+50

 

Awesome, have a citation?

 

It's the stock offset...

 

Right, but offset is a variable that only has meaning in the context of width.

Ie, oem wheel (16x8) has 52mm front spacing and 152mm back spacing---- call it 2 inch front and 6 inch back.

17x9.5 +50 yields 71mm front spacing and 171mm back spacing---- 2 3/4ish front and 6 3/4 back.

you're basically adding on 1.5 inches of width, and retaining the same offset splits the difference...... 3/4 of an inch on each side.

If this is retaining the scrub radius at zero then maybe it's party time

 

The offset is what the tire sees in relation to the suspension. Having the same offset will keep the same scrub radius even with different widths as the tire "grows" around the scrub radius in equal directions.

Most people run positive scrub radius (lower offset wheels) which when large enough can lead to the tire grabbing the road and pulling the steering wheel, especially under braking.

Try to run a minimal positive scrub radius or run stock where possible. with aftermarket suspension a 17x10 +50 is a good bet or 18x10 +50. +45 is also pretty minimal...I am running +42 with 11.5" wheels up front and I don't have any negative effects from it that I can tell.

 

GoodfellaFD3S

If this is retaining the scrub radius at zero then maybe it's party time

Well, we should not assume the stock FD has zero scrub radius.

In fact, that would be impossible because it supposes one could keep the tire contact patch (not even just the wheel) perfectly parallel to the road surface and have the front and rear steering axis move with the contact patch to maintain a static scrub radius.

I think what most people are interested in is very little change in scrub radius in the range of suspension movement they drive in for normal/relaxed driving.

Yes, if you stick to stock alignment settings, stock suspension height, stock tire diameter, stock bushings, stock tire sidewall and stock offset you will have zero change in scrub radius from stock (and a stock FD ).

Stock offset is a good start if you are trying to maintain (excellent) stock driving characteristics though.

 

Citation needed. Assuming something without data is counterproductive.

I'd like to see where about zero would be, want to see where I'd lose all negative scrub that I'm assuming is built in to +50 offset.

 

Sorry, I took "zero" to mean "the correct (stock) amount."

 

This video explains the terms nicely:

 

17x10 +38 over stoptech ST40 Kit

20160621-DSC_1277 edit by chronumn, on Flickr

20160621-DSC_1263 edit by chronumn, on Flickr

 

Show us how well they fit in the fenders!

 

I will report back Saturday once I get my tires mounted!

Edit: in the air with no tire, probably doesn't help much though.

20160621-DSC_1274 edit by chronumn, on Flickr

 

Same wheels and specs as above...

 

Would those tires touch a plum bob suspended from the edge of the fender?

Any rubbing on the track?

 

I'll take a better pic tmrw. Track duty coming soon, I'll let you know how they do.

 

What tires are you running and sizing?

 

Bridgestone Re11a 255/40/17 all around. Really nice tires

 

Yup RE11's are a great street tire. My RS3's that I'm getting should be just a hair wider to fill out the wheel.

 

I considered RS3s but since I have 17s I decided against them because of their softer side walls. Might be overthinking it tho

 

I didn't get the plumb bob pick but here is a better pic showing fitment from the rear.

 

I only asked about the plumb bob because it's illegal to have tires that poke out past the fenders here.

 

shop manual lists the offset. i'll have to look, but scrub radius might be mentioned in the yamaguchi book.

you're kind of right, we are assuming scrub radius is near zero, but not sure we actually know its curve, and its a PITA to measure

 

I checked it with a straight edge on the wheel extending up past the fender, and it does stick out about 1/4"

 

I'd greatly appreciate if you'd check to see what the stock scrub is/ where zero is at in that book. Thank you!

 

Stock scrub radius specification is not in the Yamaguchi book.

The only incidental mentions of scrub radius are page 98 where it is mentioned that there is positive scrub up front.

"Braking force generated by the car's brake system is applied rearward at the tire's road-surface contact. When subjected to this force, the suspension produces toe-out moment which stabilizes the car's braking attitude because of its kingpin axis position, which is inside the point of application of the braking force."

And page 99 where it is mentioned the rear scrub varies from negative (under braking) to positive (under throttle) as the kingpin axis is behind the center of the contact patch and the rear has the variable toe bushings.

"During braking, the combined effects of geometry (the kingpin axis is outside of the braking force input point of the tire's road-surface contact), the vehicle's tail-up (lift) attitude and the tire's slight rearward movement caused by the trapezoidal link effect (formed by the toe-control link and the lower transverse I-arm, induces a toe-in moment. This enhances stability under braking."

"The toe-in moment induced by the combined effects of geometry (the kingpin axis extension is now inside the force's input point) and the vehicle's slight squat attitude when driving torque is applied to the rear wheels, is moderated by the toe-out from the trapezoidal link effect which displaces the wheels slightly forward. This produces an optimal toe attitude for combining agility and stability when the driver "plays with the throttle" while negotiating a corner or a bend."


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Is finding out what the stock scrub radius was really important?
Keep it as close as stock (50 offset) as possible and you will have stock like road manners.

The FD is an old car that was on 225/50-16 tires and anything other than cruising that contact patch is moving inches away from the steering axis anyways (scrub radius changing).

If you haven't worked trackside and watched tire sidewalls, it looks like this (watch 1:20-1:30)-


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More interesting to me is to read pages 98-103 in the Yamaguchi book about how the front and rear goes through constant toe changes to keep bad drivers from crashing in an era before Dynamic Stability Control.

Well, some people would call a car that does what the driver intends rather than what they command it to a good handling car. I say those people should learn how to command a car or get a non sports car or better a self driving car.

Replace those variable toe bushings with single axis bushings and you get a more pure handling car (like a modern Mazda sports car with DSC turned off).

Replace the multi-axis spherical bearings with bushings (rear toe and lower transverse arms) and you get a car with bound-up rear suspension that tries to kill you...

On an old car like the FD it is possible to improve the handling, but it is also still very easy to worsen it.