in shortest terms what makes the offset for the RCF? Is the rear axle by default longer horizontally than the front? How did Lexus come up with a stock +50 front and +41 offset for the rear?

If i were to run a +35 offset all around my car, even though both front and rear are a +35 offset, would the rear still stick out further toward the fender?

 

Stock off sets are 42 R and 50 front.
its more than axel length its about suspension geometry as a whole.
you can run 35 square and yes the rear will still be 8mm wider than the front.

 

Yes, and all the work the factory did to give you mechanical grip would be compromised. But if you want stance, you'll have it for sure.

 

how would the grip be compromised? would't I be making more grip due to the tires being pushed outward from a stock offset?

the Apexi time attack RCF is running a +35 offset all around as well, and from the postings they came in 3rd. Obviously there are a lot more factors, but wheels are a big one.

 

 

here are the specs they have listed under the clip

Yokohama Tire ADVAN Neova AD08-R -
Front: 255/35R19 92W -
Rear: 275/35R19 96W

WedsSport Wheels SA-15R -
Front: 19.0 x 9.5J, +35, 5x114.3, Color: GBC -
Rear: 19.0 x 10.5J, +35, 5x114.3, Color: GBC

basically a stock car, just different offset and lighter wheels. If that offset compromises grip why would they choose to go for that especially on a track?

 

Actually no. Tazz was correct, the stock rear offset is +41mm, for both the RCF and the GSF.




And to the OP, the outer rim position of the wheel, as compared to stock, is made up of two factors, Offset as you correctly mention, and wheel width. Looking at the stock wheels. The fronts are 1" narrower than the rears, so the outer wheel rim would be .5" closer to the inside, and the offset difference is 9 mm, .35". So the total rim would be .85" further in than the stock rear wheel, if you mounted a front stock wheel on the rear of the car.

So, to answer your question, we must also know the width of the wheels you have in mind.

Hope this makes sense to you.

Edit - Just read your third post. Both wheels are .5" wider than stock, and both have a 35mm offset. This means the front rim is + .6" (15mm) + .25" (half the wheel width) or .75" total. The rears would be +.28" (7mm) +.25" or .53" total.

Lou

 

so with those new numbers, would the rear still poke out further due to the original measurements?

would you recommend this kind of setup?

 

Not necessarily, and you probably have no idea what else was done to that car to make those spacers work. Chassis tuning takes a whole lot more than "lower is better" and "wider is better" because the factory did a whole bunch of work to make the car handle at the ride height and width the car has from the factory. Arbitrarily changing this means you are ignoring all the work they did to make the car have predictable handling and grip. So making a change like this will all depend on what wheels were they running, what tires were they running, who else showed up to race that day, and what other compromises did the driver have to work around to get 3rd.

Doesn't sound like you've actually gone car racing yet and definitely doesn't sound like you've won a championship in a car.

This guy knows a couple of things about chassis tuning. It's a good place to start. And yes, he has won a few championships.

 

My calculations above say yes. The measurement numbers are there.

Lou

 

From the tone of his question he appears to admit he has limited knowledge of this subject and is asking for clarification.....not to get **** on. I also didn't see where he claimed to race his car nor brag about any championships he has or has not won.

 

Lance

Don't be so hard on the OP. He's trying to understand what would happen with a wider stance and the role that offset plays. I have nowhere near the expertise that you have on the subject, but IMHO, increasing the offset within reasonable limits does in fact enhance the handling of an automobile. As you may know, I have RCF wheels on my ISF running 255/35 and 295/30 S-04s. My goal was to:

1. Maintain the Factory ODs as closely as possible.

2. Increase the tire width, front and rear. IMHO, the stock ISF is both undertired and underwheeled. Factory suspension tuners or not

3. Move the track out a bit, again within reasonable limits. Remember what Pontiac did in in 1959.

To that end, my 9" front wheels stick out .3" and the inside is actually .7" further in. Front offset is actually 5mm more conservative than the ISF wheel. I've had the RCF rear wheel hubs milled 5mm and my fenders are rolled flat. Therefore the new offset is now 46mm vs stock ISF 9" rear wheel @ 55mm. So the rear wheel rim is now .86" further out than stock and the inner rim is .14" further in.

The only other suspension mods that I have made, are an F-Sport rear sway bar and RRRacing front A Arm bushings. To my uneducated butt, handling is vastly improved from when the car was stock.

I also don't believe in spacers. You have also modified your suspension. You have, AFAIK, Figs A Arm Busings and wider later model ISF wheels with 255s in front and 275s in the back.

So, in my mind Tazz is on the right track to learning and understanding the dynamics that are at play with widening a vehicle's stance.

Lou

 

He would be wise to understand roll axis, roll center, instant center, camber curves, and scrub radius before making arbitrary changes for fashion. I do not understand why people believe they know better than the engineers who spent a great deal of time solving the measurements to make all of these parameters deliver a car that handles predictably with software that costs more than two of these cars and sufficient testing it to ensure it delivers what they intended.

Yes, I put the later model wheels on my IS F with offsets the factory determined would work well. Width of the tires is inconsequential when the centerline of the wheel and tire is in the right place so all the same success parameters are maintained.

As I have said before, there is visual performance and mechanical performance. They are not the same. If you worry about stance, poke, and appearance, you will very likely compromise mechanical performance for visual performance. No one here is privy to the other changes made to the time attack car that placed THIRD (second loser to cross the finish line) to fix the problems caused by moving the wheels out for a wider track. Nor do we know who else ran at the event or how well they prepared to achieve the results they did. Racing is a funny thing - the thing that won last week won't win next week, and it is imperative you constantly innovate to be the front runner.

Lou - I don't doubt your assessment of your car's performance, but if you are not competing, feel does not mean much. I've said it before, a fast car is NOT easier to drive, and the fastest car is probably a LOT harder to drive than second or third place.

It all boils down to a simple question - what do you want to prove, and to whom do you want to prove it?

So I default to my original response -

 

Lance I respect you too much to argue so I won't. However:

1. In 2012 when the "factory" went with the wider wheels on the ISF, there were also changes to the suspension from earlier models, the factory never said go forth and install the later wheels on earlier ISFs.

2. You are running wider tires that were never specified by the "factory".

3. As I mentioned you are also running harder A Arm bushings that are not "factory". And, AFAIK you have also replaced other bushings that are not "factory"

Now IMHO, we are all here for a couple of reasons:

1. Try to learn from others.

2. Share our own knowledge and experience.

3. Make our already remarkable Fs better.

And, again, IMHO, leaving the F stock does not accomplish my third point!

Lou

 

Does anyone know what the factory scrub radius is for the RCF on OEM 19x9 ET+50 fronts with 255/35 tires please? I'm guessing that it will already be positive, given that positive scrub radius is generally better for RWD cars, however all the aftermarket wheel options with any degree of concave move the wheel centreline outwards causing increasingly positive scrub radius which cannot be recovered even with messing about with tire width and profile. I'm using the calculator on wheel-size.com to explore this before committing to my wheel choice (you can flame me now if you must . )

One thing I found when trying to search for the info is that Lexus had a JDM 20x9 listing, again using ET+50, but a 265/30 tire. I guess it should come as no surprise that the scrub radius is unmoved because diameter, width and profile have all been taken care of. The nagging question in my mind is just how much difference another 9 / 12 / 18mm of scrub radius is going to make to how the car handles on corners, under braking, and how much it is likely to worsen tramlining on lines in the road surface.