What causes heavy steering at racing speeds

[Ross Nicol]

Yes JacMac I do have the smaller bearing uprights.I think I will talk to Paul & Owen at RF as I think I'd better upgrade.They have a new alloy upright and I'm sure it has the larger bearings.The old RF were concerned about the small bearings when building their race car that never happened.Probably help my unsprung weight too.
Ross

[Jac Mac]

If we are talking about the same upright Ross, your existing stub axle will be 1.0625" dia for the inner wheel brg. Thats scary when you compare it to a Holden @ 1.250" & Ford @1.375" and probably something that RF owners who have this part fitted to their cars need to bear in mind before taking part in competition or fitting larger offset front rims etc. While I have never heard of a problem with these uprights failing, I do know from the racing I have done in the past that they do flex/bend under load which is a variable you can do without.

Jac Mac

[Ross Nicol]

JacMac
Thanks for those measurements.From them I am relieved to say the inner bearing has an ID of 1.25" and after talking to Will a standard Holden front bearing set was used in RF cars. So it seems a Cortina upright has been used with a 1.25" inner bearing shaft.Will says the only mods RF did to the upright were a tweak to the steering arm ( to straighten it ) and alteration to the inner seal surface. Are you comfortable with me to continue with what I have?
Ross

[Jac Mac]

Thats a relief, hope we have not got every early RF owner in a panic! I assume from your comment on the brg kit that the hub/rotor assy is a Holden /aftermarket part.

You have mentioned that your latest measurements indicate that you have around 6deg of castor,but that to acheive that you moved the top w/bone inner rod ends rearwards by means of spacers which would also have reduced the amount of negative camber on the wheel in question as the rear rod end could not wound in to compensate-sooo if this is the case one change has canceled out the other ( ie- the reduced negative camber will produce the same effect of shifting the tyre contact area to the outer tread portion in the high speed turns as per your video ).

Jac Mac

[roaldin]

I have a vague recollection of a conversation with the old RF about these uprights. I think the bearing set may be Commodore of some sort but don't quote me.

Tim.

[Malcolm]

Whilst you guys decide if you have Aussie or UK cortina uprights, you mentioned what I would call spindle flex. This has been suffered by some UK GTDs in the past. Some owners who used their cars exactly the same as others did not suffer it. So if you get a Friday afternoon casting then you may get spindle flex but a monday morning spindle may be ok! The result of this flex was brake pad knock back, a very obvious symptom indeed! Your brake pedal goes very long and requires pumping up between corners, just one or two pumps and it is firm again. I had this on my car about 10 years ago and well remember left foot braking, albeit very gently, between corners whilst mashing the throttle to get me to the next corner as quickly as possible! Road users found this problem between roundabouts (traffic circles!). It was for this very reason that Ray Christopher began design on the GTD racing upright which used bigger bearings which eliminated the problem altogether. He also used it to alter other geometry as well, such as KPI and ackerman, on the prototype items which were tested on my car. When these items went into production, now produced by Roy Smart if anyone wants a set, I think (so don't quote me) he made it a one for one replacement item as far as the suspension arm side of things went. On the prototype he gave me longer bottom suspension arms. However you do need to play with brake mounting etc etc. All the UK aftermarket uprights for 40s use the bigger bearings as standard. But I would not say this causes heavy steering, unless the little bearing fails, but then heavy steering becomes the least of your worries.

[Trevor Booth]

Ross N,
spot on with the idle description. If you have the workshop manual for the cortina , usually they have wheel alignment data. You could get the KPI from that. You can then work out your installed KPI i.e. say cortina KPI is 7.5º with a static camber of + 0.5º and your camber setting is -3º your KPI is 7.5-0.5+3=10

Jim C has mentioned a figure of 6.5º but does not say if this is a corected figure or what it states in a manual. JIM ???

Russ N,
the mass transfer may be part of the problem

If you cant start the engine--thats what tow cars are for -))

[Russ Noble]

Trevor. Silly me! I must be the only guy not to have realised that when you change your camber you automatically change the KPI!

However in your example shouldn't the maths go 7.5+0.5+3=11? I know, I know, you just slipped that in to see if anyone was paying attention!!

Cheers

[roaldin]

Assumed Standard Cortina KPI = (-)7.5
Static Wheel Camber with KPI @ (-)7.5 = +0.5
Actual Wheel Camber set at -3

-7.5+0.5-3= -10

Actual KPI = (-)10

Things may be different over there though

No allowance has been made for idle speed.


Tim.

[roaldin]

Trevor,

When you do get a chance to plot the front of an RF I'd be interested to know what effect (if any) the angled inner pickup points (distance between L & R front inner pickup points is shorter than distance between L & R rear inner pickup points) have on things: bump steer for instance.
What does the relative advantage/disadvantage turn out to be.

Hopefully the pic' kinda sorta shows what I mean.

I've just realised this may have strayed too far off topic, hopefully it's ok - apologies otherwise.

Tim.

[Jim c]

Trevor

The manual I got my info said 6.5kpi
It maybe a typo but thats what it quoted.
that would not be a corrected figure.

Jim

[Russ Noble]

Originally Posted by roaldin

Assumed Standard Cortina KPI = (-)7.5
Static Wheel Camber with KPI @ (-)7.5 = +0.5
Actual Wheel Camber set at -3

-7.5+0.5-3= -10

Actual KPI = (-)10

Things may be different over there though

No allowance has been made for idle speed.


Tim.

Doesn't work Tim!The wheel camber is being reduced by 3.5 degrees. Therefore the KPI is being changed by 3.5 also!

Things are different over here Tim. We get it right!

Maybe the real issue that Trevor is trying to draw attention to though, in his cryptic way, is that Ross states his installed KPI is about 7.5 degrees with Cortina uprights. Yet he has (I think) -3 degrees camber. We have a discrepancy here. Why? The installed figure should be nearer 10 degrees, I think. Is something bent?

Cheers

[roaldin]

Doh!

No really, I was confused by Jim and his 6.5. No? Would you believe...

Trevor, you're on your own.


Tim.

[Ross Nicol]

I think it's amazing there isn't a king pin in sight but it's still there in spirit, well as an imaginary pin anyway.
Ok to set the record straight since the video and race meeting at PI I haven't adjusted anything.All I've done is measure the caster which I found to be 4.75 LH and 6.75 RH degrees. So I've identified the heavy steering probable cause, and am now deciding what to do about it in consultation with you knowledgable chaps. I understand now (with Jac Mac's help ) that altering the inboard rose joint spacers on the top wishbone will seriously alter the camber so caster adjustment will be by screwing rose joints in and out of the wishbone in future.
Malcolm I've never had any brake pad knock back problem thank god.
Does the GTD use Cortina uprights as in the RF? and could the spindle flex be due to the smaller spindle shaft at 1.06" as Jac Mac described? Anybody got the procedure to measure scrub radius in steps. I'm interested in how to get the Kpi to a position under the tyre. I presume you mark the floor with the tyre centre and then take the wheel off, with jack holding the wheel at ride height.Then use a straight edge through upper and lower ball joint to the floor.
Ross

[Ross Nicol]

Russ while I was typing my post I see insults have been traded across the Tasman, oh well I'm on the fence. No the KPI 7.5 degrees I measured was taken from this spare unfitted upright I have.I sat it on the bench with the 2 ball joint holes vertical and measured the angle of the spindle face.I'll check my fitted KPI when I can fully understand how to measure it. I'm slowly getting there though. Have a feeling I'll come away from this thread a much wiser front suspension fiddler.
Ross

[Russ Noble]

That would work.

Did you see post#72.Probably went up while you were typing. While you're doing all this measuring of the scrub, do an accurate measurement of your KPI and your camber too.

Cheers,

Edit. Beat me to it while I was typing this time!!