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Discussion Starter #1
I currently have a set of 17&#

I currently have a set of 17" Speedlines (profile pic) but ran across this photo ( http://www.ferrari-fahren.de/ )of a 3.2 with the 16" qv wheel that also looks pretty good.

The qv wheel offset would have the wheel out a little on a 3.2, as you can see. Intuitively I would guess that the sl different offsets would cause a 1. little bit more wear on the bearings and 2. change the steering response, but not sure how.

If the wear was negligible and the steering response was appreciably increased, this may be a consideration. Ideas?

thanks
rt


 
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Discussion Starter #2
off setting the wheel out &#40

off setting the wheel out (less back space ) will change the scrub radius of the suspension. If you draw a line through the center of the upper ball joint down through the center of the lower ball joint and continue to the ground point,ideally that line would intersect with the center line of the tire width at the ground. That would be a zero scrub radius setup.when you turn a zero scrub radius setup,the inside half of the tiretread will move back and the outside half of the tiretread will move forward,(outer tire in a turn) that is the position of the least scrub of the tire on the road surface.If the offset moves the tire out past this line, you will increase the tire scrub by causing the whole tire to be dragged across the surface in turning. Small increases in offset will help with a wide track effect,but steering wheel turning force will increase, and tire scrub will increase.IE,when viewed from the top, the tire is required to pivot outside of the suspension pivot point.That creates alot of drag and increases tire heat and wear. Look at any current race car and you will see front rims with little positive offset. Offsetting the tires out can look way cool, but the effects of doing may hurt hurt performance if you go to far. High offsets also put higher loading on the outer wheel bearing.I don't see a problem with what you want to do, but just wanted you to know that you are changing many more things than just the wheels. Bob
 
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Discussion Starter #3
Bob
Great discussion and post


Bob
Great discussion and post.
Will probably just keep what I have then as the offset difference is about 1.5 inches, which I assume is a big difference.
Comments?
Many thanks
rt
 
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Discussion Starter #5
Just an old tired gear head, w

Just an old tired gear head, wondering what to do with 30 years of racing experience. R-Turner, Jack up the car, remove the wheel, set the jackstand under the lower a arm, try to get the jack stand to carry the car at normal ride height, and use a straight edge ruler aliged wih the ball joint centers. Carry that line to the ground.Mark it. Install the wheel and drop its centerline mark to the ground. See where the increased offset is moving, in or out of the suspension line. 1.5 inches may be better? or worse? don't know until you check it.
JRV, thank for the kind words, I will drop in when I think I can help if that is OK!
 
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Discussion Starter #6
Bob
Many thanks. The stock AB


Bob
Many thanks. The stock ABS 'convex' wheel has a 55mm offset while the non ABS convex 308 wheel I would like to use has an 11 mm offset. Will try to measure from the axis. If it moves toward the line, reducing scrub radius, then I should be ok?
I figure that the adjustable suspension settings cannot compensate as this is built in geometry.
Again, many thanks - This is very interesting.
Russ
 
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Discussion Starter #7
Bob,

I always look forward


Bob,

I always look forward to your input, recognized your level of experience & expertise very early on...
.

Regards, JRV
 
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Discussion Starter #8
R-Turner, correct, it is built

R-Turner, correct, it is built into the suspension. Only thing you can change is the wheel offset. you are looking for "close as possible" to the scrub radius point. I am not sure how close your car is to that point with stock setup.
JRV, So called "experience" gained with many self installed bullet holes in my feet.
 
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Discussion Starter #9
Bob
Thanks very much for goin


Bob
Thanks very much for going through this. From your or anyone's experience, what are some real world numbers you have seen with offset from scrub point?
 
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Discussion Starter #10
>>So called "exper

>>So called "experience" gained with many self installed bullet holes in my feet>>

Here also...I was told those holes in the feet and the blood on the knuckles builds character...
 
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Discussion Starter #11
R-Turner.
That is a good que


R-Turner.
That is a good question. I have never measured scrub radius on stock street cars. Most of my race stuff is less than .5" even with a 12" wide rim. Remember that most setups are not perfect and will still work to some level of performance.I would try the larger rims, measure them to understand where they centerline at and drive the car and see if it will perform better.A few track day laps should show the differences.
JRV,
Managed to smash a thumb nail this morning moving an engine block. Question: Do I file this under "Experience or Expertise?" I fear there may be one more category!
 
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Discussion Starter #12
Bob
Perhaps another angle on


Bob
Perhaps another angle on the same topic -
the book says that they went to the greater + offsets for ABS. Is there some geometric reason required for ABS or is it just to make room behing the wheel for the sensor ring?
many thanks
 
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Discussion Starter #13
Checked the workshop manual an

Checked the workshop manual and found the scrub radius on a stock Mondial listed at 10.8 mm (0.39 inch) positive (outboard). I saw where someone measured the stock wheel offset at 38mm. Using the qv wheels with an offset of 11mm, this would give a difference of 27mm (1.06 inch) that the wheel centerline is moved outboard. Added to the stock scrub radius of 10mm, that would give a scrub radius of 27+10, or 37mm (1.46 inches) using the qv front wheels.
That sounds like a lot to hang the centerline outboard of the bearings to me. Will try to compare. If anyone has some other examples that would be great.
thanks for all the help!
rt
 
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Discussion Starter #14
1.46" on Each wheel. total

1.46" on Each wheel. total increase of 2.92" (both sides) seems like a very big change to the front suspension. Might be ok for the rear suspension if you don't start the rubbing on the fender lips deal. I don't know the reason Ferrari increased the offset on the ABS, best guess it more clearance as you stated. If your numbers are correct,I don't think this much increased offset on the front will make the car faster.Might look faster! the outer wheel bearing is going to take a beating, not to mention that the spindle may start to flex(bend) under cornering loads and reduce negative camber over to positive camber, and that would be a big understeer problem. Anyway,Got ya thinking about this and look at all you have learned.My work here is done. Bob
 
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Discussion Starter #15
Bob
Many thanks - I know a lo


Bob
Many thanks - I know a lot more about steering geometry now than I did (which was squat). Apparantly that much more scrub radius would increase bump steer and slow speed turning effort immensely - something that Mondials don't need any more of! And while one can make correction with increased rolling diameter or toe, not sure that it makes up that much. Will probably just plan on keeping my wheels to factory offsets unless something else comes up. I know my bearings will thanks me.

It's been a big year for learning, first Webers, then camshafts, then distributor advance, and now moving into suspension. Thanks Bob, JRV and everyone!
rt
 
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Discussion Starter #16
Russ,

you might consider sp


Russ,

you might consider spacers if all you want is increased track width. With spacers of various thickness' you can mod to specs you can work with.

Bob, I think smashed thumbs, fingers & toes get filed under Expertise...
 
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Discussion Starter #17
Russ,
Bump steer, here


Russ,
Bump steer, here we go... Larger offset rims would increase "the effect of bump steer" due to the longer leverage arm of the wheel. The bump steer is a function of the arc's of the a arms relative to the steering rack location,steering arm angle, and tie rod length. Bump steer is measured with a surface plate bolted to the wheel flange on the car and moved up or down while measuring toe with 2 dial indicators on the plate,(one in front of the spindle and one behind.) Bump and droop are measured for a given suspension movement, and if good will produce an S curve if plotted. You can change the "Bump" by raising or lowering the outboard end of the tierod at the steering arm on the spindle. The sweet area is the first 1 or so inches (street car) of bump. That is the area where you want the smallest toe change possible. on the old production race cars, we ground, heated and cut, welded, and shimmed the arm to get to the correct setting. On the Formula race cars, we can dial the bump down to under 2 thousands per half inch of travel. It will make the car much more predicable and easier to drive faster.
 
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