Offset Question?

[robd]

Hi, just wondering, if you were to put say 16x8 wheels on the rear of the Mk1 with an offset of +15 or +0 what affect would this have? Would it just change the way the car feels or could it actually damage anything or mess up the handling for the worse?

What size wheels do most people use on the rear of their Mk1.5's/Mk1.6's when going for wide wheels & what offsets?

Hope someone can help as I really want to get some lovely deep dish wheels on the rear. :thumbsup:

Cheers
Rob :)

[Jim-SR]

it would mess up the handling, increase steering effort, increase wheel bearing loads, and cause the car to tramline quite badly as well. stock offset is ET38, try to stay as close to it as you possibly can from a performance standpoint.

[robd]

Cheers Jim, would it really mess up the handling if it was just the rear wheels that had a lower offset? The fronts I will be keeping close to the standard offset so they can clear the ST205 calipers. It's just the rear I want deep dish wheels on. Just thinking(I know it's different) but lots of drift cars have like 9" rear wheels with massive dishes, just trying to figure out if they get away with larger dishes because they have wider wheels or if you want serious dish that you have to sacrifice the offset! I can live with the car feeling different, would just have to adjust my driving...main thing I want to make sure is that it won't damage the suspension or anything like that. Wheel bearings I could live with if that was the worst it would get!

What about those people that have used the Borbet A's? Think they are like 16x8 or 16x9 with an offset of +15 or +25?

Comments from people running large deep dish wheels on the rear would be great & feedback on what the car is like? Got my eyes on some 16x8's at the moment with an offset of +0 & a 3.5" lip!! :D

Cheers
Rob :)

[Jim-SR]

at the rear its less of an issue as at the front, youre going to adjust the track width (wider at the rear isnt generally a great idea. look at any great handling car and the front track is almost always wider) and youre still going to alter the geometry a great deal. most importantly probably the roll centre height. it can be accounted for more easily at the rear, but then its not as simple as just fitting wheels. youve got to account for it in spring rate at the very least.

i expect lots of people have fitted low offset rims and dont even notice the difference. its not going to make a good car suddenly awful. its just a question of whether the visual benefits outweigh the performance ones. also youve got to consider traction vs cornering performance and drag. wider tyres give you more drive traction, upto a point. they also give more lateral grip, upto a point. but they have more friction in a straight line that is going to sap power, and also youve got diminishing gains (doubling tyre width wont likely double the benefit, but it will more than double the drawbacks).

it all depends on what you want from the car really. make an educated choice based on the above.

[robd]

Cheers again Jim :thumbsup: I would prob only want to go to 225's at the most on the rear, mainly because of the cost. Front I want to have no larger than 205's.

Thanks for the advice, I'll keep looking into it & see what wheels I can find. This car here is what I'm basing my look on...standard body wise but a nice stance & nice deep rear wheels. :thumbsup: Clicky (http://i6.photobucket.com/albums/y233/JigenVW/AW11.jpg)

[jason00122]

jim-sr

all very valid points but heres somit 4 consideration. fact: a block of metal weighing 10kg with a contact patch of 10 square cm has no more traction than that of the same block with a 20 square cm contact patch therfore the friction difference between both is none. somit 4 every one to get there heads round :) same theory applies to aplys to a wheel

[robd]

Lol Hi Jas, not to stomp on your parade but I don't think physics clearly applies to tyres because of the variants or coeffeciants they call them!! Also a tyre has tread so the more surface area on the tyre means the more tread the tyre will have, meaning more grip/traction.

Section taken from Wikipedia..

"In the case of tires, although traction increases with load, it increases less than linearly, and coefficient of friction decreases with load. A larger contact area reduces the load per unit area, resulting in more grip. There is a point of diminishing returns, such as weight and aerodynamic drag. It's also true that a larger tire dissapates heat and with a lighter load involved, it wears less. Another reason for a larger contact area is to compensate for debris or track imperfections interfering with the tire and pavement contact.

Note that load sensitivity is commonly used to adjust the understeer or oversteer of car. When a car turns, the downforce on the outside tires is increased and the downforce on the inside tires is decreased. The body of a car also rolls a bit, and the supension can be used to unequally distribute the load between the front and rear tires. If the front end is relatively stiffer, then more of the downforce is exerted on the outside front tire, and the relative grip is reduced because of tire load sensitivy. A stiffer front end causes the front end to lose some grip in turns, resulting in understeer. A relatively stiffer rear ends results in oversteer. Street cars are generally setup with understeer, while race cars are setup with a small amount of oversteer.

For a variety of reasons, such as deformations, molecular bonding type reactions as well as surface roughness, real world friction isn't the simple thing described in physics books. From the wiki article on friction:

in general the relationship between normal force and frictional force is not exactly linear (and so the frictional force is not entirely independent of the contact area of the surfaces) "

Think what that is basically saying ir rubber is different to normal solid surfaces so Newton's theory does not apply in the same way. Also as I said about tyre tread, the tyre is not just one whole solid object, it has tread & lots of other stuff I'm sure Google can explain a lot better than I can lol :D

[jason00122]

yup i agree with what ur saying its not entirely true for a wheel as there are other factors like dynamic weight transfer etc however the above statment is still fact ;) that cant be disputed

[robd]

lol well yeah, but a tyre is not the same a a solid block of metal, therefore the same theory does not apply in the same way. I know it's fact, I'm not disputing that, just saying that a larger tyre will have more traction than a smaller tyre of the same type, we all know it. Otherwise F1 cars & racing cars would all be driving around on bicycle tyres! lol ;)

[Jim-SR]

yup i agree with what ur saying its not entirely true for a wheel as there are other factors like dynamic weight transfer etc however the above statment is still fact ;) that cant be disputed

ever wonder how something as incredibly complex as friction could be made into an equation with 2 variables? F = uR

because its a gross over simplification!

tyres dont exhibit the same friction characteristics as hard flat surfaces put through a simplified formula. try telling the F1 designers that they are wasting their time with huge wide tyres and that they should run bicycle wheels instead, theres the same amount of traction available lol.

there are 3 main modes of tyre friction that are generally accepted by engineers - adhesion, deformation and wear.

adhesion is pretty much what it says, rubber (and synthetic equivalents) creates momentary molecular bonding with the surface it is in contact with, basically gluing itself to a surface momentarily until a force breaks the adhesion (thus producing friction). this is repeated constantly all over the tire surface. this is also related to surface area, the more surface area, the more adhesion. the more normal force acting on the tyre, the more surface area is created by pushing the soft rubber into the irregularities of the road surface. adhesive friction is the main contributor to tyre grip. however it is drastically reduced by lubricants on the road, e.g. water, oil, etc. so when it rains adhesion is basically out of the equation, hence why tyre grip reduces so much. you then depend on deformation to do most of the work. hence softer tyres give substantially better grip in the wet!

deformation is simple to explain but very complex in reality. essentially the road surface is rough, and the tyre is soft. force pushing down on the tyre causes it to deform into all of the gaps in the road surface. if you then try to push the rubber in a horizontal direction, in order for it to move the rubber in the gaps must deform. this creates an opposing force and thus also grip.

finally wear is similar to deformation, but at an extreme. when the forces pushing on the deformed rubber exceed the tensile strength of the rubber then it will tear. sometimes only partially, sometimes it will completely remove material. look on a race track at how much rubber there is laying on the road. sometimes it will collect up into large balls. the rubber tearing absorbs energy and increases surface friction.

so as i said, tyres are a LOT more complex than your simple F=uR formula. surface area most definitely does play a huge part in tyre grip, in all of its forms. ive got a book by a bloke called Hans Pacejka, hes one of the leading authorities on tyre simulation in the world. this book is probably 250 pages of some of the most complex maths ive ever had the displeasure of experiencing, just to mathematically simulate tyre behaviour. compare that to dampers, which most people would consider the most complex part of a typical suspension system and the formulae would probably barely fill 10 pages lol.