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by Dan Wright
“Dude I was sideways out there for almost a mile, we definitely have a traction problem!!”
~Overheard while eating lunch at the Red Flame.
Where the Rubber Meets the Road.
The racing surface at Bonneville is funny stuff.
Hard to describe. As it's a natural surface, it can exhibit great variability.
You can get an idea of the nature of this racing surface by doing your own
experiment. Take a whole box of table salt and pour it onto a pie plate.
Spread it out smooth, so you have a layer about ½” thick. At
this point it is like sand. Loose, granular, and not anything you would
want to race on. Now fill the plate with water. Don’t just dampen,
fill it so you have standing water above the level of the salt. Mother Nature
fills Bonneville with water to well above the salt every year. Now, let
it sit to dry. It will take a few days or even a few weeks to really dry.
When it is truly dry, the salt will have changed. It will no longer be loose
and granular like sand. It will have become a single solid mass. Hard, crusty,
white, abrasive, gritty, it would seem like an excellent traction surface.
But is it?? You will notice that while it is hard, and seems to withstand
a blow or major direct assault, if you rub it gently, it sheds a powdery
granular material. It is that loose, shed, surface material that is the
major problem with Bonneville Traction. Just a bit of the loose granular
material goes a long ways. If you imagine that powder as behaving like just
a slight dusting of sand over asphalt, you begin to get an idea of what
must be going on at the tire/track interface at 200 MPH.
If you rev up a car and drop the clutch at Bonneville, you can lay some pretty cool rubber stripes on the salt. The salt surface is plenty tough enough to pull rubber off the tires. But you will also notice that that same process digs trenches in the salt. Depending on the particular spot you choose for your burnout experiment, the trenches might be 1/16” of an inch, or one and a half inches deep. Like I said, the salt surface varies. It's not much like pavement.
The USFRA regularly gets questions from rookie racers, about how best to apply big horsepower for maximum acceleration. We suggest that throttle modulation is critical to success at Bonneville. If you can’t tell when you’re spinning a tire, and react to control that spin, success will be elusive. I know of people who didn’t have a good feel for tire slippage and burned up a set of expensive LandSpeed tires in a single meet. Roasted the rubber off clear down to the cords, in just a few runs. Throttle control, finesse, and driver awareness are the keys to success.
Over the years, I have heard the traction qualities of the salt variously described as: “about like pavement”, “like wet pavement”, “just like ice”, and pretty much everything in between those extremes. Yet it seems to me that none of those describe it well. I personally think it is much like a good well groomed, hard packed dirt track, or maybe a well packed smooth gravel road. Not like freshly graded loose gravel or dirt, but one that has been packed smooth and hard, maybe wetted and dried a few times. Like your local dirt racing oval, that hard packed surface that gets shiny, and streaked black with rubber in the racing groove. It is pretty good traction, but not like pavement.
One other aspect of this surface is its “Rolling
Resistance”. Again, it is much different than pavement.
If you ride a human powered machine on the salt, you find it harder work than it takes to go the same distance/speed on clean pavement. Not as much resistance as sand or loose gravel road, but more than pavement. Riding a bicycle is ok, but not great. It takes more effort to peddle the same speed and distance on the salt than it does on pavement. That problem is compounded for anything with small diameter wheels. Skateboards pretty much don’t work. Go Karts are significantly slowed, small diameter tires have a hard time here. I watched a kid with one of the little “Razor” type of scooters. It had about 3” diameter wheels, and he looked like he was working pretty hard and not having too much fun.
It is not that the salt is soft, you need a
hammer to drive a nail into the salt to hold down a tarp. The salt is hard
enough to create a lot of work when setting up racing infrastructure. In
order to build fences that mark the pit area, access roads and course boundaries,
prep workers need to insert 4’ tall posts of rebar to string up safety
tape barriers, the USFRA uses a gasoline powered drill, spinning a 1”
diameter drill bit. Even with the power tool, that job is hard work. I spent
an afternoon drilling holes, each takes about 90 seconds of drilling, then
carry the drill 30 paces and drill another. Just keep drilling holes till
you have fenced off pits ½ a mile long X ¼ mile wide, or collapsed
from sunstroke, which ever comes first. The salt is hard and tough. I remember
the drilling that afternoon as being much like manual labor. It was an excellent
opportunity to learn about the thickness of the salt crust. I needed holes
about 10”-12” deep for each rebar fencepost. In many places
the salt layer was thinner than that. When you ran the drill thru the salt
into the mud below, you knew instantly, because the mud underneath drilled
like warm butter.
As most racers know, significant preparation is done on the race course at Bonneville. A device called a “Drag” is towed behind a truck or tractor, the Drag is heavy steel, 20 feet wide and sometimes weight (barrels full of water) is added. The device scrapes any loose salt along with it, and deposits that collected salt into divots, cracks and low spots in the natural salt surface. The process also tends to collect any moisture on the surface and moistens the salt that is being redeposited. A few hours after the dragging process ends, that deposited and smoothed salt surface dries to a smooth hard surface. When conditions are favorable, this process results in a near perfect racing surface with excellent traction characteristics.
One more aspect of Bonneville traction needs to be considered here. When the racing surface is abused by high powered vehicles, very heavy vehicles, or simply by too much racing traffic, the hard packed prepared surface begins to break down, and return to its pulverized granular form. This can take the form of holes, sand traps, washboard areas, or large soft areas. If you have ever spent much time driving off road, the feeling a driver experiences when running thru these “loose” areas will be familiar. It feels like running from a good smooth hard packed dirt road into a sand bog. It can really get your attention at 200 MPH.
Bonneville Racing Tires
Typical Bonneville tire (left) compared with performance street tire (right).
Of course, tires are the other factor in the Bonneville traction equation. Unlike the discussions often overheard at dragstrips, I have never heard anybody claim that one brand or model of tire had better traction charachristics than another. Street tires speed rated “V” and “Z” are permitted on cars up to 200 MPH. There are several different brands of tire commonly seen on faster cars. Several brands of “Frontrunners” which are actually Funny Car front tires, are often used on light weight cars, and usually have a manufacturers maximum speed rating of 250 MPH. For speeds above that, LandSpeed racing tires built for Bonneville concentrate on building tires that will survive the high speed centrifugal forces. There does not seem to be any discussion among the racers about tire “compounds” or harder and softer tires. Those kinds of discussions which dominate the pit conversations at NASCAR races, local ovals, SCCA events, seem completely absent at Bonneville. Yet there is general agreement that the top speeds of most very powerful cars and motorcycles running at Bonneville are limited by traction rather than any inability to make horsepower.
I overheard a discussion between a couple of
motorcycle guys a while back. Both these guys were speed limited by traction
on the salt. They both rode very powerful Nitrous fueled bikes. They were
comparing the differences between two popular “Z” rated tire
brands. Dunlop and Metzler. While Traction was a significant problem for
both of them, the only discussion I heard was about which tire was most
resistant to “chunking”. That is, throwing pieces of rubber
out of the face of the tread face, due to power loading and the immense
centrifugal forces. Concerns about running a tire that will simply survive
outranks traction problems. Catastrophic tire failure at high speed often
results in serious wrecks.
Experience shows that narrow tires provide better traction at Bonneville. Bonneville traction strategy requires just about the opposite to what works at the local drag strip.
My own unproven theory explaining this fact is that, at speed, wide tires sort of “Hydroplane” on this bit of loose material. I also suspect that wide tires at speed are near “Hydroplaning” on air alone. It must take “some” time to force the air outward towards the side of the tire, and at “some” speed, air would pile up and be forced under the tire, eventually providing about the same traction one finds on an Air Hockey table. The narrow tires, especially ones with little tread, a rounded profile, run at maximum recommended inflation pressures, have less tendency to trap air and seem better able to bite thru the loose dusting on top. The proof of this is in the pudding. Look at the photos of the Bonneville cars that have proven to be fast. Drive tires on the successful cars and bikes are, narrow with a very pronounced round profile. That round shape, combined with high tireair pressure yields a very small “tire contact patch”. As unlikely as that combination sounds, it is the proven winner on the salt.
Traction by the Numbers
A more objective approach to the discussion
of the traction characteristics of various surfaces is possible. . I have
reproduced one chart of various Traction Coefficients below. None of the
available information discusses differences among tire brands or tire compounds,
only differences of the ground surface.
Coefficient of Traction Defined: The degree of traction between the tire and the ground is called the Coefficient of Traction.
A spinning wheel does not deliver useful power to the ground. The two factors which keep a wheel from spinning are the mass it carries and the traction available from the ground conditions.
Since there is never 100 percent adhesion, the coefficient is always less than 1.0. The result of multiplying the weight on the drive axle times the coefficient of traction represents the maximum force which can be transmitted before the tire spins. It is worth noting that in the all sources of information I found while researching this subject, the only variable considered was the ground surface. Tire type, construction, tread style, or rubber compound were not given any consideration. Probably the reason for this is that the variations in the ground surface overwhelm any variation of tires in this matter.
Coefficient of Traction for Rubber Tires on Various Surfaces
Source: Forestry Handbook by Karl F. Wenger (1984)
There seems to be general agreement among experienced Bonneville racers that the Coefficient of Traction for a best case prepared course at Bonneville is about .45 - .50 and can range downward to as low as .25 when the hard groomed surface breaks down into sandpit like conditions.
I realize that much of this information is pretty subjective. We at the USFRA regularly get questions about the racing surface, hopefully this will help newcomers to prepare. I welcome additional information from Bonneville veterans, whether scientific or anecdotal. If you have anything to add to this traction discussion please email me. I would be only too happy to include your experience.