carbonRX8

iTrader: (1)

This is a question for those handling gurus out there. I recently put on some wider tires (w/ appropriately wider rims.) Initial turn in is still as sharp as OE wheels, while even with out breaking-in the tires, grip through corners is noticeably enhanced. (There is some very limited greasiness that feels like sidewall roll, but that might be due to the fact that the tires have seen <100miles)

My problem is that during highway driving, just going straight and not fast (70 or so), the steering feels like it has centering slop and is a tad numb. I am constantly finding myself making tiny corrections just to go straight. Also, while going straight, the car doesn’t seem to respond to slight input as quickly as it once did. It is very subtle and is nowhere near, say a 1979 Lincoln continental. While in a corner, the steering feels at least as responsive as stock. Have I lost some toe by changing wheels? Is this possible? New tire size is slightly (like 1.5%) smaller than stock and 22% wider. An alignment is in my future for a coilover install, but that might be several months.

NoCones

How different from 50 is the offset of the new wheels? You probably altered the "scrub radius"...here's a nice discussion:
http://www.hrsprings.com/site/techni...rubradius.html

"When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock."

That's not really "numb" steering...more like front wheels that are too responsive to the road (thus the frequent corrections)...like having too much front toe out. (which shouldn't be the case here...the wheels don't impact your toe settings)

Gord96BRG

Details please?

What wheels; size (diameter, width); offset; weight?

What tires; Brand; model; size?

carbonRX8

iTrader: (1)

Sorry, I was being purposefully vague to avoid suggesting that this was a review. Tires are BFG KDW2 275/35-18 on Enkei RP-F1 18x9.5 with 45mm offset, 17.5 lbs.
Thank you. Yes this does not feel too sensitive. I actually dont feel this numbness while in a corner like you would with incorrect toe (at least I dont think i do). I didnt think that a smaller wheel should effect toe like it could camber. Could this be a consequence of the wider tire?

carbonRX8

iTrader: (1)

That link was rather hard to think about with out pics so I am posting this from my copy of Chassis Engineering by Herb Adams (HPBooks, NY, NY)





So stock is 203.2mm wide and +50mm, I now have 241.3 and +45mm offset. Now if I knew the Kinpin inclination angle, my tire diameter and width (275/35-18) on the 8 I could figure out what I did to my scrub radius, correct?

swoope

iTrader: (5)

do not think or pass jugdement till the molding compoud wears off...

~200 miles or less if you push them, but if you push them they may bite.

sounds stupid, but if you ride a motorcycle you know for sure. and yes straight line is involved.

beers

carbonRX8

iTrader: (1)

Ok dude. See you in a week or so.

dannobre

iTrader: (3)

[QUOTE=swoope]sounds stupid, but if you ride a motorcycle you know for sure. [Quote]

Funniest thing that I ever saw was a guy taking off from the dealer on a brand new GSXR 750 and being cool....and droping it about 200 yards from the road entrance cause of just the above What a tool !!!

Wider tires will " tramline" more than narrower ones as well on any little ridge in the road....

carbonRX8

iTrader: (1)

That I expected. The stock tires did that quite a bit, and I thought it might be just a little worse. Yet when it happened for the first time. It was a little suprizing, the violence of it.

swoope

iTrader: (5)

release compond is about heat cycles....

beers

TeamRX8

iTrader: (25)

mine are the same or worse straightline with the Yok's; you're just putting a lot of rubber on the road is all.

at high speeds though the stability is unreal, I was cruising to an event in the morning and looked down to see 100 mph on the speedo , only felt like 60 mph

TeamRX8

iTrader: (25)

also, the width doesn't matter for scrub radius, only the offset and diameter changes, our change is minimal, especially compared to the guys runing offsets in the 30mm range with 19"+ sizes

TeamRX8

iTrader: (25)

here's a revised pic, PLEASE replace the one above

carbonRX8

iTrader: (1)

Thanks for the pic resize.

Ok, I have only taken the car to triple digits once (I havent retorqued the lugs yet) and I would agree that at actual speed, things feel smooth and as responsive as the stockers at "normal" speeds. Why then does more rubber feel slightly smooshy at straightline cruise?

I understand what scrub is and I think I understand how it effects the car. From what I have read, this is not the issue that I am feeling (I think.) How did you calc. it?

Thanks for the reply, BTW. esp considering your setup.

TeamRX8

iTrader: (25)

that's just the Azenis 615, they have soft sidewalls and if you have full tread it's just that much squishier

TeamRX8

iTrader: (25)

the stock offset is 50mm, the RPF1 is 45mm, if the tire OD is identical you just increased scrub radius 5mm over OE, my tires are a bit smaller OD the the factory tire and this increases scrub radius depending on the magnitude of the change, however that range of change is nothing to be concerned with

to calculate it out fully you have to know the exact inclination angle (which changes with alignment, etc) and then use geometry based on the wheel offset and tire diameter change

StealthTL

iTrader: (1)

Perhaps your problem is actually the KDWs? I find them a little squishy and soft, compared to others in their class....

S

Imidazole

Its quite simple as this:

The wider your tires, the more you're affected by the road, and the more the road is apt to steer you in one direction or another.

carbonRX8

iTrader: (1)

After a week or so of riding on these, I have to completely agree with this statement. I think I have come to "understand" the new wheels is a "feeling" way. What I think I was feeling was the variation in the crown of the road. At normal speed, the crown varied enough that I could actually feel it. At faster speeds this variation averaged out. I think the data all fits. 1) the tire "tramlines" more in ruts and 2) I can feel the crown of the road more. Yet the car definitely sticks better. Obviously, this is subjective without a skidpad, but it is not hard to demonstrate. My best buy yet.

Spin9k

"Ah Grasshopper, perhaps you are simply experiencing *Road Suction* where your tires belong more to the road than to your car, an enviable state of being!

May the downforce be with you."

Imidazole

Just wait til you start running over small potholes or other 'odd' things in the road.

You'll be fighting the road even more then, with your wheel.

I used to think that the wider tires, the more it would roll over and ignore such things. Completely the opposite.

rabinabo

The flip side of the wider tires = more grip equation is that it also gives you more drag, so that might slow you down on takeoff.