Originally posted by Jasper
Been offered a pair of 13" Compomotive wheels with an offset of 30mm, is this ok or will they rub????
Hi Jasper
this may either help or totally confuse you,i know it did me. ST told me that i needed an offset of ET24, after speaking to someone in the alloy wheel trade it became clear that ET24 was impossible to get, modern FWD cars seem to use huge offsets up to around ET50 or so, older RWD fords as quoted by Mark,use ET18,interestingly Peugot 205's are FWD and use ET19 and the same PCD as ford.
Phew! with me so far?
Anyhow i borrowed a wheel and tyre off me mates Westie ( ET35 ) and tried it for size, it fitted the front Ok, but on the rear it fouled the shock, and looked like it needed a spacer of about 10mm ( ET35 minus ET24= 11mm so maybe ST were right. Iv'e bought a set of ET35 and am using spacers on the back, i don't have a clue about testical stuff like scrub radius and reading Rortys site i may have dropped a huge bollock, time will tell.
One thing i can say is that i'm sure some lads are using Ford Focus alloys, surely these will have a large offset?
Sorry loadsa info but not much help
Al
Rorty - 22/1/03 at 03:09 AM
This was addressed in another post a few weeks ago, but I'll just reiterate a warning again here.
Be very careful which wheel rims you fit to any car. Just because they physically fit on without fouling any part of the steering, suspension or
bodywork, doesn't mean they are suited for your particular car.
Where they probably won't have much affect on the rear, apart from maybe overstressing the wheel bearings, the wrong offst or inset on the front, can
have disaterous results.
If too large an offset is used, the steering will be heavy, and will make parking a real pain in the butt.
If the offset is too little, or if inset is present, then you will end up with an over sensitive, twitchy car. You'll spend all your time, trying to
keep it on the straight and narrow. The car may, for no apparent reason, at a certain speed, decide to head for the shrubbery.
To keep it all safe, find out what offset (or inset) was originally fitted to the front uprights you are using, and only fit wheels of the same
dimensions. 
Stu16v - 22/1/03 at 10:44 PM
quote:
Originally posted by Syd Bridge
How many times must this be addressed?
Steering 'feel', or Weight, is down to CASTOR. The more castor, the heavier the steering, and the greater the self-centreing effect.
Steering feel is different to steering weight. IMO 'feel' is the information sent back through the steering wheel, whereas 'weight' is the heaviness of the steering.
In an ideal world, road surfaces are perfect, there is no ice, and you dont get punctures. Thats when zero scrub is ideal. And it makes the suspension designers job a lot easier.
Scrub is carefully designed into the suspension geometry, to help give the required feel if the car starts to skid, or if it has a soft tyre etc, etc. A lot of FWD drive cars run with negative scrub (one reason for the high offset) on the front, which apparently helps control torque steer, and in the event of a blowout, helps conteract the nasty 'snatch' that occurs.
Castor geometry supplies the steering self centering effect, but there are other factors which influence the effect. Kingpin inclnation, tyre pressures, and scrub play a part too. Get the scrub wrong and you end up with a car thats a pig to drive on Blightys roads, it will either dart all over the place as you fight to correct the wayward steering, or it will be as dead as a hedgehogs flatmate, you just wouldnt tell if one wheel was skidding, or you have a slow puncture.
IMO Rortys advice is good, although may I add that it is only a good starting point, because the effects of scrub are in turn effected by the weight on the front axle. A light car such as a Locost, *may* benefit from altering the original offset specification wheels for the uprights.
But then messing with the offset in turn effects the roll centre geometry, as will increasing the rolling diameter of the tyre. And thats another can of worms.....
[Edited on 22/1/03 by Stu16v]
Rorty - 23/1/03 at 02:20 AM
Stu16v, you're absolutely correct, and in fact there is quite a bit more to it, but I was merely trying to keep to the basics for simplicity's
sake.
Syd, Castor and scrub contribute to a car's steering feel and responsiveness.
I've never in my life seen a production car with zero scrub radius.
quote:
Don't anybody take my word for it though
I won't!
Liam - 25/1/03 at 12:18 AM
Syd - is this car you have drawings for a FWD maccy-strut car? It is apparently common practice for them to have zero or negative scrub radius. Zero
eliminates one cause of torque steer, or going negative helps with stability under brakes (because the front wheels want to toe in I presume). Zero
scrub would be fine for a comfortable shopping trolley where feel isn't important, but easy steering and reduced torque/brake steer are important.
Most RWD cars (i.e. locost) have a positive scrub radius to provide feel and less twitchy steering.
quote:
I've never in my life seen a production car with zero scrub radius.
Aaah well...at school, a mate of mine got hold of some ancient Citroen. A DS I believe. He threatened to restore it and actually drive it until we laughed him out of it and he got an uno/nova/fiesta like the rest of us. Anyway - apparently they were famous for their 'centre-point steering' or zero scrub radius. Advertising blurb mentioned how they wouldn't veer off under braking, or if a front tyre blew out - cos they had 0 scrub. That was the only thing 'good' about it. So there =)
Liam
[Edited on 25/1/03 by Liam]
Rorty - 25/1/03 at 01:29 AM
My appologies, I should have made my last statement a little clearer.
I will now state: I have never seen a rear wheel drive car that handles properly, with zero scrub radius.
Liam - 25/1/03 at 02:02 AM
Ah but the citroen was RWD! Completely stupid car though - it's as if citroen deliberately came up with the most bizarre solutions to engineering
problems. I mean even though it was RWD, they went to the trouble of putting in a complete front driveshaft assembly just so it could have inboard
brakes!!! There is a single hydraulic pump that operatesa number of systems. That pump failed and you had no steering, brakes or suspension!! My
mate got rid of it pretty soon after he drove it back into his garage after disconnecting said hydraulic system to fiddle with the suspension. One
brake system short of a brake sytem, he was lucky he didn't destroy his garage! We laughed about this for a week.
You may still be dead right though with your statement - it probably handled like a bag of sh*t.
Liam
fastenuff - 25/1/03 at 08:32 AM
Liam do you have other citroens than we have on the mainland. Even the the Traction Avant was FWD last time I saw a DS it too was FWD, unless you'd drive backwards then it would be rear engined and RWD and you 'd be facing the wrong way
Liam - 25/1/03 at 01:40 PM
Hang on - I'll just ask Mr Internet...oh it is FWD. I could have sworn it was RWD. It was a while ago mind, and I didn't really see it much - just
heard amusing stories about it. It was blue. Maybe I assumed it was RWD just cos it was so old? Hmmm, oh well.
Liam
Alan B - 25/1/03 at 05:39 PM
And to complicate things further....
There is the self-centering effect caused by king pin inclination, whereby the car tries to rise as the wheel steers off-center.
Damn....this is fun...
Mark Allanson - 25/1/03 at 09:50 PM
Is anyone in a position to say exactly (in mm) what is the ideal scrub radius for a locost running cortine front uprights?
The cortina I took the uprichts from was running 165 13 tyres on rostyles, I am fairly sure the offset of these is ET18 (can anyone confirm?)
I have a set of rostyles of a Mk 1 1300GT escort, a set of pepperpots of a sierra 2.0i ghia, but would rather buy a set of 14" or 15" alloys
specifically for the car, before I part with £400 I would really like to know I am buying the right wheels!
Has anyone got a set of drawings for the cortina uprights (sierra DWG drawings are widely available but I havent found any for the cortina), from
these and knowing the scrub radius, I can work out the offset for my chosen tyre size.
Hopefully,
Mark
Rorty - 26/1/03 at 03:23 AM
Syd Bridge:
quote:
I know Rorty will want to expound his knowledge on this further.
No Syd, I don't WANT to, but neither will I sit by and see other members, who are trying to decipher what's the best direction to take, be subjected to nonsense and untruths, which are totally outside the parameters of general concensus.
Syd Bridge 22/1/03:
quote:
Steering 'feel', or Weight, is down to CASTOR. The more castor, the heavier the steering, and the greater the self-centreing effect.
Rorty 23/1/03:
quote:
Syd, Castor and scrub contribute to a car's steering feel and responsiveness.
Syd Bridge 26/1/03:
quote:
Neither scrub nor castor, on its own creates feel, but they work together.
Now we're getting somewhere!
Syd Bridge:
quote:
With Castor, the setup is viewed from front and side.
How the hell do you view the effects of castor from the front?
Mark Allanson:
quote:
Is anyone in a position to say exactly (in mm) what is the ideal scrub radius for a locost running cortine front uprights?
I will shortly be designing a composite upright for a couple of people on this list. When I have all the data I need for the drawings, I will confirm, and post, the original Ford factory scrub radius for the Cortina.
I doubt if it will be less than 10mm. If it is, I'll eat this old squished, fluffy toffee lying here!
I think, as a Sevenesque Locost is so light, 15+mm wouldn't be out of the question.
[Edited on 26/1/03 by Rorty]
Stu16v - 26/1/03 at 06:55 PM
Mark Allanson, as I said on my earlier thread it is impossible to say with accuracy what the offset you (or anybody else) requires. The offset does
alter the 'scrub', but so does the overall diameter of your chosen wheel/tyre, and the chosen camber adjustment, which is probably going to be a bit
different from Fords original spec. To keep it simple, best do as Rorty suggests, and go with original to begin with, that way you are not going to be
*too* far away...
*waves hands in air* Alan B, I got that one in already..... 
Syd, I have an excellent book that goes quite in-depth into all this kind of stuff. Recommended by one of my Lecturers when I was studying MV
engineering, some really interesting info inside. Trouble is, a mate has borrowed it at the moment. I think it is called Motor Vehicle Engineering and
Technology by Hienz Heisler. If I see him soon, I will find out exactly.....
Mark Allanson - 26/1/03 at 07:22 PM
Stu,
What I meant to say was, irrespective of what tyres or wheel size, spacers etc etc... What is the ideal scrub radius.
I know (or will know) which tyres and wheels I will use, I will choose my offset and profile of tyres based on the ideal scrub radius, I can work out
the rest on a calculator, but what I need to know is what is the ideal scrub radius, not the ideal offset
Rorty - 27/1/03 at 04:30 AM
Syd Bridge:
quote:
Yes, this is fun isn't it!
Syd, you're a very sad individual. I don't see this as a game. There are members here wanting sound (and I mean main stream) advice on how to go about building their cars.
I don't profess to know much, but what I do know, has been gathered from 30+ years of experience.
I don't have many books on cars, less than 6 I think. I do remember years ago, reading a book by someone by the name of Jute, who had made the car industry his life. I think the book was called Building Special Cars. He stated IIRC, scrub radius was seldom more than 32mm, and was usually between 10mm and 25mm. If someone has this book, maybe they'd be kind enough to confirm this.
I have never seen any production car with as much as 32mm, but I have used roughly that amount on a few off-road cars. I would still say the norm is more than 10mm by design, not by accident as you state.
Syd Bridge:
quote:
(Composite uprights..... did a few for a company doing R&D for the red car team, in Guildford about 12 yrs ago. In their application, the constant heat from the braking made the situation untenable, so they stayed with spark eroded ali. Should work with road cars though, but do not handle constant shock loads without being grossly overdesigned, so back to ali. Oh, if any one wants cfrp gearcasings, did those as well. Problems with constant high temps again. CFRP conrods are good fun; but oilpans, rocker covers, waterpump housings, bellhousings, propshafts, ...well you name it, all seem to work. Did Kevlar as well. Have fun!)
Syd, you need to get out more.
Rob Lane - 27/1/03 at 10:34 AM
I don't wish to get involved in the full technical discussion but I will add some common sense observations.
Most builders use either the Cortina or Sierra hubs.
The commonly available wheels for these are 6J ET19 offset from either Ford or Peugeot for the Cortina or ET35 for the Sierra. (I know there are
other offsets up to 44 used on later FWD Fords)
Now, trying for theoretical scrub figures is fine but the S/H wheels are usually a set of four and no option for different offsets.
It would also be a pain using different front wheels to rear, unless you want to of course. Not saying it's not advisable but it does simplify
matters, such as tyres and fitment.
The actual limiting factor for purchasing these wheels is the rear hub offset from whatever you are using, be it live Axle or IRS.
In the case of the LA it's the clearance from the tyre to the trailing arms. In the ET19 wheel at 6J wide this is about 10-15mm on an escort axle.
Fortunately this is also the size and offset that was originally used by Ford on the Cortina front. So the front wheel scrub should be just as it was
designed by Ford.
The real problem is the Sierra hub and how it is modified for the Locost.
I know MK has spent quite a lot of time getting the front wishbones correct for the odd angle of the upright if used with an adaptor, hence his angled
top wishbone. He has obviously also taken into account the wider track to suit the Sierra IRS rear.
I can't speak for anyone else's design but MK have used ET35 wheels I believe, from memory when we spoke last.
Other offsets such as Focus will fit but will definitely alter scrub.
Rob Lane
Stu16v - 27/1/03 at 06:41 PM
quote:
Originally posted by Mark Allanson
Stu,
What I meant to say was, irrespective of what tyres or wheel size, spacers etc etc... What is the ideal scrub radius.
I know (or will know) which tyres and wheels I will use, I will choose my offset and profile of tyres based on the ideal scrub radius, I can work out the rest on a calculator, but what I need to know is what is the ideal scrub radius, not the ideal offset
Sorry mate, I was thinking that you were looking at the problem from the other way....
But what I said before is still valid (sort of). Your ideal 'scrub' will be based on vehicle weight (esp. on front axle), your chosen camber setting, (dont know for sure, but I would guess that Ford would of probably run zero to slightly positive camber, whereas most builders normally choose to run slightly negative), the width of the chosen wheels etc etc. Everything interacts with each other so it is impossible to state the 'ideal' figure. Hence the reason why I would advise that it's best to stick with Ford's dimensions as regards rim width/offset, even if you choose larger diameter rims, as long as the rolling radius is similar to the standard 13's, at least you will be in the ballpark.
HTH Stu.
Mark Allanson - 27/1/03 at 09:24 PM
Considering that most of our front suspensions are based on Cortinas, there seems to very little info on them. My data books dont go back that far, I
can tell you anything (suspension settings, engine data, alignment data, part numbers for just about anything) about any car built in the last 12
years from the data bases at work, but I cannot even find out the KPI of a once common Cortina, or the offset of the original wheels, buggers (should
that be beggers) belief!
Is there an online reference site with this kind of information that I have not found yet?
Answers on a postcard.............. or post it here
Mark