ATV STATIC STABILITY TESTS
FINDINGS & OPINIONS
FOR:
THE OREGONIAN
1320 SW BROADWAY
PORTLAND, OR 97201
MAY 1, 2007
Page 1
FME, Inc conducted static stability tests (tilt testing pitch and lateral). The ATV’s tested included
a Yamaha 450, a Polaris Trail Blazer 250, a Honda Rancher 400 and a Polaris Sportsman 700.
Testing was not done with riders. Calculations were done to predict how a rider and load will
change stability. We also performed five braking skid tests to determine the coefficient of
friction of ATV tires on various surfaces.
The ANSI/SVIA 1-2000 standard requires a pitch stability coefficient (Kp) of at least 1.0. The
standard does not address lateral stability (Ks/Ktt). ATV manufacturers reportedly agreed to
meet a minimum Kst of .89.
FINDINGS & OPINIONS
1) Coefficient of friction values (.58 to .88) are similar and generally a little higher than
passenger cars and SUV’s. Results shown in Table 1.
2) All ATV’s tested had a Kp much better than 1.0. Lateral tipping tests resulted in tilt table
measurements (Ktt) between .751 and .986. Calculated (Kst) between .876 and .930.
3) Two ATV’s failed to meet the Kst .89 minimum. The Yamaha Sport .876 and Polaris
Trailblazer .877. The Yamaha has a Ktt of .92.
4) Kst is generally higher than Ktt, and overstates stability by 10 to 15%. This was also true for
the 4-wheel ATV’s tested in SAE paper 891108. The lone exception was the Yamaha Sport.
Ktt is a test value and would be a more representative number for lateral stability.
5) Kst numbers for 12 ATV’s from SAE paper 891108 (paper presented May 2-4, 1989) ranged
from .86 to 1.07, with an average of .94. The ATV’s tested in 1989 had better lateral stability
than the ATV’s we tested.
6) Kst of .89 and Kp of 1.0 are big improvements over the 3-wheel ATV’s of the 1980’s.
Unfortunately, these 3-wheel ATV’s were grossly unstable and were likely a factor in
establishing such a low Kp. The Kp requirement (shall be at least 1.0) is the same as the
requirement from the original 1990 standard.
7) A Kp of 1.0 is not an acceptable criterion for safety. As a practical matter, a Kp coefficient
for a 4-wheel ATV is unnecessary, providing the ATV has acceptable lateral stability Kst/Ktt.
8) A more important measure of ATV stability is lateral stability. If the Kst/Ktt number is
acceptable for safety, the center of gravity will be low, and the Kp will be much greater than
1.0.
9) ATV‘s would be safer and less likely to tip over if they have a Kst greater than 1.0. None of
the ATV’s we tested came close to a Kst of 1.0. Two of the ATV’s tested in 1989 (SAE
paper 89118) exceeded a Kst of 1.0.
Page 2
10) ATV’s with Kst values less than 1.10 are dangerous and will roll over. Rider experience and
skill levels, are factors in any ATV rollover.
11) With the current design configuration (wheel base & track width) it is not feasible for ATV’s
to have Kst values similar to passenger cars or SUV’s.
COEFFICIENT OF FRICTION
1) Coefficient of friction tests were conducted with two of the ATV’s on two different surfaces.
A Vericom VC2000 braking test computer was used to determine coefficient of friction
between the tires and various surfaces.
2) Peak values ranges from .84 to 1.1. Average values ranged from .58 to .88. Table 1 is a
summary of the results. Passenger and SUV vehicles on dry surfaces will have comparable
values. On wet surfaces they will have lower values.
3) Coefficient of friction translates to the cornering forces acting on the tires of the ATV to tip it
over. Higher coefficient of friction will result in higher tipover forces.
STABILITY TESTING
1) Static stability tests provide a relative measure of stability with which to compare various
ATV’s. Tilt table tests do not incorporate the rider, cargo, or other dynamic considerations
(tire and suspension deflections, tire coefficient characteristics, ATV weight, or machine
performance).
2) Placing rider and cargo on the top of a machine significantly raises the center of gravity and
reduces stability (Ktt and Kp numbers are reduced). Obviously, seat design, placement, and
height are important factors in determining stability.
3) The rider, by changing position, can add or subtract to stability. Calculations assume a fixed
rider position, as shown on the diagrams 2 & 5.
REARWARD TIP-OVER/PITCH STABILITY
1) Pitch stability Kp, balancing angles, and Kp calculations with rider are shown in Table 2. All
test values without rider exceed the 1.0 performance requirement of the standard.
2) Pitch stability (Kp) is a measure of the ATV’s propensity to tip over backwards. Kp is the
tangent of the balancing angle (rear wheels and drive axle together). The tip over formula
[Kp = L1 ÷ h]: where L1 is the location of the center of gravity from the rear wheel forward, h
is the height of the center of gravity.
3) The formulas and calculations for the Yamaha without rider are shown on diagram 1. The
calculations and formulas for the Yamaha with rider are shown on diagram 2.
Page 3
4) When tipping upwards (as specified in ANSI/SVIA 1-2000) the tip angle is about the drive
axle. When accelerating (real world application) the ATV will rotate about the drive wheels.
The formula for Kp, as specified in the standard, is not a direct calculation from the tangent
of the tip angle. The formula is modified using the vertical height between the drive axle of
the rear wheel and the ground (Rr). The formula for Kp from the standard para. 9.3.1 is: [Kp
= (L1)(tan alpha) ÷ L1 + (Rr)(tan alpha)]. The Kp calculation should be the same as the
tangent of the balancing angle if the rear axle and wheels are locked together.
5) The ANSI/SVIA pitch stability test conditions, paragraph 8.1, requires suspension
components “be fixed by a means of locking procedure such that they remain in the same
position and displacement as when the unladen vehicle is on level ground,….” Such locking
procedure is not defined and likely unrealistic. When the suspension and tires flex and
deflect the at rest location of the center of gravity moves. Tip angle testing to determine
center of gravity location will not precisely compare to the unladen vehicle on level ground.
EVALUATION PITCH STABILITY
1) Rearward tipover occurs when the forward acceleration plus uphill grade exceed the Kp
(with rider and cargo). For clarification, acceleration is not speed; it is rate of speed change.
Higher accelerations usually occur at lower speeds, and the rate of acceleration is based on
engine performance. Thus, the mechanism to tip over an ATV going uphill is much greater
at a lower speed. For example, a .35 g’s acceleration going up a 35% grade (this is both a
steep grade and high acceleration) will tip over ATV’s with a calculated Kp (with rider and
cargo) of less than .7. As table 2 shows, no calculated Kp is less than .7. If the rider
moves backwards the Kp will be less, and tipover may occur.
2) Of the four ATV’s tested, none should tip over for the hypothetical example given above.
Not because they just meet the standard, but because they far exceed the standard by 19%
to 25%. Any ATV that just meets the standard Kp 1.0 when adding a rider will have Kp of
about .60, and will easily tip over.
3) The ATV’s tested far exceed the Kp standard. When establishing the Kp number, it was
likely influenced by 3-wheel ATV’s which had more weight in the rear and resulted in a low
Kp and they were easy to tip over backwards. As already discussed, a more meaningful
number is Kst/Ktt.
SIDEWAYS TIP-OVER/LATERAL STABILITY
1) Lateral stability is a measure of the ATV’S propensity to roll over (tip over sideways). Kst is
calculated, Ktt is from tilt table testing.
2) Kst is calculated according to the methodology used in SAE Paper 891108. The formula:
[Kst =T/1 ÷ h]: where T1 is the distance from the tip axis to the center of gravity (resisting
tipping moment), h is the height of the center of gravity. This method uses (T1) the distance
from the tip axis to the center of gravity as ½ (T/2) the track width (T/2 measured to the
center of the tires). The formulas and calculations for the Yamaha are shown in Diagram 3.
Results for all ATV’S are shown in Table 2.
Page 4
3) From the Kst value balancing angles are calculated by taking the arc tangent of the Kst
value. The calculations for the Yamaha also shown in Diagram 3. Kst values and balancing
angles are shown in Table 2.
4) Lateral balancing angles were determined by tipping the ATV’s. There were different
balancing angles to the left and right. This means the weight of the ATV is not symentrical.
Ktt values were determined by taking the tangent of the balancing angle. Formulas and
results for the Yamaha are shown in Diagram 4. Results for all ATV’S are shown in Table 2.
5) Using T/2 in the formula (T is the track width from tire centerline) is making two general
assumptions. One is the ATV has weight symmetry. The second is that tipping occurs
about the tire centerline. Neither of these assumptions is precisely correct. This is apparent
when comparing the lateral stability calculations with lateral tests. Tip angles to right and
left, Diagram 4 (using h from pitch stability) indicates T1 left side is 19.83” which is 2.2” more
than T/2. T1 right side is 18.56” which is about 1” more than T/2.
EVALUATION LATERAL STABILITY
1) Lateral tipover occurs when the cornering forces (centrifugal acceleration) plus or minus the
side slope, exceed the dynamic Kst (rider and dynamic considerations). As the vehicle
corners, the location of the center of gravity will actually change relative to its static test
condition. The cornering forces are a relationship between the sharpness of the turn and
the speed. The resistance forces, which cause tipping, are generated by the tires
(coefficient of friction with the surface).
2) When cornering forces are greater than the Kst, the ATV rolls. Ideally, the Kst value would
be high enough, i.e. center of gravity low so that the ATV would side slide or spin out, not
roll over. Side sliding is what typically occurs with passenger cars when the cornering forces
exceed the coefficient of friction forces (tires and road surface).
3) Passenger cars typically have a Kst of 1.30 to 1.65. The Kst for SUV’s, generally 1.05 to
1.25. Those SUV’s with a Kst lower than 1.10 are the ones considered tippy, and will roll
over during severe turning maneuvers. The Jeep CJ5, one of the least stable SUV’s, had a
static Kst of 1.01.
4) ATV’s have a significant change in the Kst when in operation. The rider sits on top of the
seat and has a greater effect on center of gravity than the driver and occupants of
passenger vehicles and SUV’s. A calculated Kst, because of the center of gravity change,
is shown in Table 2. Sample calculations are shown in diagram 5.
5) Given the basic configuration of the current ATV, establishing Kst numbers near those of
passenger cars becomes unrealistic. A Kst value 1.0 would be an improvement, and 1.10
would significantly increase stability. Rollover would still occur, but would be less likely.
6) The Kst of the Yamaha is .876. If track width is increased 1” and center of gravity lowered
1”, Kst would be .948. If track width increased 2” and center of gravity lowered 2”, Kst would
be 1.03. If track width increased 3” and center of gravity lowered 3”, Kst would be 1.12.