UNIVERSITY OF CALIFORNIA 
 
 COLLEGE OF AGRICULTURE 
 AGRICULTURAL EXPERIMENT STATION 
 
 CIRCULAR No. 267 
 June, 1923 
 
 THE TENDENCY OF TRACTORS TO RISE IN 
 FRONT; CAUSES AND REMEDIES 
 
 By A. H. HOFFMAN 
 
 Since the use of lighter tractors has become more general, a number 
 of serious accidents due to tractors 'rearing up' in front have occurred. 
 The causes of tipping up are found in the reaction forces necessarily 
 present in a tractor when it is in operation. The purpose of this 
 circular is to explain the action of these forces and to suggest methods 
 of controlling them. 
 
 When any tractor is pulling a heavy load, the teeth of the drive 
 pinion push with very great force against the teeth of the large gear 
 directly attached to the drive wheel or track sprocket. If a worm 
 drive is used, the threads of the worm push against the teeth of the 
 worm wheel. The shaft of the drive pinion (or worm) is held to the 
 tractor frame by its bearings, enabling it to exert its driving force 
 and still maintain its position. It is only by 'bracing itself,' by push- 
 ing backward against its bearings and the tractor frame, that the 
 pinion can exert its forward force. In figure 1 the arrows, 1, 2, and 
 3 show the directions of the motion of the tractor, drive wheel, and 
 drive pinion, respectively. Arrow 'a' shows the direction and point 
 of application of the driving force. Arrow 'b' shows the direction 
 of the reacting force with which the pinion pushes backward against 
 the frame. In a rear-drive tractor this force 'b' tends to revolve 
 the tractor frame up in front and over backward about the rear 
 axle as an axis. It makes no difference whether the pinion (or the 
 worm) is placed above or below, to front or to rear, this tipping-itp 
 tendency is present just the same in any track drive or rear-wheel drive 
 tractor when pulling forward. In a front drive tractor the effect 
 of the driving force is to press down on the rear carriage, or on the 
 implement if it is directly connected, when pulling forward; and 
 to tip up in the rear when in reverse. In a tractor driving equally 
 on all four wheels there is little tendency to rise in front, since the 
 center of weight is farther forward and half the force is applied to 
 the front wheels. 
 
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Fig. 1. — The tendency of a tractor to rise in front is due to certain 
 forces which are necessarily present when the tractor is working. Arrows 1, 
 2, and 3 show directions of motion; arrow 'a' direction and point of appli- 
 cation of driving force; arrow 'b' the reacting force with which the pinion 
 pushes against the tractor frame, thus tending to make it revolve about the 
 axle of the drive wheel. 
 
 Fig. 2. — A track type tractor has the same tendency as a wheel type to 
 rise in front; however, the higher it rises the less becomes the tendency to 
 rise. Many track type tractors are steered by the tracks, hence may be 
 turned even when the front wheel is up off the ground. 
 
Circular 267] TENDENCY OF TRACTORS TO RISE IN FRONT 3 
 
 In most tractors this tendency to tip up is not great enough 
 to throw the tractor over backward unless: (a) it is going up a very 
 steep grade, (b) the drivers have been made to dig themselves 
 in by revolving when stalled, or (c) the clutch is thrown in 
 suddenly when the engine is racing. Track-type tractors are less 
 likely than the wheel-type to go over backward, though the fronts of 
 the track types start to rise when the load is very moderate or even 
 when there is no load at all (fig. 2). This stability of the track type is 
 due to the fact that the middle of the tread area still touching the 
 ground moves backward as the tractor tips up in front. This means 
 
 Fig. 3.— Fin flange for Fordson tractors. Attached by clamp bolts requir- 
 ing no drilling of holes. Adds weight to front end of tractor and makes steer- 
 ing easier. 
 
 that up to a certain height it is increasingly difficult to throw the 
 tractor over backward. With wheel-type tractors, on the contrary, 
 the higher up the front of the tractor is, the less force it takes to 
 turn it on over, in general. 
 
 It should be remembered also that the load by its resistance pulls 
 backward on the tractor just as hard as the tractor pulls forward. 
 This resisting force tends to revolve the tractor over backward on 
 the line through the places where the drives grip the ground. 1 This 
 tendency to revolve is doubled by doubling the height of the drawbar 
 above the ground. Hence with a low drawbar it would be small; 
 but it would not be zero unless the drawbar were down at the level 
 
 iThis effect is, of course, present in tractors that drive on all four wheels. 
 
4 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 of the grousers or lugs in the surface of the soil. This would, of 
 course, not be practical. A horse when pulling hard has the same 
 tendency to tip up in front. Often a horse is enabled to pull his 
 load out of a hard place by the assistance afforded him when his 
 driver hangs onto the hame or rides on his shoulders. 
 
 Careful tests have shown that on level ground it is practically 
 impossible to turn over backward any tractor now on the market 
 unless we have all or nearly all of the following conditions present : 
 
 Fig. 4. — Loading front of tractor by casting masses of concrete in the front 
 wheels. Use rich concrete mixture, reinforcing with heavy iron wire fastened 
 to the spokes. 
 
 (1) Too little of the tractor's weight carried in front. 
 
 (2) No governor, or a governor not working properly. 
 
 (3) A very high maximum gear reduction ratio (i. e., when 
 in low gear) between engine and drive wheels. 
 
 (4) A high drawbar. 
 
 (5) Engine racing to its limit. 
 
 (6) Clutch thrown in quickly, or grabbing. 
 
 The manufacturer is responsible for the first four points; the 
 operator for the last two. With the last two present it may be en- 
 tirely possible to make certain tractors rise in front and turn on 
 over backward when they are on level ground and even when not 
 attached to any load other than the weight of the tractor itself. 
 
Circular 267] tendency of tractors TO risk jx fkoxt 5 
 
 A tractor having the first four characteristics may be operated 
 safely (1) by care in handling the clutch and throttle and {2) by 
 lightening the load before the drivers have dug themselves in. It 
 may be made still safer (3) by adding weight in front, figs. 3, 4, 
 and 4A. This is best done by adding a flange fin to each front wheel, 
 riveting, bolting, or clamping it to the fin already on, if there be one. 
 These extra fins may be obtained from manufacturers, from most 
 tractor service stations, or may be forged from % // x2 1 / 2 // iron. The 
 added weight of the flanges and their deeper penetration will also make 
 
 Fig. 4A. — Loading front of tractor by bolting into the 'front wheels pieces 
 of cast iron, which may be filled with concrete if desired. 
 
 Patented 
 
 Fig. 5. — Auxiliary drawbar for Fonlson tractors. Furnishes a drawbar free 
 to swing between horizontal guides and helps to prevent tipping over back- 
 ward. 
 
6 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 steering easier and more certain. Another and cheaper way of adding 
 weight is by casting masses of concrete in the front wheels, as shown 
 in figure 4. A rich concrete mixture should be used, reinforcing with 
 iron wire fastened to the spokes. The concrete must be kept moist at 
 least a week while setting or it will speedily go to pieces from the 
 jarring. 
 
 Cast iron pieces are obtainable (fig. 4A) which may be bolted 
 into the light tractor wheel to add to the weight. They may be 
 filled with concrete if desired and would effectively prevent the 
 concrete from shaking out. The castings shown in the figure weigh 
 sixty pounds to a wheel. 
 
 Fig. 6. — Auxiliary drawbar for Fordson tractors. A swinging drawbar 
 also makes for easier turning with such loads as disk harrows, drags, pul- 
 
 The hitch point may be lowered by bolting on an auxiliary draw- 
 bar, either stationary or swinging. The markets afford several kinds 
 of extra drawbars designed for use with certain tractors to make 
 them easier to steer and to assist in overcoming sidedraft effects. 
 Some of these, also, while not especially designed for that purpose, 
 render the tractor safer against tipping over backward when 
 operated by careless drivers. 
 
 The drawbars shown in figures 5 and 6 are in part designed to 
 serve this purpose. A drawbar extending out some distance back 
 of the rear axle may prevent tipping over backward by the anchorage 
 it affords when attached to a heavy load, for example, a subsoiler. 
 It is entirely possible to cause certain four wheel tractors to run 
 along for considerable distances with the front wheels up a foot or 
 
CIRCULAR 267] TENDENCY OF TRACTORS TO RISE IN FRONT 7 
 
 more in the air, the tractor drawbar and frame lining up with the 
 line of resistance of the load. It would of course be impossible to 
 steer the tractor under such circumstances. Tt should also be pointed 
 out that a drawbar extending rearward, if pinned, increases side- 
 draft effects. 2 
 
 Fig. 7. — Extra swinging drawbar designed for Samson tractor. 
 
 There are numerous devices 3 obtainable which are designed to 
 ' ' kill the engine " or to release the clutch of a tractor when the front 
 wheels have risen a certain distance above the level. Some of these 
 
 2See Bulletin No. 349 Calif. Agr. Exp. Sta. A study of Sidedraft and 
 Tractor Hitches. 
 
 3 Local dealers in tractors and farm machinery are in general able to 
 furnish tractor accessories that help to overcome rising in front, or at least 
 they can supply addresses of the manufacturers. Manufacturers' addresses 
 may also be obtained upon application to the Division of Agricultural Engin- 
 eering, Branch of the College of Agriculture, Davis, California. 
 
8 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 devices may prove of value under certain circumstances, but the 
 remedy for the trouble is rather to be found in a thorough under- 
 standing of the causes and a careful avoidance of them. In general 
 it may be said that loading down a tractor with supposed safety 
 devices makes for greater complexity and hence for greater chance 
 for accident. 4 
 
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 Fig. 8. — Under view of the drawbar of figure 7, showing details of construction and 
 
 attachment. 
 
 -^Discussions of this subject from a mathematical standpoint by E. B. 
 Hewitt and D. L. Arnold may be found in Transactions of the Society of 
 Automotive Engineers, 1919, Part 1, pp. 83-95; and by Adams in Common Sense 
 Instructions on Gas Tractor Operation, p. 209