ends mounted to the chassis? I have previously altered caster by altering the chassis bushes on either side of the rose joints on the top wishbone.I hope your ok with this cause I've run out of rose joint thread on the LHS.
Ross

 

[Jac Mac]

Ross, when you shift spacers to move the rod ends towards the rear on your car you also remove some of your negative camber due to the divergent axis of the top w/bones (pivot line of top w/bones is not parallel to car centerline).
For the purpose of the test you may have to reduce the lock nut thickness on the rear rod ends in order to get the castor adjustment reqd.
If the tests prove successful I suggest that for a permanent fix that some lower w/bones with inner rod ends so that the lower ball joint can be moved forward to achieve the desired castor on a permanent basis & still give you some room for adjustment might be in order.

Russ, sorry if we a hi-jacking a bit here, but if nothing else its becoming obvious that each type of Replica is going to have its own traits largely due to the choice of upright as this dictates the included angle and front wheel offset. Given that the original 40's ran front tyre widths in the order of 8", moving up to 10"/11" fronts requires a careful rethink to achieve optimum handling.

Jac Mac

 

[Russ Noble]

Hi Jac, I don't think it's a hi-jack, ay Jac, Mac!:lol:

If we could get Malcolm and Ross to compare notes, measurements and maybe photos and if Ross tries a few different things, as I'm sure he's going to, we might just find out what the problem is.

Also Flatchat campaigned his old DRB quite extensively. It would be interesting to get some feed back and comparisons from him.

Cheers

 

[Frank Catt]

I have been reading all this with interest, and as you know I am the "engineer" useing the extreme settings mentioned. I think of all the replies JacMac is the guy thinking along the right lines and should be considered. I am not going to try to justify what I do, only to say it is proved and highly successful, and has been tested by some extrememly competant and knowledgable drivers on the circuits, but as we all know there is more ways of killing a catt than one, find the one that works for you. Frank

 

[Fred W Ballinger]

A question for Trevor re his set up.

Scrub radius 110 mm

Is this a typo - 110 mm seems huge for a scrub radius? I understand scrub radius to be disctance from the point the KP line intersects the ground to the tyre centre, and that anything much over 25 mm was to be avoided?

Not trying to be a smart ass, im really intereststed in this setup as I am also using a standard escort rack

Cheers

Fred W B

 

[Russ Noble]

I was hoping you would chime in Frank

Yours is a very interesting setup. I take it you have no problem with heavy steering and that your steering ratio is 'normal' for a high performance car?

What you have done would tend to disprove the common theory that high caster causes high steering input effort. I presume you have a high caster figure due to the high KPI? Although, from memory, you have never actually said that on here. I wonder whether the KPI and caster figures cancel each other out as far as steering effort goes?

Trevor, do you have any comments on this?

The replies so far give me confidence that my setup will be OK. For my initial settings I'm going with 3* caster and KPI, 1* camber, 80 mm scrub radius and full Ackermann.

Not very scientific, but the Ackermannn was chosen to make the car easier to push around the pits, (The geriatric pit crew is already complaining about the prospect of a welded up diff, so I don't want to make life too difficult for them!:lol. I don't think Ackermann or anti-Ackermann has a lot of effect on lap times or anything else in a racing environment.

Cheers

 

[Trevor Booth]

Hi Fred W,
110mm is correct.

Russ N,
I am with Frank C
listen carefully to what Frank C has to say, he is not the only one to explore high KPI. BUT, every thing else has to match. It can have some benefit. I have played with it as has some others, Frank c probably has played with it more than I have.

It is all about what the wheel / tyre is doing in the turn. what is good for one car may not necessarily be good for another. You need to remember that this forum is about 1965 model cars.

High castor does not necessarily cause high steering input if the KPI is matched to it so that you dont jack the car excessively, however, saying that some cars/tyres do benefit from loading the front O/S tyre by some jacking of the front end in a turn.

If You look carefully at some of the F1 and high end sports cars you may also note that the axle centre is ahead of the king pin axis. Just think about that for a minute and draw a force diagram, also bear in mind the high down force that the modern era experiences!!

What is unknown about Ross N's vehicle is the roll centre heights from front to rear and the mass transfer front to rear, this knowledge is fundamental to achieving good handling and good steering response. He could have a massive mass transfer. He has not got ( or posted) known wheel behaviour on a wheel aligner.
There is no tyre temp data - basic requirement, there is no tyre press rise data also a basic requirement. What is the LSD breakout torque , what is his engine idle speed-- how much trail braking does he use (if any) too much unknown stuff.

I am not familiar with the RF front suspension but if the rear is any indication well........

These days 3D software does it for you and 3D mechanism dynamics tells you everything in terms of kinetics. I have long since abandoned the pencil and paper method. I have made the offer before and some forum members have taken it up, provide me with your XYZ coords and I will do a 3D plot FOC.

dont laugh but I have applied racing car suspension theory to trucks buses miniing trucks and mobile cranes-- the saving in tyre wear was worth the effort!!

" Ackerman chosen to make the car easy to push around the pits "

Thats what they have engines for.

 

[Russ Noble]

Thanks Trevor, a very thought provoking reply. Shall sleep on that one......

Hopefully Ross can come up with some of the missing info to help clarify his problem.

What was your reason for looking at high KPI? Is not the inside wheel attitude a problem with that scenario?

Cheers,

Edit. Sorry, I knew I should have slept on it before I said anything stupid. Obviously the KPI/caster on the inside wheel helps negate the normally largish negative camber induced by body roll with camber compensating suspension. Duh... I'm starting to see benefits....

 

[roaldin]

I perceive that to be a very useful post Trevor.

" Ackerman chosen to make the car easy to push around the pits "

Thats what they have engines for.

ROFL! I bet Russ got a giggle from that.

FWIW the rear of my car seems to be behaving in a reasonably friendly manner these days, using about 5 degrees rake, 3 degrees camber (figures from memory) and Jim C's components which allow such settings (and allow for much easier adjustment).

Maybe you could come up with a simple email data sheet that interested parties could fill out. This way you'd get exactly what you need in a form that's immediately understood and you could send back a (hopefully) frustration free plot.

Tim.

 

[Trevor Booth]

hope that stirred up some grey matter But no body picked up on the idle speed

Russ N
There is no such thing as a stupid question ,thought or statement- many a scientist has been ridiculed for seemingly stupid questions, thoughts and statements - the world is a far better place resulting from the work of oft ridiculed scientists

There are 4 wheels on a car ))

Hi Tim,
Next thing you need to do is change the forward end of the rear suspension trailing arms to rod ends. With no disrespect to the designer, the rear suspension does not "track" very well, the need for 3º of camber demonstrates this

a spread sheet for data input is probably the way to go, I shall attend to that in due course.

 

[Ross Nicol]

Ok here is the first piece of data from my car.
Caster/castor I like castor but I use both.
Houston we have a problem here as measured tonight

LH - 4.75 degrees positive

RH - 6.75 degrees positive

No doubt this discrepancy came about after my camber mods.
My natural reaction is to reduce the RH side to match the LH and test but what is the panel's recommendation.

Ross

 

[Trevor Booth]

That explains something, the video appeared to indicate more effort for LH bend.

My two bob ( more value than two cents) 5º both sides

 

[roaldin]

...There is no such thing as a stupid question...

Yeah but there are some that sure sound stupid, like - Idle speed?

Tim.

 

[Jim Cowden]

RF specs
Steering arm 120mm long
KPI 6.5deg
Scrub radius around the 80-100mm never had an original wheel to 100 percent clarify that.
The steering rack is STD escort.
Even though it has 2.5 turns lock to lock it is STD.(it has lock stops built in)

I read this with interest
I have spent countless hours changing my RF an have enjoyed the process of the engineering and the education.

I still stuck with the 6.5kpi
The reason is it helped with my scrub radius.
It is know 13mm wide.
To acheave this I had to make a new upright.
I also changed the wheel offsets on the front.
this has allowed a 235 to be fitted with room to spare on the inner gaurds.
I did what you suggested Frank and work in from the wheel I was able to make the upper and lower arms 75mm longer.
This has stabilized the roll centres and with the new upright are adjustable.
Bump steer is down to .004"
It will still require setting up but I think it will all work ok with a tweak here and there.

Jim.