Just completed my first autocross event with the Boston chapter! Wow, what a blast. Finished 4th in my group which I'm very happy with. But now I'm wondering what I can do to prep my car better for autocross. Wheels and tires are the obvious things - but I don't want to move into a different group. What upgrades can I do to my car without moving up a group? What would you do first?
Start by looking at your auto-x class rules - most mods that up hp will likely bump you into a higher class. Modifying a car for any particular purpose can go from mild to insane, depending on how much you want to spend, what you want to do, and why. It's easy to get sucked into the more, more, more.... & more... path, so starting off with some advance planning & budgeting might not be a bad idea, lest you find yourself down the road with $35k in mods, a ridiculously competitive auto-x car, and a vehicle so heavily changed you can hardly stand driving it on the street to & from your auto-x events. That said - the single biggest thing you can do, besides upping your own driving skillset, is tires - which would be running the widest & stickiest your class rules allow... which might lead to different wheels... the lighter, the better (which also typically means more expensive), although there are relatively affordable lightweight wheels (see tirerack.com for Kosei wheels, for instance, and also google), which may not be the lightest, but aren't a bad value compared to realllly expensive lighter-weight wheels. After that, getting belted more securely in the car can help, which can be as simple as using the belt inertia-locks to hold the belt tight and the power seats to lock into them, to simple things like CG-Lock belt tighteners, Schroth street belts, or full 5-6 pt. harnesses. Other items your rules _might_ allow - front &/or rear shocktower stress bars, which can sharpen steering response. Possibly some brake mods, and so on - your auto-x rule set should specify allowed modifications. For auto-x, larger dia. wheels & tires won't help you get out of the box faster, they tend to act the same as running a numerically-lower differential ratio, which makes for slower acceleration (but gives a higher max-speed, or allows for running lower rpms for a given speed for any given gear). Never mind being a heavier wheel/tire combination with more unsprung weight, and all the negatives thereof. Another thing to keep in mind, if part of the rationale is improvements for driver's schools/track performance, BMWCCA national track event insurance regulations don't allow convertibles - I think the Porsche club driver's schools may, as well as others, but if 'CCA track driver's school may be the part of the planning, that constraint may be a consideration on how far to go with mods, etc. A 'CCA track driver's school or even adult driver's clinic (if any chapters in your area offer either) can go a long way towards improving your driving skillset, and complements auto-x nicely by building smoothness, training to look ahead, anticipate, and so on. Reading up on both driving & auto-x basics can also be helpful, you'll certainly want to be looking at how your tires are rolling over & wearing, and tweaking tire pressures for optimal grip & handling balance.
Thanks for the great suggestions. I've already done couple of ADSS courses (one with the BMW Performance Driving School during the UDE at Gillette Stadium, and another with the CCA). I agree on the slippery slope on mods ... hopefully some sage advice from forum members will help me avoid that. The Boston CCA does have some guidelines on points within a class: The E85 has 107 points to start with and the class allows up to 120.9 points. So I can add up to 13.9 points of upgrades. The points impact by upgrade are: Engine Non-stock fuel injection components [add 3 points] Eta motor camshaft plus chip upgrade (eta engine only) [add 10 points]Camshaft upgrade [add 6 points] Pistons - high compression over 9.5:1 [add 7 points] Lightweight flywheel [add 5 points] Underdrive pulley for crank/fan belt [add 3 points] Supercharger pulley without LS diff [add 5 points] Supercharger pulley with LS diff [add 10 pts] Tube exhaust headers [add 2 point] Removed catalytic convertor [add 3 points] Cat back exhaust replacement [add 1 point] Valves, larger than stock [add 1 point] Aftermarket Turbocharger [add 15 points]Aftermarket Supercharger [add 15 points] Water injection [add 2 points] Porting & Polishing [add 2 points] E30 M3 Exhaust cam gear upgrade [add 3 points]Non-Stock Air Filter Housing [add 3 points]Non-Stock High Air Flow Meter (HFM) [add 3 points] Non-Stock intake manifold swap [add 2 points] A computer, piggyback, or software upgrade (whether turned on or not): ETA Motor [add 12 points] 1995 and Before (non-ETA) [add 7 points] 1996 to Current (non turbo) [add 3 points] Stock Turbo BMW or Mini [add 15 points] ’ 95 M3 – 5 pts Increased displacement or non stock engine points (see autocross points committee for points assignment) Suspension and Brakes Front Sway bar (added, larger than stock, or adjustable) [add 2 points] Rear Sway bar (added, larger than stock, or adjustable) [add 2 points] Springs, cut or non-stock, set of 4 or less [add 4 – 2 if sport susp.] Coilovers, cut or non-stock, set of 4 or less [add 8 points], if selected, do not select shock & springsShocks, non-stock, set of 4 or less [add 2 point – 1 if sport susp.] Front Stress bar [add 1 point] Rear Stress bar [add 1 point] Rear subframe, relocated or adjustable (camber) [add 2 points] Increased caster [add 2 points] Quick ratio steering [add 2 points] Brake system improvements (e.g. larger rotors, drum to disk) [add 4 points] X-brace for E36 (except convertibles) [add 2 points] Increased negative front camber points (3 points per 0.5 degree increase beyond stock: -1 non-M, -1.5 M-car measurement starting points) Driveline limited slip differential in my non-M car: Stock Limited slip differential (non-M car) [8 points] Custom aftermarket limited slip differential (non-M car) [add 10 points] Non-stock limited slip differential in my M car [+2 points] Factory Turbo or Factory Supercharged limited slip differential [add 13 points] Non-Stock differential gear ratio [add 5 points] Wheels and Tires The points to be assessed by a tire width change are computed by subtracting the increase in width in mmdivided by 10 to get the number of cm increase. The points are calculated by the cm difference times amultiplier. The multiplier is 2.0, 2.5, or 3.0 depending on the upgraded tire's aspect ratio. width multiplier new tire aspect ratio 2.0 80 - 70 2.5 65 - 55 3.0 50 - 30 Points = ( [new width mm – stock width for each axle] / 10 ) * (width multiplier) If you are just increasing the width of the front or the rear tires, enter half the points from the formula. Tire width change points Points = (new width in inches – stock width for each axle) * 2 If you are just increasing the width of the front or the rear wheels, enter half the points from the formula. Wheel width change points A decrease in tire diameter influences the weight-to-thrust ratio. Tires that are smaller in diameter canresult in an acceleration advantage. The tire diameter change points are computed by multiplying the tirediameter decrease (in inches) by 4. Therefore, there is one point assessed for each quarter inch decreasein the tire diameter. Tire diameter change points R compound tires are soft tread compounds are defined by the tire's tread wear rating of less than 110.These tires are also referred to as racing tires. Harder compound tires are referred to as street tires. The tires that you run are a major factor in determining your car's class. Note: The Autocross Committeedoes not allow the use of rain duty specific R compound tires in the AR class. Use of rain Rcompound tires is only permitted in the M (modified) class. Chassis and Bodywork Battery moved to trunk [add 1 point]Relocated motor mounts [add 1 point] Sport or racing seats [add 5 point] Extensive lightning such as fiberglass body parts, removed seats, and so on (see autocross committee forpoints assignment) Class AR* Unlimited Class A 138.0-158.0 Points Class B 121.0-137.9 Points Class C 104.0-120.9 Points Class D 87.0-103.9 Points Class E < 87.0 Points
eek, so many choices! What's the stock tire size for your car? Tire Rack says 225/50-16; looking at Apex wheels, they don't offer any for non-M Z4 smaller than 17"... what have you found out about the max width tires that fit your car?
My car has 225/45R17 RFT's on now. Tire Rack says I can go with 225/40R18 front and 255/35R18 rear within the OE specs (so presumably zero points for tire changes). I've read there is an acceleration loss with larger diameter wheels vs smaller ones, though I confess to not understanding this as the lower profile tires presumably compensate for the larger diameter wheels to maintain an overall tire+wheel diameter of approx same dimensions. So i'm thinking of getting new 18in wheels with wider tires (non RFT) and upgrading to adjustable coilovers (8 pts). That would leave me 5.9 points of additional changes I could make.
The difference comes from changing overall diameter. The plus-sizing concept attempts to maintain same overall diameter, as you go up in wheel diameter, lower aspect-ratio tires are used, keeping overall diameter approximately the same, as you've noted. If overall diameter is reduced using lower aspect-ratio tires on the same rims, or smaller-diameter rims with comparatively lower-aspect ratio tires, that makes for an effective change in rear gearing, meaning for any given speed in any given gear, the engine will be at higher rpms & higher in the torque band, translating to slightly faster acceleration, compared to what rpm's the engine would be at for a certain speed & gear with larger overall-diameter wheel/tire combo. The increase in inertial mass from a slightly larger-dia. wheel hurts acceleration, although I would think it's somewhat nominal, unless the larger-dia. wheels are substantially heavier. For auto-x &/or absolute performance considerations though, everything counts. Coilovers - expensive; primary benefit, quicker left/right transitional response, and less lean. Presumably 10th's of seconds difference on an auto-x course, but, might make car easier to hustle around an auto-x course. I'd think the first best thing to do is get to a square setup, with the same size & offset wheels, & same size tires, on all 4 corners of the car. Narrower tires at the front likely means the car's handling balance is heavily biased towards understeer, and if that's the case, as the front end is scrubbing around the course and you're compensating & struggling for front end grip, your losing time. Bigger contact patches on the ground mean more lateral grip, which makes for faster times. Since any tire less than 140 treadwear rating puts you into the highest class, I'd start with looking at tire options, and what the points start to look like as you go up in width and down in aspect ratio, if there's good tire choices available (think max-performance summer with big treadwear blocks). Like, if 255's could fit on the front without rubbing, that might be a starting point. Taking a quick look at Tire Rack (and this kind of searching sometimes is better in a conversation w/ someone who can look up things more quickly & comprehensively, but with some patience and a variety of searching, you can be pretty thorough) - Entirely possible this is entirely moot, if 255's _don't_ fit without rubbing, but - Looks like, best I can tell, the only 255's in 16" dia. are either r-comps, or one that won't be competitive, too high a treadwear rating (looking for 140 treadwear rating). - so, for that width tire, 16's are out. Also, 16's would only be worth considering if 16" wheels clear your front brakes, however, the limited street tire selection kills that idea, at least for 255 width tires. Note, I didn't look yet what's available in 235's or 245's of different aspect ratios/diameters. Doesn't look like there's any 255/35-17's, so looking at 255/40-17's, the two most auto-x looking tires are the Bridgestone RE070 and Toyo Proxes R1R - Ok, so, there are some viable tire options that theoretically may be competitive (you have to do more googling on those to see if you can find any comments about them, plus, whatever insight the TR folks might have with them, or have heard/experienced). On to wheels - looking at the Apex wheels site, looks like they offer 3 17" wheels, 8.5" wide, or two different-offset 9" wheels. The Apex guys are also good to talk to about wheel/tire fitment, they've dealt w/ a lot of specific BMW fittings. So - IF 255's fit the front without a problem, AND 17" wheels clear your front brakes, you have a combination there that might be pretty good for auto-x, except you have to figure what kind of points that add up to. There may be other wheels to consider - not sure if there's a Kosei in 9" width, although I think for your car there's 8.5" width; more stuff to google, but Kosei's are light, and not priced insanely high. They're also easy to clean. There probably are some BBS wheels that are lighter, but probably cost a small fortune per wheel - there may be others, there are a lot of sport/racing wheel manufacturers. In case you're wondering why to consider smaller dia. wheels, less unsprung weight, which allows the suspension to work better, and, less inertial mass translates into faster acceleration, plus the gearing effect of an overall diameter decrease. Also, runflats are heavy - for auto-x purposes, that extra weight is undesirable - so ditching the runflats saves weight, &, time on the auto-x course. Then, looking at whatever points that takes up, brings where to go from there - need to do the same thing for the tires you have available in 18" sizes, and different widths in 16", who knows, maybe there's good tire options in 235 or 245 16's (I'm guessing not though, as presumably the bulk of the tire market these days is in 17" dia. tires. If, for whatever reason, you can't run square all around, the closer, the better. I'm not sure I'd run narrower width rears, or at least much narrower, unless there's some points gain that might allow you another mod that would be beneficial. Another thing to consider - if you don't have a limited-slip differential, looking at that, points-wise, & expense wise. Also find out what others say they're finding by adding or not having it, particularly if you find anyone else who auto-x's the same model. I think lack of a lsd on-course has the potential to be a disadvantage and might help especially with navigating slaloms, and getting power down, particularly coming out of any turns. But, that might be too much of a points penalty. That's a start........
Awesome details ... thanks so much - a lot to ponder here! I'm thinking larger diameter wheel gives a greater contact patch with less roll on the wall due to the lower aspect ratio - though I agree weight is everything and so I'm looking at very light wheels only. Yep, the car has quite a lot of roll right now, so I'm thinking this may be worthwhile. The car comes with an option for wider rears presumably to give greater contact under acceleration. Right now I have the same size on all 4 corners, but TireRack suggests a front/rear setup with larger wheels on the rear. I'll have to investigate pros and cons on this some more for autox. I was hoping that if I stick with the OE wheel/tire combos I can upgrade here without a points penalty - since OE go up to 18in front/rear, with a slightly wider (8.5" or 9") rim on rear. Guess I'll have to check with the club. Since 16" aren't standard OE I'd get a points penalty for reducing the wheels to 16" so trying to avoid that. But I get what you're saying about size and weight ratio. If I can get the same width on a 17" rim that sounds like the best option. I'm thinking O.Z.Leggera HLT may be ideal because of the manufacturing process and the low weight but they are expensive. The Kosei's look great! I'll have to rethink my bias here! Limited slip diff would take up most of the available points so it would be a choice between that and new adjustable coilovers. You're awesome - thanks for putting time into this - really appreciate your perspective.
Besides the tires wheels and coil overs which are really good advice for auto x. I would look into sway bars they make a big difference in your car from the ones I have driven.
I get your reasoning on the larger diameter - the nominal differences in wheel diameters we're talking about here though aren't going to make a practical difference for having more rubber on the ground; I'm thinking it would take having a wheel 3 or 4x larger, or more, before diameter might make any practical difference, and the nominal gain would be more than offset by the weight of a wheel that's taller than your car! Wider tires = increased lateral grip. True though that the lower aspect ratio tire, the less sidewall flex/rollover will be - obviously less sidewall height = less sidewall flex/deflection. Points-wise, lowering springs & sport shocks might be less expensive; also less $$$ - maybe worth considering, particularly if it leaves points for swaybars, as Charlson mentions. I think the reasoning behind BMW offering staggered setups on most of its cars is to reduce tail-happiness, as most average drivers aren't likely to have superb skid-recovery skills. I'm sure BMW highly values it's reputation of having safe vehicles; all the electronic nannies go a long way towards helping everyday drivers get around without losing it in low-traction or emergency maneuvering. The BMW's of the 70's & 80's, with no traction control and less-evolved trailing-arm rear suspension designs, were prone to lift-throttle snap oversteer (ask me how I know). You can imagine how unsettling this might be to someone with little-to-no skid-recovery skills, and how it could be a problem if too many owners were having accidents, close-calls, or suffering damage. The development & refinement of the electronic systems has gone a long way to make fun-to-drive cars like 911's & BMW's easier & safer to drive, particularly at speed. Not sure about today, but 30 years ago, a German driver was required to have much more training to be licensed - more skills, higher age limit to get a license, & much more expensive than here - BMW's have always been great, fun-to-drive cars, but the older ones required a higher skillset to successfully drive them quickly, even though their absolute handling limits were far below today's BMW's. I think those are the kinds of factors w/ BMW's rationale fitting larger rear wheels, as traction control also manages any slip under acceleration. For auto-x handling balance, I'd think a square setup is better - rears larger than the front _likely_ means the car will push, and you'll always be fighting the inherent understeer of a staggered setup. I qualify that because you know your car - if, say, it's particularly tail-happy no matter what you do, then maybe 255's on the rear would give you the front/rear handling balance needed. Generally, BMW's are fairly neutral-handling out of the box, biased towards manageable understeer when pushed hard, and transitioning to oversteer if pushed harder, particularly if equipped w/ the same size tires all around. However, the narrower-front-than-rear tires BMW seems to be putting on all the models these days bias the car to understeer more extremely - which is fine for the majority of its customers, and, generally speaking, likely safer for most drivers; but, most car mag reviews have the reviewers commenting about them having too much understeer when being pushed on a track. An ideal handling balance for an auto-x car is not completely neutral, but a slight oversteer bias, as a little bit of oversteer will make the car rotate, and you can get around a super-tight auto-x turn quicker. Generally, this is more oversteer than you want driving around on the street, or for high-speed track driving, where that much oversteer just means the car will be twitchy & hard to manage, if not a somewhat unsafe handful. For an auto-x, you don't want so much oversteer that you have to be wasting time with steering corrections & catching the back-end sliding, just enough for some rotation, without needing correction. Powering out of tight turns when one side of the car is unloaded is where not having a lsd limits your ability to get going, but, as you mention, that option would kill most of the points available - boohiss! So, my rationale for suggesting square at all 4 corners is so you can rotate the tires/rims all around to even out the wear pattern, (no dismounting, remounting, & re-balancing needed, at least not for awhile, saving $$ & effort) and to have a more-even handling balance. I'm guessing your 225's are overworked for an auto-x w/ your car, so you found you have a lot of lean (car suspension - springs, shocks, & swaybars), & slide (tire compound/grip), & sidewall flex (if noticeable in such a short time as an auto-x run), but probably didn't find yourself having to go to extreme measures to compensate for understeer & lack of front-end grip - if so, that, I think most likely, would be because you were running the same size all around. All of which is useful for decisions - if you're happy w/ the understeer/oversteer balance, what you've experienced so far can be considered for what to do. The best thing is feedback from others that have auto-x'd your model and see what they say (google) A couple threads I found on tire sizes - Anthony's reply (starting with reply #5) may be useful - I googled this up, but, coincidentally, Anthony is also one of our local active club members in my area. http://forums.bimmerforums.com/foru...s-wheel-and-tire-size-question-for-06-Z4-3-0i Here someone was asking about lightweight wheels, & a few piped in, including a guy from Apex; btw I don't mean to sound like I'm pushing the apex's, I have a set on my E92, I like them, they're nice wheels, not the cheapest, but reasonably light, so I say they're worth checking out. Hard to beat the weight/value of the Kosei's though - the lighter the wheel you want, the more expensive it will be. This thread is from 2010 and one poster mentions the Enkei's - if you look at those, check for reports of failure, I thought I'd come across posts about cracked spokes on some wheels, maybe the Enkei's. Also a link to the Apex wheels for Z4 non-m fitment. http://www.zpost.com/forums/showthread.php?t=406525 http://www.apexraceparts.com/apex-products/z4/ - continued in next post...
hit the max character limit for postings! Anyway... To narrow it down, to leave as much points available as possible, one given is to keep same o.d. as stock, regardless of rim size. Tire Rack shows 225/50-16 as one of the stock sizes for a 2006 Z4 3.0i roadster, but, 16's are out for a variety of reasons including competitive tire availability & points assessed, IF there were some tires to downsize to. 17" wheels are the best option in my opinion, since you have competitive tires available in a variety of sizes, 17" wheels are lighter than 18's, 17" tires are probably a little less expensive than 18's, and a 40 or 45 aspect ratio, depending on tire width, is fine performance-wise, compared to a 35 a/r on 18's, and the negative performance penalty from running heavier 18" wheels. Looking at realoem.com, it looks like 8.5" wide was the largest rear fitment, & 8 was the front. Considering 245/40-17's, you could probably stuff those on 8" rims for no points, possibly at a cost of increased sidewall flex. Trying to decipher your chapter tire-change points system is, uh, challenging. It looks like they assess separate points for increasing rim width & tire width, decreasing aspect ratio, and any decrease in o.d.? Then, there becomes the question of negative points - if you use a _narrower_ tire on the rear than the max fitment, looks like that gives a negative number, and can that be used to offset other points? Ok - looks like 8.5" rims on the front = .5 pts for rim width change on 1 axle So - a question for your auto-x peeps - one of the front tires spec'd for your car has an aspect ratio of 40 (being 225/40-18) - when changing the front tire, if your increase in width _does not_ exceed the aspect ratio of another tire spec'd for the front, does it have a points assessment? In other words, is a wheel/tire change considered on the whole, or do the points formulas apply only to a specific that's changed. Logical fairness would dictate on the whole, which I presume would be meant to be interpreted within the spirit of the rules and the event, but, sometimes there are technicalities to take advantage of - something you might want to ask someone about. The way I read the aspect ratio change formula, the front, opting for a 245/40-17, can be: (((245-225)÷10) x 3) ÷2 = 3 pts However, apply the same formula to the max rear fitment of a 255 and running a "downsized" width... (assuming they let you assess it this way) (((245-255)÷10) x 3) ÷2 = -1.5 pts Keeping the same o.d. (use the tire-size change comparator linked in one of Anthony's replies above to doublecheck, but, presumably, we're close enough) makes for no points there, so... (.5 + 3) - 1.5 = 2 pts total, for 245/40-17's on 17x8.5" wheels, leaving you 11.9 points to work with. Assuming you can do the neg. points that way. Looks like 255's on the front on 8.5" rims would be 4.5 pts, + .5 for the wheels, and there would definitely be no offsetting neg. points, , which would leave you with only 8.9 pts. to work with otherwise. Coilovers are 8 points - that route leaves you .1 pt shy of the 4 needed to do front & rear swaybars, so only the front swaybar could be done. Shocks & springs though are only 6 pts. all together, which would leave you room to do swaybars - hmm, the question becomes how comparable shock/spring setups are to coilovers, and whether this route is preferable since you can do the rear swaybar. I dunno, adjustable coilovers do a lot, you can get them with really stiff springs (even too stiff), and lower the car a lot, much more options than w/ conventional springs & sport shocks, and presumably you can tune the car's handling with them that would compensate for any lack of rear swaybar. Worst case scenario, you could always sell 'em & go another route. And - if it were the case that, as mentioned above, there's the seemingly unlikely scenario of no assessment for aspect-ratio change on the front, that would open up more points availability, possibly making 9" wide wheels a possibility, etc. Aren't auto-x rules fun?? If there's no points, stainless steel brake lines are a no brainer. Hard-core race pads are likely a neg. for auto-x, not sure if I saw anything in what you posted about points for pads. On the Tire Rack site, looks like you have 3 245/40-17 140-treadwear-rated options, the Toyo Proxes R1R, (looks best to me for both price & tread pattern), Pirelli P-Zero, & Bridgeston RE050. The O.Z.'s are pricey & weigh the same as the Kosei's - not worth the extra expense, in my opinion. The only Apex wheel that weighs less than the Kosei 17x8.5 is 17x9, which might not be an option, 9" wide wheels all around would be 3 pts. total. The Kosei 17x8.5 wins on price & weight. You'll have to google to find a lighter wheel off other sites, and they'd almost assuredly be more expensive. Me, I'd to a tire package w/ the Kosei's & Toyo R1R's, 245/40-17 (assuming no o.d. pts. penalty) and 17x8.5 wheels. Then you have pts. room for the coilovers & a front swaybar.
Forgot to add - making sure that combination would fit - tire rubbing too much = bad! Looked at the tire size comparer - .2 o.d. difference from stock 225/50-16 (also found this as a stock size listed on realoem.com), or .3 from 225/45-17, making for 1.6 pts. for both axles, or 2.4; Looks like arguably the lower number could be used, meaning the wheel/tire change with 245/40-17's would be 3.6 points total accounting for a calculated diameter difference. As an aside, it doesn't look like the rules specify how the difference should be measured... you might get a closer actual-diameter measured difference if, say, you measure the stock tire, oh, not-quite-noticeably nearly completely deflated and the new size with.... 68lbs of air in it - and preserve a few points (while exploiting technicalities and probably not really complying with the spirit of the rules... which are meant to keep things fair & fun, not incentivize 1000 ways to creatively interpret them!). I kinda doubt that's worth trying to argue the point, 13.9 - 3.6 = 10.3, still enough for coilovers & a front swaybar, and either way, you can't do much of anything w/ 1.3 or .3 pts. If you can't do neg. pts as appears might be possible (prior post) and you can't get away with avoiding the a/r pts. formula, then presumable worst case for my suggested wheel/tire change, 3.5 + 1.6 = 5.1, leaving 8.8, enough for coilovers but no swaybar. Or shocks/springs for 6 pts. & a front swaybar for 2 pts.
