P4
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| posted on 30/12/02 at 09:11 AM |
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Wheels and ride hieght
I am using Cortina uprights and would like to know the following if anyone can help:
What rolling radrus of wheels should be used?
What is the ride hight from the floor to the under-rail of the chassie?
How much off set should the wheels have to give a satisfactory kingpin inclination angle.
Is there a simple way of checking the caster angle?
P4
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Mark Allanson
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| posted on 30/12/02 at 11:03 AM |
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Tyre diameters are as follows, the Cortina that I got my uprights from was running 165 13's
135 13 21.5
155 70 13 21.54
165 13 23.39
165 70 13 22.09
175 13 24.02
175 70 13 22.65
175 14 25.02
185 14 26.65
185 60 14 22.74
185 65 14 23.47
195 60 14 23.21
185 65 15 24.47
195 65 14 23.98
195 60 15 24.21
195 50 15 22.68
195 65 15 24.98
195 55 15 23.44
195 60 15 24.21
205 55 15 23.88
205 65 15 25.49
225 55 17 26.74
I think the correct offset should be ET19, but stand to be corrected on this. Don't be tempted to use wheels with a large offset, it will destroy
your handling (see Rorty's website - all is explained).
Ground clearance varies, but acording to a previous thread, about 4.5" is pretty standard.
Caster angle can only be checked accutately on an alignment system, but accurately measuring the ball joint centres and doing the maths will give a
good idea, but this assumes the chassis is dead level
You have asked a few questions, are you trying to check out the feasability of a cunning plan - why not share it, it may give us some new ideas as
well!!!
Hope it helps,
Mark
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Rorty
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| posted on 31/12/02 at 02:20 AM |
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As viewed from the front of the car, the point where the line through the ball joints meets the ground, does NOT necessarilly have to coincide with
the centreline of the tyre. If it does, it has zero scrub radius....not a good thing.
If the line through the ball joints meets the ground on the inside (car side) of the tyre centreline, then it is said to have positive scrub radius.
As you're building a rear wheel drive car, this is what you should be aiming for. Most front wheel drive cars use negative scrub
radius.....er...another time!
As to the actual amount of positive scrub radius you should have, is, as Syd points out, built into the wheel/tyre/upright package by the
manufacturer....Ford.
As a guide, a positive scrub radius of around 10mm would be a good starting point. The scrub radius can be varied by fitting wheels with a different
off set, and also by fitting tyres of a different height.
My advice would be, unless you really know what you're doing, is to stick with the package the original manufacturer designed and specified. In other
words, if you are using Cortina uprights, then use either original wheels, or aftermarket ones specifically for the Cortina, and fit tyres of the
originally specified height. That's not to say you have to use skinny tyres, but choose a diameter/width/aspect ratio to achieve the same height.
This can be a dangerous area to mess about with, and apart from the risk of the finished car being unsafe, it could also end up being an absolute pig
to drive.
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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Stu16v
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| posted on 31/12/02 at 06:41 PM |
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Basically scrub radius is there to let the steering 'communicate' with the driver. Zero scrub radius results in a dead feel and doesnt allow the
driver to 'feel' when one of the front wheels is losing traction from braking etc. It also accentuates the pull from having a soft tyre on the front
axle too. Having too much may result in a lethal car if you were to have a blow out on the front at speed.....
Dont just build it.....make it!
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Duncan
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| posted on 31/12/02 at 08:14 PM |
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When using Cortina or Sierra uprights, it is worth noting that the King Pin Inclination (KPI or SAI) and the Camber angles are set by Mr Ford, (this
is the included angle) and cannot be altered proprly by adjusting the length of the top ball joint. If camber is adjusted by this method it
automatically affects KPI and scrub radius.
I would stick with Rorty's advice and keep everything to the settings for the donor vehicle. At least you know it works adequately
ps. scrub radius is also the angle that attempts to prevent the vehicle pulling violently when a tyre suddenly deflates.
[Edited on 31/12/02 by Duncan]
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Rorty
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| posted on 1/1/03 at 05:29 AM |
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Syd, I did wonder if you had been born within sight of the Sydney Harbour Bridge! I'm from N. Ireland BTW, I'm only a recent temporary Aussie.
What Stu16V says about positive scrub is spot on. The biggest worry with drastically altering it, is obviously that of safety.
There's also the aspect of compliance. If the radius is too large, then the steering will be much harder to turn. Try standing on your heel, and
spinning around. Now do it again, but drag the ball of your foot on the ground as you do. It's the same thing really.
Negative scrub has applications in front wheel drive cars, which basically has to do with the pull exerted by the nature of
FWD.
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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Mark Allanson
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| posted on 1/1/03 at 11:46 AM |
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Anyone got an accurate drawing of a Mk5 upright so we can burn our calculators out?
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Stu16v
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| posted on 1/1/03 at 03:17 PM |
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You dont really need one mate. Basically, fitting a similar wheel offset and rolling radius, set the wheel camber properly and the maths is worked out
for you.
It's surprising as to how much the steering/suspension geometry is affected by bolting on different sized wheels and tyres. Throw into the equation
the possible change to castor, ride height and roll centres, and it is easy to see how simple it is to mess things up big time.....
[Edited on 1/1/03 by Stu16v]
Dont just build it.....make it!
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