, and the inability to house any more inset on the escort strut, gives loads of lovely thumb ripping kicks on rough, tarmac rally type
roads!!! That's at a fairly low speed too, (well, below 90mph ish)
As the title says. Click here.
Scrub radius and it's effects seems to be an area that the experts can't agree on
quote:
Originally posted by WIMMERA
Scrub radius and it's effects seems to be an area that the experts can't agree on
Ingalls Engineering state that the greater the scrub radius(positive or negative) the greater the steering effort required.
Jeffrey Daniels in Car Suspension at Work states, Where the steering axis is in line with the centre of the contact patch (zero scrub) the front wheel
can only be turned by scrubbing the whole patch around it's centre, this makes the steering very heavy at low speeds, he goes on to say, one
answer is to introduce some offset so that at least part of the contact patch will roll around the steering axis instead of scrubbing, and just to add
a bit more flavour, Staniforth in Competition Car Suspension states "centre point steering" provides both lightness and lack of kick back
through the steering wheel that is still a laudable aim but hard to achieve
OK, I now see what you were talking about.
I disagree with Jeffrey Daniels; zero scrub offers the least resistance. Staniforth seems to agree with this too.
I've never read or heard of Jeffrey Daniels take on the matter. I've never heard a similar argument from anyone else for that matter.
Even if the tyres were rock hard and there was a lot of negative camber involved, any scrub radius would require more effort to turn the wheels as
they'd be taking the long way around. So to speak.
I understand what Jeffrey Daniels is getting at, it will be heavier "at low speeds". By low speeds I think he means stopped and the difference will be minimal anyway. As soon as you reach the dizzying heights of, say, two miles an hour zero scrub is the way to go. I only have one vehicle with zero scrub and when it is stood still the wheel is very hard to move, but it is a 3&1/2 tonne van.
Daniels is an engineer he's written a couple of books on suspension and handling. I've seen the Daniels argument before on a few occasions.
quote:
Originally posted by WIMMERA
Daniels is an engineer he's written a couple of books on suspension and handling. I've seen the Daniels argument before on a few occasions.
quote:
Originally posted by Syd Bridge
Very odd!! I seem to recall that you argued vehemently to the contrary, when I suggested that centrepoint steering, or as close as you can get, is to be aimed for. This was more than a year ago.
quote:
What else are you going to come to my way of thinking and experience on?
quote:
Most (if not all) topline racecars have centrepoint(or as near as can be achieved), to make the drivers life easier, avoid 'kickback', and changing steering loads with lateral weight transfer.
oooooh meow!!!
less scrub = less kickback (less moment between contre of contact patch and centreline of steering axis)........simple fact!
A seven won't have heavy steering whatever the scrub radius at low speed, because it weighs f*** all...........simple fact!
Too much scrub radius will reduce the feel through the steering of the grip at the contact patch because the feel will be masked by the moments
imparted on the wheel by uneven road surfaces........simple fact!
All thanks to simple physics!
Beyond these I don't know, and to be honest, a) more people set up suspension and wheels because " that was what they could get that was
nearest to ideal" rather that what was ideal and b) those that worry unduly about these things are usually those that don't get cars built!
quote:
Originally posted by NS Dev
oooooh meow!!!
quote:
less scrub = less kickback (less moment between contre of contact patch and centreline of steering axis)........simple fact!
quote:
A seven won't have heavy steering whatever the scrub radius at low speed, because it weighs f*** all...........simple fact!
quote:
Too much scrub radius will reduce the feel through the steering of the grip at the contact patch because the feel will be masked by the moments imparted on the wheel by uneven road surfaces........simple fact!
quote:
Originally posted by NS Dev
oooooh meow!!!
less scrub = less kickback (less moment between contre of contact patch and centreline of steering axis)........simple fact!
A seven won't have heavy steering whatever the scrub radius at low speed, because it weighs f*** all...........simple fact!
Too much scrub radius will reduce the feel through the steering of the grip at the contact patch because the feel will be masked by the moments imparted on the wheel by uneven road surfaces........simple fact!
All thanks to simple physics!
Beyond these I don't know, and to be honest, a) more people set up suspension and wheels because " that was what they could get that was nearest to ideal" rather that what was ideal and b) those that worry unduly about these things are usually those that don't get cars built!
Yes, all the Locosts I've seen with Cortina hubs and uprights(including my own) have a mile of scrub radius, but not many consider it a fault or they keep quiet about it
quote:
Originally posted by WIMMERA
Yes, all the Locosts I've seen with Cortina hubs and uprights(including my own) have a mile of scrub radius, but not many consider it a fault or they keep quiet about it
You are quite right about the escorts britishtrident, you do want to keep your thumbs out of the way, yes, with decent width rubber on the front
(7.5", and the inability to house any more inset on the escort strut, gives loads of lovely thumb ripping kicks on rough, tarmac rally type
roads!!! That's at a fairly low speed too, (well, below 90mph ish)
quote:
Originally posted by Syd Bridge
Give an open wheeler or sports proto any scrub at all, and the driver will rip your head off. That's if he has any ability to use his arms after a lap or two.
On the other hand, not enough castor, and they complain of 'dead' feel. With castor, the more you turn the wheel, the more straightening load is generated at the tyre, thus more 'feel'. The added benefit is straight line stability and steady 'feel' when cornering, without the juddering and banging that scrub imparts into the steering and suspension. Castor is also the geometry that allows the car to self steer on a banked corner. No castor, no self steering. Lots of castor, heavier in flat turns, too light on the banks.
My experience here seems to contradict some of what is being mooted.
Someone's got it back-to-front. If I'm wrong, then an awful lot of highly experienced race drivers don't know what they are talking about.
The only proviso to this is that these effects are magnified as speed increases. Below 100mph or so, the frequencies are such that kickback(except for big hits) is manageable, though still not desirable. The buggies which Rorty has his major experience with probably fall into the lower speed category. Compared to other effects from the solid back ends coupled with bike engines and light weight, and moreso the surfaces (??) they run on, kickback from scrub is the least of their worries.
[Edited on 22/1/05 by Syd Bridge]
hmm, interesing, talking of offroad buggies, have a look in my photo gallery, you may see an interesting likeness to some of the things on
Rorty's web pages!
I see some power to weight ratios mentioned too, well be way of comparison, mine is around 400hp per tonne (195 hp in 485kg), and that's
certainly not enough! National champ has nearly 600hp per tonne in my class, and in the top class they have approaching 900 hp per tonne.
I have some off-road experience too, and yes, although the setup is different to go quickly, the same parameters are relevant!
(seem to work, even
with a crap driver i.e. me!, got first in club, 2nd in league this year)
[Edited on 23/1/05 by NS Dev]
quote:
Originally posted by NS Dev
hmm, interesing, talking of offroad buggies, have a look in my photo gallery, you may see an interesting likeness to some of the things on Rorty's web pages!
I see some power to weight ratios mentioned too, well be way of comparison, mine is around 400hp per tonne (195 hp in 485kg), and that's certainly not enough! National champ has nearly 600hp per tonne in my class, and in the top class they have approaching 900 hp per tonne.
I have some off-road experience too, and yes, although the setup is different to go quickly, the same parameters are relevant!
[Edited on 23/1/05 by NS Dev]
How much offset did the Rostyle wheels have? The Cortina wheels that I've measured have 41.4 mm offset (stamped on them)but that still gives 3+" of scrub. I have a set of Mondeo wheels that have 49.5mm offset. The fact that Cortinas have such a small KPI angle doesn't help the cause either, but it is a good feature in other respects. I have fabricated a set of uprights with a live stub for the new car, which should address most of the problems and they have a weight advantage.
What's the standard tyre size for those wheels?
Can't help with standard tyre sizes, they were bare wheels when I got them, the Mondeo wheels are 15x6 the Cortina 13x5.5 also have a set of aftermarket Peugeot 7x16 with 20mm offset all 108 PCD
quote:
Originally posted by Rorty
quote:
Originally posted by NS Dev
hmm, interesing, talking of offroad buggies, have a look in my photo gallery, you may see an interesting likeness to some of the things on Rorty's web pages!
I see some power to weight ratios mentioned too, well be way of comparison, mine is around 400hp per tonne (195 hp in 485kg), and that's certainly not enough! National champ has nearly 600hp per tonne in my class, and in the top class they have approaching 900 hp per tonne.
I have some off-road experience too, and yes, although the setup is different to go quickly, the same parameters are relevant!
[Edited on 23/1/05 by NS Dev]
900 hp per tonne is pretty impressive. I presume they're some of the twin engined cars?
How times change. My fond memories of autograss are of a RV8 on a Renault transaxle mid-mounted in a Fiesta.
We race on circuits like motocross tracks, complete with all the jumps and table tops. 400 hp per tonne coupled with 14" of travel is quite a buzz. It makes a CR500 seem tame!
So, what do the experts reccomend as a maximum scrub dimension for fast road and track day use, om a midengined car??
Lets learn from all this debate
Regards
Fred WB
No idea Fred, have heard that 25% of tyre width is a good starting point but no doubt others will have a different slant on it, it's not the easiest thing to change which I suspect is the reason it's generally overlooked
Yes Syd the new uprights are built to achieve zero with the Mondeo wheels I mentioned earlier, but , until I drive it I won't really know. A Ford suspension designer suggested the 25% of tyre width as the max, (as Fred asked for) to me some time ago, so thats what I've done, two sets of wheels with different offsets and an assortment of wheel spacers should see me right. Now a question for you Syd, what components do you use to achieve zero scrub.
I should add a bit to that question "on a Locost, not a racecar"
Wimmera
From memory, some Fords run about 20-25mm +ve on road cars. Bear in mind those are front engined cars. As per usual, Syd is off on his own and
misrepresenting me.
I think I mentioned in a post some time ago, though I can't find it, that 10-15mm was a good Locost figure for the road and is probably a good
start for the midi you refer to. I'm told some older cars ran much more scrub than 25mm.
On track, where suspensions are tauter and more responsive and the drivers are more attentive with adrenalin in their veins, then yes, closer
to zero is the way to go.
quote:
Originally posted by Syd Bridge
Rorty,
Please explain this one perplexing question: Why MUST you always be seen to be correct?
What you have just written regarding dimensions, is more or less the same as what I wrote just previously. The comment on teaching came from the people who actually teach this stuff. One of whom I spoke to on the weekend. If you know better, write a paper and get it published. Then go on a lecture tour to inform the world, and make a million doing it. In the meantime, I'll stick with simple physics, and accepted engineering and automotive convention and teaching.
Wimmera,
If you're using Cortina uprights and hubs, you're stuffed.The offsets are huge, much bigger than most cars before or since. The only wheels I've seen close to the same offset are on the Austin/Morris 1300's. There was a last minute wheel size change in the design process, which stuffed up a lot of geometry. (Or so the Ford engineers told me.)
Anything further I'll continue on u2u.
Please, anyone wishing to ask me questions or further discussion on this do it via u2u. This is getting petty, trite, and downright stupid.
Syd.
quote:
Originally posted by Syd Bridge
The essential difference in our outlooks is that I know scrub as an evil and should be zero but up to 25-30mm is tolerable, though not desirable. You say it is necessary.
Now, if all of this has been anything more than an ill conceived wind up, Mr.F-S, you are a very sad and lamentable person indeed.
I know that thing in you demands you have the last word, but I'll be saying no more.
The rest of you can continue on the u2u, if you so wish.
Syd
The unsuitability of the Cortina hubs & uprights was established early in the piece Syd, and I went into some detail about what I'd done to overcome the problem, but my fix is not for everybody as it involves a fair amount of machining and general stuffing around, so I'll repeat the question "what components do you use to achieve zero scrub?" the stage is set for you here Syd to really impress those of us who are trying to build a better mousetrap and on a personal note would your middle name be Harbour by any chance.
I think this thread should finish now, the moderator on here should have pulled it long ago, trading of personal insults is daft so lets finish
now