Doing a a bit a Googling I came across some interesting tyre test data put on the web by RWTH Aachen university.
Of particular interest is a graph shown the effect of vertical load on self-aligning torque on a 195/50x15 tyre, this is a tyre sized
used by many locaost builders.
Self-aligning torque is created by distortion of the tyre sidewalls important for steering self-centering.
The area of interest is the results from at and below 3000 Newtons vertical force as this represent a typical corner weight for our sort of
car.
The rest of the page contains some other data of interest.
It probably not what the OP was looking at, but anthor interesting view on self aligning torque is.
In order to build lateral force on tyre you need a vertical load and a slip angle. Tyre tests are done with a fixed vertical load and they then sweep
through the slip angle.
As the slip angle increase so does the lateral force until it reaches a maximum. depending on the tyre you may get a small drop off of lateral force
or it holds steady, ie it can't generate any more force so you've reached the limits of the tyre.
Why self aligning torque is relevant is it contributes to what your feel at the steering wheel (its one of the forces trying to put the tyre back in a
straight line, dont confuse this with the effect of castor angle as thats a different force/torque)
The interesting bit is the self aligning torque drops off before the lateral force peaks. This means on a car with good steering feel you can feel the
steering wheel go light just before your reach the limits of lateral force. So you get a warning you've reached the limits before dropping over
the edge!
The effects of suspension geometry (caster and wheel offset) and power steering can mask this feedback, drive a vauxhall and you'll know what I
mean :0
quote:Originally posted by liam.mccaffrey
Basically I think the graph shows that lighter vehicles like ours have significantly less self aligning torque reaction than heavier vehicles.
They don't self centre as well as heavier vehicles.
But the shape of the curve also says that the self-aligning torque increases with slip angle until you get to a peak at about 5 degrees at which point
it decreases again dropping to zero at around 12 degrees.
Agreed Bob, just noticed you are based in Swansea. Up until recently I commuted to Swansea from Milford Haven every day. Now self employed but doing
a lot of work with a Swansea base customer. Was there yesterday actually. Small world
quote:Originally posted by BobM
But the shape of the curve also says that the self-aligning torque increases with slip angle until you get to a peak at about 5 degrees at which point
it decreases again dropping to zero at around 12 degrees.
Photo Archive
Building: Dax Rush - very, very slowly....
posted on 16/8/12 at 10:52 AM
quote:Originally posted by mikebThe effects of suspension geometry (caster and wheel offset) and power steering can mask this feedback, drive
a vauxhall and you'll know what I mean :0
I know what you mean but thankfully I don't have to drive them that much...
somehow my focus always 'felt' a bit better and now I think I know a little bit of why that is (this it the point where someone tells
me the focus is just as bad and its all in my head :lol: )
Focus is a lot lot better.
Last time I was involved in steering systems, vauxhall were set up for motorway driving and safety (good straight line tracking and good at staying
straight ahead under adverse conditions). Unfortunately this set up compromises steering feel compounded by the early electric assist systems. They
might be better now but I haven't driven one to find out!
quote:Originally posted by mikeb
Focus is a lot lot better.
Last time I was involved in steering systems, vauxhall were set up for motorway driving and safety (good straight line tracking and good at staying
straight ahead under adverse conditions). Unfortunately this set up compromises steering feel compounded by the early electric assist systems. They
might be better now but I haven't driven one to find out!
IIRC the high offset on GMs was also part of the self centering (in a blow out situation etc), i guess this would would effect our kits as they tend
to have lower offsets.
On the GM front I have had a few early cars and thought they handled better than the early fords I had, abd my brother had a 04 vectra estate for 3
years and said it was one of the best handling and funnest 'big' cars he has ever driven.
Interestingly ford has since followed suit on the high offsets, the wifes Fez STs had 55mm!
When I said safety I meant things like blow out, split mu braking.
From recollection early vectras had negative offsets (scrub radius) to counter act blow out etc.
The problem with car handling is its very subjective, my friend has always had Vauxhalls and thinks they handle well, When I first drove a focus I
thought the steering was to twitchy but I just wasn't use to the response compared to my ageing motor!
Note: Scrub Radius isn't quite the same thing as the ET offset on wheels btw
Photo Archive
Building: Dax Rush - very, very slowly....
posted on 16/8/12 at 02:14 PM
quote:Originally posted by loggyboy
Interestingly ford has since followed suit on the high offsets, the wifes Fez STs had 55mm!
my focus has ET52's - standard ones for the original Mk1 pre-facelift (i.e. from 98 onwards)
and assuming the hub design stays the same doesn't a high offset wheel reduce the scrub radius just like the fancy new setups from ford and GM ?
(you just can't go as far with a normal strut setup as the wheel rim will hit the strut)
as far as the handling of the earlier cars goes it depends so much on the models your comparing
e.g. I used to drive my dad's 1.8D mk3 fiesta everywhere before I got my own car - it was a shockingly badly balanced car with big wide tyres
and all the weight on the front and no real feel to it at all, as I discovered the back end whould sudenly let go with almost no warning if you pushed
it
one of my mates had a 1.7D Nova and it was much nicer to drive (never thought I'd say anything good about the nova's )
another example was my second car, a 92 1.0 Pug 106, it had next to no grip but had a lovely feel to it - you knew how much grip you had left.
at the same time my brother had a 98 1.0 saxo, same car but with EFI, 5 gears, 10mm wider tyres and a front anti-roll bar (even the same type of
tyres) - it had next to no feel and the same snappy lift off oversteer that the 205 gti's are famous for....
I've driven my focus for about 40k now (funnily enough I used to think the steering was twitchy too) and I've had shots in the astra F, G
& H but the old focus feels much sharper and more responsive/alive. my mate said the same thing and he's the vauxhall nut who owns the astra
G and H
but thankfully I don't have to drive them that much... 
)
) 
