• No results found

Discrepancies between longitudinal high slip and lateral low slip friction measurements on prepared ice surfaces

N/A
N/A
Protected

Academic year: 2022

Share "Discrepancies between longitudinal high slip and lateral low slip friction measurements on prepared ice surfaces"

Copied!
1
0
0

Loading.... (view fulltext now)

Full text

(1)

Discrepancies between longitudinal high slip and lateral low slip friction

measurements on prepared ice surfaces

Niclas Engstr¨om 1 (niclas.engstrom@ltu.se) ,

Henrik Andr´en 2 , Roland Larsson 1 , Lennart Fransson 2

1)

Department of Applied Physics and Mechanical Engineering

2)

Department of Civil, Mining and Environmental Engineering Lule˚a University of Technology

S-971 87 Lule˚a, Sweden

1 Introduction

Road surface conditions impact the safety of transportation on the Scandinavian road systems, especially during the winter months.

One of the main challenges is to monitor and predict the danger- ous conditions. High road grip, i.e. the total friction between road and tires is of great importance in avoiding hazards. The ability to decelerate and change direction quickly is affected mainly by speed and road grip, therefore knowledge of road grip and ways of measuring it are of great importance. There are many measur- ing devices, utilizing different methods to measure road grip. One challenge is to find out how relevant these measurements are to traffic.

In an ambitious attempt to compare different kinds of road grip measurement devices the Swedish Road Administration and Lule ˚a University of Technology’s Center for Automotive System Tech- nologies and Testing (CASTT) conducted a 3 day field experiment together with Pon-Cat, ViaTech, Coralba, SRIS, Halliday Technoli- gies and Racelogics to examine the likeness and differences of a wide collection of friction measurement devices.

The main goal of the tests was to discern rather than evaluate the different devices. Examining in what aspects the measurement devices and methods correlate and discern.

2 Equipment

This presentation centers mostly on how the two longitudinal mea- surement devices Traction Watcher One (TWO) and RT3 differ and correlate in the measurements.

The RT3, pictured in Figure 1 below, measures lateral low slip fric- tion via a wheel, turned a few degrees in comparison to the line of driving. No braking or accelerated force is used for friction mea- surements, so only side forces are measured.

Figure 1: RT3 tow hitch model mounted on Mitsubishi L200, in upper left corner tire pattern and in upper right corner dis- play unit.

The TWO, seen in Figure 2 below, uses a reference tire, rolling on the ground. This tire drives another wheel via gears, WHICH spins at 80% of the reference tire’s rotation; creating a relatively high slip of 20%. To ensure that the reference wheel is rotating with the surface, the normal force is twice as large as on the mea- surement wheel.

Figure 2: TWO device with type 523 Trelleborg friction tires.

Since the RT3 gives measurements in Halliday Friction Numbers (HFN) and the TWO calculates a friction coefficient µ a standard had to be chosen. We opted to use polished ice as normalizing measurements to be able to compare the different results.

For comparison with the friction values a VBox system consisting of very accurate GPS positioning devices as well as gyroscopes measuring jaw and tilt and thus calculating retardation and from this data evaluating the road grip is also used.

3 Track

The test track was professionally prepared on Lake Kakel, Arje- plog, by IceMakers. For these tests we used a track divided into three parts, as seen in Figure 4, each part consisting of a man made well defined surface according to Figure 3.

Figure 3: System 2000 tool, Old System 2000 ice, Brushed old polished ice and New System 2000 ice with new snow.

• Old system 2000 ice: Ice surface aged through weather and wear to micro smoothness while maintaining its macro rough- ness.

• Brushed old polished ice: Zamboni polished ice that had been covered with snow and then brushed off. The aged snow crys- tals fused with the ice gave it a micro smooth surface, while being macro rough.

• New system 2000 ice: Freshly prepared surface with the Sys- tem 2000 hard metal teeth cracking the ice and making it both micro and macro rough.

Figure 4: Test track layout on Lake Kakel.

4 Tests

The tests were conducted in the following fashion:

• Line up in the left or right lane, according to what type of tires the towing vehicle had.

• Distribute target speed to drivers. Drivers used the speedome- ter in the vehicle to maintain the speed required. Speeds used were 30 km/h, 50 km/h and 70 km/h.

• Accelerate each vehicle to target speed and maintain it through- out the run.

• RT3 drivers started logging data manually at the “Start” posi- tion, marked with a marker. The TWO units started the logging around 100 meters before the “Start” position, see FIGURE 5.

• Logging of data was stopped at the “End”, also marked with a marker.

Time Run Speed Units on Units on [km/h] right side left side

10:26-10:44 1 30 G065,G055 G053,G056,TWOV,TWOT 10:45-10:57 2 50 G065,G055 G053,G056,TWOV,TWOT 10:57-11:10 3 70 G065,G055 G053,G056,TWOV,TWOT 14:12-14:24 4 50 G065,G055 G053,G056,TWOV,TWOT 14:25-14:34 5 70 G065,G055 G053,G056,TWOV,TWOT 14:34-14:42 6 30 G065,G055 G053,G056,TWOV,TWOT 15:08-15:18 7 70 G065,G055,TWOV,TWOT G053,G056 15:19-15:26 8 30 G065,G055,TWOV,TWOT G053,G056 15:27-15:37 9 50 G065,G055,TWOV,TWOT G053,G056 Table 1: All the tests relevant for the data gathered. As can be seen, the TWOs changed sides by the end of the day, for comparison.

Weather conditions were recorded and stable during the day. Ice temperatures were between -4

C (24.8

F) and -3

C (26.6

F).

5 Results

There are two main results presented from this report. The most important one is shown on Figure 5, where the difference of the RT3 and TWO is obvious. Both observations and measurements show that the grip on both kinds of System2000 ice is adequate.

But careful measurements show that the RT3 reports a better grip on Old System 2000 ice compared to the New System 2000 ice, whereas the TWO claims the opposite.

Figure 5: Normalized weighted road grip comparison, all speeds included, with RT3s G053, G056 and TWOs TWOV and TWOT. March 18th, 2008.

The discrete VBox system correlates with the results of the TWO, which makes sense since both systems use longitudinal high slip for measurements. The RT3 on the other hand can rather be com- pared to steering rather than braking.

Figure 6: All road grip values from VBOX at 50 km/h together with average and standard deviation.

The other observation from the data is that velocity did not affect the measurements in any discernable way. The measurements from one RT3 represents this in Figure 7, but no measurements from any RT3 or TWO in the test could display any speed depen- dencies whatsoever.

Figure 7: Normalized weighted road grip comparison, all speeds included, with RT3s G053, G056 and TWOs TWOV and TWOT. March 18th, 2008.

6 Conclusions

The main conclusion from these tests is that there are discrep- ancies between longitudinal high slip and lateral low slip friction measurements on prepared ice surfaces with furrows.

• Longitudinal high slip devices measure the absolute grip to be higher on new System 2000 ice compared to old System 2000 ice.

• Lateral low slip devices measure the road grip to be lower on new System 2000 ice compared to old System 2000 ice.

• Lateral low slip devices report larger relative difference between surface contacts with high road grip and surface contacts with low road grip compared to Longitudinal high slip devices.

• Speed did not affect road grip in a significant way during these tests.

7 Acknowledgements

This work is supported by: The Center for Automotive System

Technologies and Testing – CASTT project at Lule ˚a University of

Technology, The Swedish Road Administration, and the Kempe

foundation We would like to acknowledge Pon-cat TWO for their

assistance with TWO units, personnel and software during these

tests and Halliday Technologies INC. for their assistance with per-

sonnel during these tests.

References

Related documents

The results from these tests showed that the rating scales describing perceived safety and balance, and the methods for observing movements, were reliable and could be used

anti-slip devices (* and walking sticks) 5 1* 0 6 Mean daily total walking distance compared with experiences of incidences/falls is similar among the subjects

The main task of this thesis work is to investigate how different oil additives influence the friction in a wet clutch, in order to gain the ability to optimize the formulation of

Survey type 1, which yields qualitative data regarding the saturation of genres, dynamics and narrative tropes, is a universally applicable tool but it doesn’t yield direct

On the contrary, when dimples are located in the maximum pressure region, load is reduced less than friction which gives a decrease in friction coefficient and improves

It is important to avoid the confusion between the units’ preferred direction described in Figure 3, which relates to the robot movement direction and the maximal firing rate of

The plastic material behaviour of the model makes it possible to model slip between glulam beams in a bridge deck of a prestressed timber bridge.. Examples of simulation

The difference is that the intensity ratio is much more instable. We also see that the signature of the HFN numbers changes slightly between each test run. The low initial road grip