A Treatise on Tire Direction

Seriously nerdy commentary follows.  You’ve been warned.

In yesterday’s post, I talked about running tires backwards on the rear, to increase traction.  That incited a very good question about what it means to run a tire backwards, and whether such a practice actually does anything.  Full disclosure: I’m a tire nerd.  I grew up in a family of tire nerds.  I used to sell tires.  I spend an unreasonable amount of time thinking about and looking at tires.  I notice when friends or acquaintances get new tires.  I like tires.

I also come from a farming background, where tires have a significant impact on operations.  In the past 20 years, tires have gone from a ‘replace them when they’re bald’ commodity in farm operations to a scientific undertaking.  The big Ag schools have found that proper tires on tractors can increase tractor efficiency by huge amounts.  Between varying tire brands, changing tire lug design can increase efficiency by 5-8%.  Going from worn tires to new tires can increase efficiency by double-digit percentage.  (Perversely, depending on ground conditions, installing new tires can also reduce efficiency.  For hard, dry conditions, sometimes going to new tires with big lugs can generate lug deformation and reduced traction.  Like I said…it’s complicated).  For farmers that are running thousands of acres, increasing tractive efficiency by those big jumps is HUGE.  It means burning less fuel, going faster, doing more work, and generating less tractor wear.  Also of note, farmers are not working to eliminate slip.  They measure tire slip in percentages, with 0% slip meaning that you have perfect traction (no slip) and 100% slip meaning that the tires are spinning but the tractor isn’t moving.  50% slip would mean that the tires are spinning at 5mph and the tractor is going 2.5mph.  Farmers shoot for around 8% slip with a modern 4wd tractor.  That amount of slip reduces tire wear (don’t ask me how) and reduces strain on the drivetrain.  Less slip means that you probably have too much ballast and are being too hard on the tractor (or you’re not pulling a big enough implement and are wasting fuel).

Like I said: nerd content.

With tractor tires, the ‘direction’ of rotation is clear.  The tires have big bar lugs and you install those to “point forward” when viewed from the top.  Here’s an example.  Note that on front and rear tires, the lugs lead in the center, and then slop rearwards

In the ag world, a lot of money is spent to develop the perfect height and profile of lug.  Firestone has staked out a position that a 23 degree angle on the lug is perfect for traction.  Other manufacturers disagree.

The purpose of the forward pointing lugs is simple: they are intended to displace mud and dig down to a hard surface.  The lugs work like shovels, pushing slop or debris out-of-the-way.

You’ll occasionally see tractor tires mounted backwards.  Usually, that means that the farmer had a flat and is running a backwards tire temporarily, or that he/she had to flip the rim around to change the wheel spacing.  When reversed, the tires are not nearly as effective for traction in a forward direction.  (It might mean that if you get stuck going forwards, you can back out of the stuck, though…)

So let’s move to bike tires, because this is still a bike blog.  In the bike tire world, some tires have pointed lugs that you can view as tractor bar tread–directional in a sense.  Here’s a good example, the Husker Du:

See how the lugs are directional?

The other thing to consider is the shape of the lugs.  On most tractor tires, the front and rear of the lug are shaped relatively similar.  On many bike tires, one side of the lug will be flat, and the other side will be ramped.  On the Husker Du above, the ‘front’ side of the lug is gently sloped and the rear side is flat.  It’s more obvious on some other tires.

Generally speaking, running with the ‘ramped’ side forward (when viewed from the top) means less rolling resistance.  Running with the ‘flat’ side forward means greater traction.

In the tractor world, as noted above, running a tire backwards decreases forward traction. In the bike world, however, we’re not necessarily trying to dig down to solid ground.  We don’t have enough horsepower or ballast to do that.  Accordingly, it is often the case that running the rear tire backwards increases traction.  For example, on the Dillingers, if you run them forwards, they look like a tractor tire.  If you run them backwards (on the rear), they’re more like a paddle…and they work better in bad conditions.

So here’s my general thoughts on tire direction.  For lowest rolling resistance, run them forwards, with the ramped side forward.  For best traction (in the rear), play around with them…but I’ve found that flat side forward, and tire pointed “reverse” often works better.  It clearly works better on the Husker Du and Dillinger.  It also works better on the Rocket Rons that I run on the Spearfish.

So after a lot of talk, here’s my recommendation: try your tires in different configurations.  Don’t be afraid to try running them backwards, don’t be afraid to try the forwards.  Mix it up.  Be a tire nerd.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s