Slicks or Street tires? What's best for driveline durability?


First, a bit of wisdom regarding slick use...the prevailing philosophy is that slicks are harder on driveline components than street tires. This is not necessarily true. Slicks, while they certainly can transmit more continuous torque to the driveline than street tires, do have one major benefit...they don't impart the torque 'spikes' that street tires can, and it's these 'spikes' that damage more axles and differentials/transaxles than many imagine. Where do these 'spikes' come from? Two conditions: 'spin and hook', and wheel hop.


Spin and Hook

In 'spin and hook', the street tires spin when power is applied, and can spin VERY hard. When they do suddenly hook up, the momentary torque transmitted can be many times the continuous torque potential. Where does all this momentary torque come from? We all know that energy can't be destroyed, it can only be converted. The kinetic energy of a wildly spinning driveline is amazing. It's all one big flywheel, and a very heavy one at that, when you add the weight of all that is spinning (wheels, tires, differential, axles, transmission shafts/gears).

Ever feel the 'surge' forward that occurs when the tires suddenly hook and stop spinning? That's the conversion of all that kinetic energy into movement. Now, another thing is also happening. Not only is the flywheel effect described above at play, but what else happens? The engine, along with its flywheel and clutch assembly, is suddenly decelerated massively, thus the RPM drop when the tires hook up. More energy we cannot destroy! So, now, not only does the driveline have to put up with the sudden flywheel effect of itself decelerating, its also getting yanked on through the other end (input) by all that energy of the sudden deceleration of the engine and flywheel/clutch. Add to this the weight of the vehicle being surged forward, and you can imagine how much momentary load is seen. Every item in your driveline has a torque limit, and if that limit is exceeded even momentarily, the damage can be instant.

Slicks typically won't allow these severe 'spin and hook' issues, and thus, can actually reduce the attendant strain on the driveline.


Wheel Hop

The other major culprit is wheel hop. Folks, do NOT ever allow wheel hop to occur. If you feel it beginning, lift off the throttle...NOW. Wheel hop is a smaller version of the phenomenon described above, but in a way, it's even worse...with every 'hop', the over-torque occurs. If there's ten 'hops' before it stops hopping, which can take but a couple seconds, that's ten death-blows to the driveline.

Slicks tend to eliminate wheel hop due to the 'resonance' of the floppier slick tire as compared to a stiff street tire. Taken another way, the slick acts as a shock absorber, stopping the wind-up and release of suspension that causes wheel-hop.


The Common Misperception

Back in the day when we were first turbocharging Neons, we regularly were told that the drivelines would NEVER hold up. We were told, "Why, my buddy with his 14 second Neon busts axles all the time! How could it ever hold up to double the power?"

Seems logical on the surface, sure! But what the fellow didn't know are the real reasons why his buddy was shredding axles. Yep...it was all about 'spin and hook' and wheel-hop.

We proceeded to run as fast as high 11's on stock axles with slicks before we needed to upgrade the axles. And get this...we ran high 9's on a stock transaxle-equipped (Quaife differential) drag Neon with 600HP, and did it repeatedly!

Further evidence is shown in the very fast Hot Rod (NHRA) or Pro 4-Cylinder (NDRA) classes of racing. Did you know that there are Hondas there that run in the eight-second zone on huge slicks with warmed-over STOCK 5-speed transaxles? Sure, they put in better diffs and axles, and don't get many passes off a stock-based trans, but they do it...all the time, with power in the 1000HP range.


FWD One-Wheel Burnouts

One other aspect that leads to early stock differential failure...one-wheel burnouts. Whatever you do, do NOT participate in this ruinous activity! I have been at the track and seen cars doing this so many times, and I cringe...why, sometimes I can even hear the spider and side gears HOWLING from 30 feet away while this is going on!

Why does this kill diffs? Bear in mind, the stock diff is designed to do one thing...enable the tires to turn at different speeds to allow you to turn left or right without binding. The speed difference between the axles under this intended use of the diff is, at worst, approaching 2:1...the diff gears barely even turn. What do you think those gears are doing when one wheel is going 60MPH and one is standing still in a one-wheel burn? These gears don't even have real bearings in them, for they should not need them...the lubrication problems alone from one-wheel burnouts can trash a diff faster than you can say 'Phantom Grip'!


What makes a stock differential stronger?

Does a product such as a Phantom Grip strengthen the differential? In straight ahead acceleration with both wheels hooked up, no. FWD vehicles have one inherent advantage over their RWD cousins with engines in the front of the car...natural engine torque reaction does not tend to reduce pressure on one wheel, encouraging the other to spin. Thus, a FWD car with slicks and a Phantom Grip is no durable than the same car with a stock diff. Slicks tend to ensure that both wheels stay hooked equally, so a stock diff on a car with slicks is not made stronger by the addition of that PG. However, a Quaife or PG is a real good idea on street tire cars that drag race, if only to discourage one wheel from taking off and spinning much harder than the other, whether in a burnout or on the track.

But there's something no trick diff can ever do, and that's change your driving habits. If it spins, LIFT. Get right back into it if you like, but never keep your foot mashed to the floor unless you like buying driveline components with money you could spend on other things.

-Bill Hahn Jr., copyright 2008