by Paul Hornback

With the development of any newArmy combat vehicle, the question,“Which is better: a wheeled vehicle or atracked vehicle?” surfaces again andagain. In order to answer this question,the U.S. Army has tested and studied themerits and shortfalls of wheeled andtracked combat platforms for the past 30years. Results indicate that no single cri-terion can be applied that will answerthe wheeled-versus-track issue for allsituations and missions. In fact, the un-derlying premise in resolving thewheeled-versus-track dilemma is deeplyrooted in the complex variables regard-ing the platform’s combat mission, ter-rain profile, and specific vehicular char-acteristics. Tests and studies, however,established a set of criteria to determinea platform’s optimal configuration. Al-though most of this information is overten years old, the basic factors which im-pact the physics of mobility have notchanged and are still relevant.

MOBILITY . Mobility, as defined bythe 1988 Mobility Analysis for the TRA-DOC Wheeled-Versus-Track Study, isthe ability to move freely and rapidlyover the terrain of interest to accomplishvaried combat objectives.1 Mobility isthus measured by a system’s freedom ofmovement (percent of the terrain overwhich the vehicle is mobile) and its av-erage speed or travel time over that ter-rain. A platform’s gross vehicle weightand its footprint (the area of track or tirewhich impacts the ground) determine theresultant ground pressure that the plat-form imparts on the soil. The soilstrength, coupled with the vehicle’s char-acteristic ground pressure, determine aparameter entitled Vehicle Cone Index(VCI), which is a key first-order dis-criminator of a platform’s mobility. Thehigher the VCI, or ground pressure, theless mobile the platform becomes. Fig-ure 1 shows that, as ground pressure in-creases, so does the percentage of No-Go Terrain (terrain over which a combatplatform is immobile) due to tractionloss in wet, temperate areas.

A vehicle’s mobility is impacted by itstractive ability over various soil types(dry, wet, sand, or snow-covered) and itsability to maneuver over obstacles, crossgaps, and negotiate varied vegetation. Asa general rule of thumb, a lower VCI not

only equates to better soft-soil mobilitybut also indicates better performance onslopes, in sandy terrain, over obsta-cles/gap crossings and when overridingvegetation.2 From a mobility perspective,tracked vehicles offer the best solutionfor a versatile platform that is requiredto operate over diverse terrain, includingextremely difficult ground, becausetracks inherently provide a greater sur-face area than wheels, resulting in alower VCI.3 Recent operations in Bosniahave demonstrated the inherent weak-nesses of wheeled vehicles with regardto mobility and protection.4 When opera-tions were conducted on roads, wheeledvehicles demonstrated excellent mobilityand speed; but when off-road usage wasrequired, and wet or snow conditionsprevailed, mobility suffered.

Wheeled vehicles inherently attainfaster road speeds and, therefore, offerthe best solution where unrestricted mo-bility is not the primary mission driverand on-road usage exceeds off-road us-age. So, vehicle weight and off-road us-age constitute two key criteria for mobil-ity. Figure 2 compares the average 100km mission travel time for both wheeledand tracked platforms as off-road usageincreases (recall that mobility was de-fined as both freedom of movement andtravel time over the terrain).

As off-road usage dominates the vehi-cle’s profile, tracked configurations pro-vide significantly better mission traveltimes. Consequently, Army studies indi-cate that when a vehicle’s mission re-quires off-road usage greater than 60percent and gross vehicle weight ex-ceeds 10 tons, a tracked configuration ispreferred for combat roles.6 However,

when the gross vehicle weight exceeds20 tons and off-road usage remainsabove 60 percent, a tracked configura-tion is required to guarantee the best mo-bility for unrestricted, all-weather tacticaloperations.7

SURVIVABILITY . A combat plat-form’s survivability is dependent on nu-merous criteria, to include mine and bal-listic protection, size/silhouette, andstealthiness. Tracked vehicles, by design,are inherently more compact thanwheeled vehicles.8 The primary reasonsfor a tracked vehicle’s compactness arereduced suspension clearance, wheelturning clearance, and the absence ofmultiple transfer cases and drive shaftsthat are integral to the design of multi-wheeled vehicles. Army studies have in-dicated that, for a comparable VCI (orground pressure) at the same gross vehi-cle weight, wheeled platforms require upto six times more volume for drive trainand suspension components than trackedplatforms. This results in up to a 28 per-cent increase in vehicle volume if thesame interior volume is maintained.9

Survivability analyses clearly indicatethat a larger size is more readily seenand subsequently hit and destroyed. Ad-ditionally, as a combat platform’s size in-creases, so does the gross vehicle weight(provided the same ballistic and mineprotection are maintained), which tendsto degrade vehicle mobility and deploy-ability.

In general, wheeled platforms are morevulnerable to small arms fire and gre-nade, mine, and artillery fragments, dueto the inherent weakness of wheeled sus-pension designs, components, and tires.10

ARMOR — March-April 1998 33

0

5

10

15

20

25

7.4 7.8 10.6 11.6 13.5 15.7 19.4 37

Ground Pressure (lbs/sq in)

% N

o G

o T

erra

in (

trac

tion)

Generally as VehicleGround PressureIncreases, Cross

Country TrafficabilityDecreases

Figure 1

The Wheel Versus Track Dilemma

Wheeled vehicles may now be able tocontinue movement for limited distancesat reduced speeds when tires are punc-tured by small arms rounds, battlefielddebris, or shrapnel, due to the advent ofrun-flat tires. Run-flat tires typically con-tain a hard rubber insert (some with ni-trogen filled cells) inside the tire. The in-sert bears no vehicle load until the tire ispunctured, at which point the load istransferred to the insert and vehiclemovement may continue for a limiteddistance and speed.

On the plus side, wheeled platformsprovide a reduced noise signature whilemoving, primarily due to less vibrationand metal to metal contact on runninggear. Improvements in track technology(i.e., Roller Chain Band Track) and de-coupled running gear have decreasednoise signatures for tracked vehicles, butnot to the level attained by wheeled plat-forms.

Tracked platforms do provide a skid-steer capability which allows the vehicleto pivot steer (or neutral steer) and virtu-ally pivot in place. This unique maneu-ver capability enhances survivability bypermitting a 180-degree directionalchange when confined or built-up areasare encountered, and while traveling onnarrow road surfaces.

From a survivability perspective,tracked vehicles offer smaller silhou-ettes, reduced volume, enhanced maneu-verability, and better ballistic protection,providing a balance that equates to amore survivable platform.

SUPPORTABILITY . A combat plat-form’s supportability is dependent on nu-merous factors, to include fuel usage, re-liability, and O&S costs. Wheeled vehi-cles traditionally offer better fuel econ-omy due to the reduced friction lossesinherent in wheel/tire suspensions andrunning gear. The better fuel economytranslates into smaller on-board fuel stor-age requirements or greater operatingranges for wheeled platforms.

Previous articles and studies have con-cluded that wheeled vehicles are intrinsi-cally more reliable than tracked vehiclesand, therefore, require less maintenanceand supply support (spare parts). How-ever, one must bear in mind thatwheeled vehicles generally have a higherpercentage of on-road usage whiletracked vehicles incur more off-road us-age. Obviously, the more severe cross-country terrain results in reduced reli-ability for the tracked vehicle. A recenttest of the Up-Armored HMMWV, run-ning a scout profile with 68 percent off-road travel, resulted in significantly lower

reliability when compared tothe same platform runningat a tactical truck profile ofonly 40 percent off-road.

Given that wheeled plat-forms offer better fuel econ-omy and reliability (to anextent), then Operating andSupport (O&S) costs arelower than those demon-strated by tracked platforms.This makes wheeled plat-forms excellent candidatesfor support roles whereoverall mileage is high andprimarily conducted on-road.

CONCLUSION . Figure 3presents an overview of the

key advantages demonstrated bywheeled and tracked platforms based onthirty years of Army tests and studies.

Wheeled and tracked vehicles each ex-hibit advantages that can be optimizedfor the 21st century battlefield, providedthe platform’s combat mission, terrainprofile and specific characteristics arecarefully assessed. For combat vehicles,vice combat support or combat servicesupport vehicles, Army studies unani-mously conclude that a tracked configu-ration is the optimal solution for tactical,high-mobility roles (off-road usagegreater than 60 percent), gross vehicleweights in excess of 20 tons, and mis-sions requiring unrestricted terrainmovement, continuous all-weather op-erations, smaller silhouettes/dimensionalenvelopes, and greater survivability.

Notes

1“Mobility Analysis for the TRADOC WheeledVersus Track Vehicle Study, Final Report,”Robert F. Unger, Geotechnical Laboratory, De-partment of the Army, Waterways ExperimentStation, Corps of Engineers, Vicksburg, Miss.,September 1988, 1.

2Ibid, 26.3“Wheels or Tracks,” Military Technology, Vol

XVIII, Issue 7, Jul 1994, 14.4“Is There Any Future for the APC,” Military

Technology, Vol. XXI, Issue 3, March 1997, 103.5“Mobility Analysis,” 48.6“Wheeled Versus Track Vehicle Study, Final

Report,” Studies and Analysis Activity, Head-quarters U.S. Army Training and Doctrine Com-mand, Fort Monroe, Va., March 1985, 1-99.

7Ibid, 1-92.8Ibid, 1-62.9Ibid, 1-62.10“Wheels or Tracks,” 11.

Mr. Paul Hornback is a general engi-neer with the federal government. He ispresently assigned to the HQ TRADOCCombat Development Engineering Divi-sion, Fort Knox Field Office, which pro-vides reliability, maintainability, and sys-tems engineering support to the Director-ate of Force Development, Fort Knox, Ky.He holds a Bachelor of Science in Me-chanical Engineering and a Master ofScience in Industrial Engineering, bothfrom the University of Louisville. His mili-tary experience stems from a six-yeartour as a UH-1N helicopter pilot in theU.S. Marine Corps.

34 ARMOR — March-April 1998

100 KMMission

Travel Time

01020

3040

5060

20 30 40 50 60 70

Percent Cross Country(Europe Wet)

Hou

rs

0

20

40

60

8045 55 65 75 85 95

Percent Cross Country(Arid Sand)

Hou

rs

Wheel

Track

Figure 2 5

Tracked Wheeled Vehicles Vehicles

Route Flexibility

Cross Country Mobility

Traction on Slopes

Road Speed

Logistics

O&S Costs

GVW, Volume, & Payload

Maneuverability/Turning Radius

Transportability

Weight Growth Potential

Gap & Obstacle Crossing

✔✔

✔

✔

✔✔

✔✔

✔

✔✔

Study Results

Figure 3