Cascade-LGT Posted October 14, 2004 Share Posted October 14, 2004 Installed BBS RKs with 48mm offset. Has the car's track has now increased by 12 mm? If stock is 54mm and replacements are 48, then each wheel has moved outward 6mm? Are there any alignment implications? The car feels more stable to me, although that is totally subjective. I also had the wheels/tires road force balanced and they seem to run smoother - again subjective. I'm running 215/45/17 Conti Extremes which are high performance all seasons. Appreciate your thoughts... Link to comment Share on other sites More sharing options...
BoxerGT2.5 Posted October 14, 2004 Share Posted October 14, 2004 No alignment implications.....you didn't change suspension geometry enough to cause you to have to adjust camber. Basically you've given the car a slightly wider base. OBAMA......One Big Ass Mistake America! Link to comment Share on other sites More sharing options...
gtguy Posted October 14, 2004 Share Posted October 14, 2004 You will have to tend the car more. It will feel dartier over highway imperfections, rutted roads, etc. You might see some effect on turn-in as well, but not much. Long-term implications on wheel bearing life will be negligible. Kevin Link to comment Share on other sites More sharing options...
Cascade-LGT Posted October 14, 2004 Author Share Posted October 14, 2004 Thanks for the replys. gtguy - A wider track "speeds" up the car's handling making it more reactive? Interesting, I'll look for that tendancy. Link to comment Share on other sites More sharing options...
Trent Bates Posted October 14, 2004 Share Posted October 14, 2004 Actually, the track measurements have increased 14mm. Stock offset is +55mm. As GTGuy said, it might have more of a tendancy to wander around a bit, etc. But we are talking about the equivalent of just over half an inch total increase since you've stayed with a stock size tire. That might not even be noticeable in the ruts created by semis. I wouldn't say that it's going to "speed up" the handling. Road imperfections are going to have a greater effect on the suspension/steering systems and the steering wheel will fight back more. This isn't the same as increasing a car's track by modifying the suspension. You've given the existing suspension "more" of a wheel to try and control. You've lengthened the "lever" that is the hub, axle and wheel by 7mm each side. Now the wheels have that much more leverage to work against the suspension. But again, it might not even be noticeable. :) Link to comment Share on other sites More sharing options...
MtnSub Posted October 14, 2004 Share Posted October 14, 2004 I assume Cascade-LGT installed 17x"7" BBS RKs given the tire size he listed. I am planning to instal 18/"7.5" Prodrive P1s with 225/40/18 tires and the offset will be "53". I understand the suspension now has more lever/wheel ,rolling mass to deal with. Question, is 53 offset ideal given half inch wider wheel and 225 tire or would it be better with the following set-up? The other choice I have is OZ Superleggras at 18x8 w/48 offset. I think the 7.5 width wheel with 53 offset should be best combo?? My expectation and the comments from others that have changed wheel/tire combo is that the steering feels more solid or slightly heavier and turn in feels better. Yes, more road feel impact on ride and some marginal loss of performance with increased rolling mass, but not significant given overall perf of car. Link to comment Share on other sites More sharing options...
Trent Bates Posted October 14, 2004 Share Posted October 14, 2004 Bear with me while I try to elaborate! Let me know if I get too far off topic too! !!:eek: Offset should not be used to adjust for wheel width and/or clearance issues. An offset of +53mm is really close so I'd say you should go with that! The purpose of offset is to place the car's driving and cornering forces in the tire's contact patch as it relates to the strut's angle. The struts are mounted inboard of the tires at the top but are at such an angle that they would run right into the tire's contact patch if they extended to the ground. There is a sweet spot within the contact patch of the tire that provides a good balance of driving characteristics. Moving a wheel inward or outward with an offset change will move the strut intersection point on the tire. I mentioned in another thread that a tire/wheel shop once put on +25mm offset wheels instead of +52mm wheels for me. In that instance, the "focal" point of the suspension went from being somewhere around the center of the tire to the inside edge by 27mm on both sides. If I'd driven that way without trying to compensate with alignment adjustments, I would have worn out the inside edge of the tire much quicker and the outer half of the tire would have never been very beneficial to lateral traction. I would have essentially ended up with much less usable tire even though they were wider than stock. The alignment compensations would have put more pressure on the outside of the tires in certain situations but would have messed up something else. (Those wheels never touched the ground before I told them to take them off BTW.) There's no easy rule of thumb in these things. You could do all the calculations you want in order to make a different offset fit ideally with the suspension, but something is going to be different than the way the desginers planned it. Sometimes you get the desired results, sometimes you don't. Unless you have the abilities and equipment to redesign the suspension, your guesstimating. Wider tires and less of an offset might give you more lateral traction, but it might also give you more rolling resistance and unsprung weight. the same combination might instead give you less lateral traction and still give you more rolling resistance and unsprung weight. The difference between the two could be 1 mm in offset per side. In reality, small changes are often not noticeable, but racers don't spend all that time getting the cars dialed in for nothing! So, here I am, off topic as usual. :rolleyes: Let me get back to offsets: If you do the calcuations and see that your new tires are going to be slightly shorter in diameter than stock, you can reduce the positive offset slightly to compensate. But be aware that all of the speed sensors are now off too because the diameter is smaller. It's my opinion that it's best to stay as close as you can to stock values unless you know what you are doing. I'd highly recommend better rubber on stock wheels for most people! :cool: Fewer problems, cheaper, and a noticeable increase in traction! Link to comment Share on other sites More sharing options...
MtnSub Posted October 14, 2004 Share Posted October 14, 2004 Interesting post Trent Bates, First, sounds like my 18/7.5 and 225/40/18 combo with +53 offset is closest to stock and the best of options I am considering. I appreciate your comments. As you stated, the best result and most practical option would probably be gained from putting some Toyo Proxie 4s on the stock wheels. I am pretty sure that the Toyos will make up for any negative effect that "could" be created by added rolling mass and resistance. As you stated, I "could" also achieve a gain in lateral traction from wider patch and better tires. Braking will definitely improve with new setup. Based on passed experience, I bet I will like the feel of the slightly wider, higher quality tire. Since I am buying an AWD car with all-weather tires for occasional ski trips and Calif rain down-poors (we tend to get our rain in short hard bursts), I have considered that I might be better off with 215 vs 225 tires on my 18x7.5 wheels. Again, past experience tells me I like the "feel" of the slightly wider tire. Regarding practical, we all left that behind when we purchased the GT version Legacy and the added Ltd package puts "practical" completely out of view in the rear view mirror. The only reason to want more than the std Legacy wagon, in most basic form, is FUN, style and or creature features. Stock Leg GT wheels are about the best looking OEM wheels I have seen and my wheel change is purely an induldged expression of personal style. The Toyos are practical and will provide better stopping, even if lateral grip is equal to stock. Wholly cow, it is mutch easier to select ski/boot combos than wheel/tire/car combos. Given how close to stock I am, I think I am spending far too much concern with this choice. Link to comment Share on other sites More sharing options...
Trent Bates Posted October 14, 2004 Share Posted October 14, 2004 I haven't done the diameter calculations on your proposed size yet, but even if I did, I don't really know your actual 18"x7.5" wheel diameter at the lips. Is it 18.0", 18.2", etc. And I don't know how much tread there is on the Toyo tires either. The two together could yield a very close match in diameter to stock or be half an inch off and it wouldn't be reflected in the specs. A wheel with +53mm offset is really a very good choice though. If I wasn't concerned that someone might half-heartedly read these posts and go get a set of wheels that they might not be happy with, I'd voice my approval of your proposed setup! I will mention that going from 17"x8" wheels back to 16"x6.5" wheels on my GTP gained me 0.4 seconds in the quarter-mile. The bigger wheels/tires were almost exactly the same weight as the stockers. That was $1400 to look a little cooler and go slower. ;) The thing about wheels and tires is that it's the only contact the car has with the ground. If they are not right somehow, the entire car will suffer in driveability! Anyway, let us see pics when you get them! Same with you Cascade LGT! Link to comment Share on other sites More sharing options...
outahere Posted October 14, 2004 Share Posted October 14, 2004 [quote name='Trent Bates'].......... If you do the calcuations and see that your new tires are going to be slightly shorter in diameter than stock, you can reduce the positive offset slightly to compensate. [/QUOTE] Compensate for what???? Link to comment Share on other sites More sharing options...
Trent Bates Posted October 14, 2004 Share Posted October 14, 2004 Haven't you been paying attention? :p If a tire is shorter than stock from the hub, the focal point of the strut hits the ground sooner... :confused: [i]Damn, I've got it backwards again...[/i] :eek: If a tire is taller than stock, the focal point of the strut hits the ground further outwards than it was designed to. Reducing the positive offset by a certain amount puts the focal point back in the sweet spot of the contact patch. Link to comment Share on other sites More sharing options...
Cascade-LGT Posted October 15, 2004 Author Share Posted October 15, 2004 [size=1]Thanks for all the info Trent. I knew it was a technical issue beyond my knowledge. I'll post some pictures soon...[/size] [size=1]MtnSub - The RKs are 17 by 7. Have you considered having dedicated winter tire/wheels? If not, are you going with an "ultra high performance" all season? I'm currently using the Conti's on my Leagacy and have Michelin Pilot Sports AS's on an Audi A6. Both are great tires, although the Michelins are expensive.[/size] Link to comment Share on other sites More sharing options...
MtnSub Posted October 15, 2004 Share Posted October 15, 2004 Cascade-GT, Dedicated snow tires do not make sense for my situation until I move full time to the Sierras. For another two years, I will spend most of my winter in SoCal and drive 300 mi. across high desert for skiing about 15-20 days per winter. I think I have decided on Toyo Prox 4, ultra-high perf all weather tires in 18/225/40. SoCal gets more rain than people think and when it rains it often rains hard for short periods of time. I destroyed a previous BMW in a May cloud burst with summer high perf tires on a flooded, downhill, slightly curving, off camber freeway transition. Since then, I have been a fan of AWD and tires that reduce hydroplane. When I honestly told the Highway Patrol officer that I slowed down to 30 mph, he said that even 30 mph on that transition road with my tires on a flooded road was too fast. I actually almost pulled out of trouble because car handled so well when I turned into spin, but rear wheel hit raised shoulder curb just before getting straight and that was enough force to send car spinning in the opposite direction. No injuries, no other cars hit, I was driving alone, but what a terrible feeling. Viva AWD and all weather tires for me now. Link to comment Share on other sites More sharing options...
outahere Posted October 15, 2004 Share Posted October 15, 2004 [quote name='Trent Bates'].......The purpose of offset is to place the car's driving and cornering forces in the tire's contact patch as it relates to the strut's angle. The struts are mounted inboard of the tires at the top but are at such an angle that they would run right into the tire's contact patch if they extended to the ground......... There is a sweet spot within the contact patch of the tire that provides a good balance of driving characteristics. Moving a wheel inward or outward with an offset change will move the strut intersection point on the tire........ [/QUOTE] Sounds like you are talking about scrub radius. For more about the importance of this factor, see this link: [url]http://www.hrsprings.com/site/technical/scrubradius.html[/url] Link to comment Share on other sites More sharing options...
Trent Bates Posted October 15, 2004 Share Posted October 15, 2004 YES! Thanks outahere! I couldn't find the right words to explain the concepts I was describing without using physics gibberish. That give a good description of scrub radius! Link to comment Share on other sites More sharing options...
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