More than just a Contact Patch
The stock VW sedan came with 15" rims and these little P165 tires. This means two things: ground clearance wasn't there, but gearing was set for such tires.
In the baja way, ground clearance is paramount. Lucky for us, there's a variety of fiberglass fenders out there that'll stay out of the way of larger tires. Gearing, however, is still a factor!
The VW engineers went through great trouble to compute proper gearing for their little 36-horsepower motors. The intent was, naturally enough, to put the power curve in the right place(s) - but also to run the motor at the correct RPM for proper cooling! Thus - when you switch to larger rear tires, the motor-speed at 70mph is lower. The fan turns less times, thus converting to less cooling air over the cooling fins.
Another complication is the fact that larger rear tires are harder on a clutch. Launching those larger tires on the same gear ratio takes a bit more effort on the clutch assembly.
Ring & pinion gears are located in the rear portion of the transaxle. Think of this as the final drive, or maybe the differential of your baja. Lowering the ratio of the ring & pinion changes the gear ratios for every gear. On the other hand, increased tire size changes the gear ratios for every gear - so hey, this is exactly what you're looking for!
Installing a ring & pinion set takes a great amount of alignment. If the ring gear is out of alignment, the pinion gear will mesh correctly on one side of the ring gear, but will be out of whack on the other 180-degree portion. The professionals use a bit of red clay-like stuff to see that the gears are meshing correctly.
Many baja people opt for the bus transmission. see the Gear Chart and you'll see that including reduction gears, the bus tranny is superior for low gearing. The drawbacks include: Cutting tin, buying adapter kits, and running your shift linkage outside of the tunnel. It's complicated, and I've never done it myself, but Baja Mike tells me it's doable. In addition, the aftermarket ring & pinion sets are technically weaker than stock ring & pinion, due to the number of teeth of pinion gear engaged at any given time. I believe that it's not the weak link of the system, however. CV's and diffs are more likely to blow.
Regarding the cooling factor - this Lizrd is in central Arizona. Cooling is paramount around here. What can I say? We need lift, but must take care to cool the motor properly. I choose to accept a shorter motor life in exchange for the ability to go where I durn well please but hedged my bets with:
- external oil cooler - helps with that #3 cylinder being hotter than all the others, due to the location of the stock oil cooler
- high-pressure oil bypass spring - keeps the pressure up
- no power pulley - have you heard of the power pulley? It's a smaller pulley that allows your fan to turn less times thus increasing horsepower. Not a good idea in this desert.
- proper timing - all the hogwash about setting your timing to 31 degrees' total advance is only good in the winter around here.
Something else to consider is the MASS of the tires you're turning. Some enterprising engineer somewhere probably figured out how to run dualies on a baja, but it takes a whole of horsepower just to turn the things!
So, along with tire height, consider the mass involved, and the stress on the clutch. Did you want good launches?
I read my HOT VW's magazine religiously, and saw an ad for TRANSFORM - makers of transmissions, gears, ring & pinion sets and even shot-peened CV joints. Their hint was: with tall gears, do not go with a lightened flywheel. Makes sense to me. Once you rev up that motor, and the flywheel is turning well, you'll have more angular momentum stored up in a heavier flywheel.
The most common sizes for rear baja tires are either 31 by 10.5 or 30 by 9.5 inches. The stock VW rim is 15", and so are most aftermarket rims, but you can get custom 16" rims if you feel like paying for the tires! It's nice to have the additional height, but 16" tires are significantly more expensive. You can run up to 33" rear tires, but it's a load on the drivetrain - I don't truly know why anybody would want to go this big. Majior mass and bad launches. Look at it this way - your front end is always lower so you'll mow down any obstacles with the front, right? (but take this with a grain of salt - determine your own baja style)
Fenders permitting, you can run any rear tire size, but must figure out what your gearing is! I happen to have the 3.88 IRS transmission, and haven't invested in a new ring & pinion yet. (Hey, it's a new tranny, wanted to break it in first!) My motor is 1600, and although I have some excellent carburetion and exhaust and such, I'm not the horsepower king. I found that 31" 10.5 tires were a bit much, and opted for 30" 9.5 rear tires. You might not believe it, but I noticed a difference and just bought a new set of the same size.
Swingaxle bajas have that camber problem. If the suspension is lifted, the tires like to angle down & cause some really funky tread wear. Swingaxle baja owners who do high mileage might consider a cheaper tire? Those are big words, but if you're only getting 30,000 miles from a rear tire, you are at a serious disadvantage from us IRS owners who may get 70,000 miles from a rear tire.
Front ends can be modified relatively inexpensively (a couple of hundred dollars, maybe). You could go with a 6" wider front end. You could get a tube frame for the entire front pan (known as a class 5 frame, but this isn't especially accurate). You can get longer trailing arms. The point here is, there's lots of ways to change the VW front tire equation!
The rules here are: narrower tires are better, for the rotating unsprung mass, and for the fact that your lightweight baja probably likes to track the ruts in the road if you have wider tires. Taller is better, to keep your front end off the rocks. The problem is, it's very difficult to find a narrow, tall tire!
Tire makers use these fancy numbres like P185-75-R15. Here's what it really means:
P rating. P=passenger car, LT=Light Truck, and I don't know what other ratings you'll find. This has to do with the load rating, or the number of steel belts in the tire, or something such.
185 = 18.5 centimeters in width.
75 = the hight is 75% of the width - in this example, 75% of 18.5 cm 13.9 cm.
R15 = the rim size. VW Bajas should stick to 15-inch rims - tires are cheaper.
The problem with buying front tires seems to be how much tire will fit?
The tire shop is bound to tell you that 185's is the most that will fit. But here's a trick: use a hammer to pound the metal lip inside of the wheel well flat. This is actually the edge of the pan. This allows a 215 tire to fit!
Your fenders may get in the way of such a large tire. To be cautious, you might consider 195's or 205's. I chose to cut the fender & use a bit of fiberglass repair material on the ragged edges. Less rock protection, but it works fine for me.
It's a good idea to chain your front end to a jack, and compress the front end as far as it will go. That way, you can check the clearances when you turn the wheels and such to make sure it won't rub & ruin your new tires.
There's turn radius stops right by the steering box. Plus, if your turn radius is OK in one direction but not the other, you can adjust your tie rods to get the maximum turn radius in both directions. Just be sure the front end is aligned correctly.
What I've noticed is that all kinds of SCORE racers use Yokohama tires. I'm not a SCORE racer myself (yet - maybe next year) and have to take their choices at face value. I must believe that Yokohama is offering the same racers' benefits as everyone else, and the racers choose their tires due to superior quality for that purpose.
I'm on my second set of BF Goodrich All-Terrain TA rear tires. I found them to be EXCELLENT!
You can get a real bargain with used tires, especially as an experiment in sizing. I've been careful to buy from a quality shop that backs what they sell. I remember that many years ago, I was told radial tires had to be run on the same side for their entire lifetime. The steel belts spin in a certain direction, and the belts would move a bit & sink in to the rubber. If you reversed the direction, they moved the other way and often bubbled and separated. This doesn't mean that the poor man can't buy a used tire - but back then he had certain odds of tread separation. Your fellow baja fanatic, The Darceman, makes a solid case against this on modern tires, however, and quotes Tire and Automobile Manufacturers to boot, so don't worry too much, just buy from a shop that'll back you.
Along the same line, a reader tells me about a tire advertised in 4-Wheeler Magazine, called the Buckshot Mudder. They are less expensive, and come in tall narrow sizes, such as P215-85-R15. Sounds like a good thing. For a few months, I stated the fact here that I had no idea if they're any good. Since then, I've had several people state that they're great! (Thanks folks - you know who you are.) The big bad baja in March's HOT VW's magazine has Buckshot Mudders on it.
The Darceman also recommends Riken Radials, and I've come to believe what he says.