FR2624451A1 - MECHANISM FOR ADJUSTING THE POSITION OF A SEAT - Google Patents

Abstract

The invention relates to a mechanism for adjusting a six-way seat. According to the invention, it comprises a base 12, a lower support means 14 linearly sliding on the base, an upper support means 16 having front and rear ends, a first axially telescopable locking means 34 actively connected between the end front and the lower support, and which can be released remotely to adjust the height of the front end relative to the base, a second axially telescopable locking means 35 actively connected between the rear end and the lower support, and which can be released remotely to adjust the height of the rear end relative to the base and a third locking means actively connected between the first means and the base, and which is releasable to replace, by sliding, the lower and upper support means relative to the base, thus a seat on the upper support means can be adjusted in height, inclination and horizontal position relative to the bottom e. The invention applies in particular to the automotive industry.

Description

The present invention generally relates to the field of

  adjustable in position and is more particularly directed to a mechanical seat support and position adjustment unit, in particular for use with

adjustable car seats.

  A major effort has been devoted to the development of mechanisms and devices for adjusting car seats, particularly driver's seats, for the best convenience, comfort and safety. Existing adjustment mechanisms include purely mechanical systems where adjustment is achieved by spring-loaded levers and locks, electrically controlled mechanisms, or threaded elements are moved along motor-driven screws, or combination of these two mechanisms. Purely mechanical systems are particularly favored at the bottom of the economic ladder for their lower price, ease of installation and greater inherent reliability. As a result, many mechanical seat adjustment units have been developed and are in use. Purely mechanical seat adjustment units rely on the weight of the seat occupant body to overcome the upward biasing of a spring and thereby obtain a desired displacement of the seat position. When unlocking is released, an adjustment of the seat position occurs under the effect of the spring biasing on the extent permitted by the occupant, who can control the degree and direction

  of adjustment by shifting the weight of one's relative body-

the adjustment mechanism.

  The existing mechanical seat adjustment units suffer from certain disadvantages. A source of difficulty for spring loaded mechanisms is the variable loading force that is provided by the coil springs which are typically employed for this purpose. A constant rate spring, as is well known, requires a greater and greater compression force while the spring is compressed. So,

  when the seat is lowered from a total position-

  the occupant is experiencing increasing resistance

  and close to the fully lowered position corres-

  laying on the almost complete compression of the spring, the weight of the body of a light individual may be insufficient to further lower the seat because of the force of the spring it encounters. This difficulty is commonly overcome in many forms

  by providing a handful of assistance, ie

  a mechanical lever that is normally mounted on one side of the seat and serves to lower the seat by adding a manual lever force to the weight of the occupant to thereby achieve full compression of the spring. However, as automobiles tend to become more and more compact, it becomes increasingly difficult and impractical to provide the space required for the assistance lever. On the inside of the front seats, a center console is frequently provided, while on the outer side of the seats, there is little space between the seat and the door panel because the cars shrink in size. The assist lever also adds bulk to the interior design of the vehicle and is undesirable for aesthetic reasons. For the foregoing reasons, it has become desirable to provide a seat adjustment mechanism of a spring-assisted seat suitable for automobile use which is more responsive to fit by the body, body and movement of the seat. body alone, without

maruel assistance lever.

  The present invention is directed to the necessities

  mentioned above by providing for an adjustment mechanism

  a seat having a base consisting of two parallel mounting rails for attachment to a seat mounting surface such as the floor panel of a passenger compartment of an automobile, two beams

  lower supports that are longitudinally sliding

  along the mounting rails and two upper beams to support a seat attached to it. Two transverse and mutually parallel shafts are journalled to and interconnected to the lower beams in parallel-and fixed relationship. Each shaft carries a rocker arm and two axially spaced lift arms, which arms are attached to and extend radially generally from the shafts. The upper beams have front ends pivotally connected to and supported between the outer radial ends of the two lift arms of the front shaft, while the rear ends of the upper beams are likewise supported on the lift arms of the rear shaft. The rotation of a tree raises or lowers the corresponding end of the two upper beams to thereby rock a seat that is supported. A locking interlocking assembly is connected between the outer radial end of the rocker arm on each shaft and is one of the lower beams. Each of the two locking members is spring-loaded by a constant rate spring to a linearly extended condition so that the expansion of the spring tends to rotate the shaft to which the particular rocker arm is connected, also rotating the rocker arms. lift arms on the same shaft in one direction to elevate the ends of the upper beams supported on these lift arms. The body weight of a seat occupant acts through the lift arms and rocker arms to resist spring loaded loading and interlocking the locks. Either the front ends or the rear ends of the seat support beams can be selectively raised by releasing the appropriate lock and moving the occupant's weight away from this end to allow the spring to extend the lock and raise. so the corresponding end of the upper beams

  by tilting the seat in the desired way.

  Each lock is normally locked, keeping the two shafts against rotation to secure the seat to a particular position. Each lock is

  individually releasable through

  release by cable or the like and allows a continuous and independent adjustment of the height of the front and

  back within the design limits of the mechanism.

  The mechanism of the invention provides an improvement over the previous forms in that the geometry of the crank lever composed of the lift arms and rocker arm on each shaft is optimized so that the mechanical advantage of the crank lever decreases while the spring of loading is compressed, that is to say

  while the loading spring force increases.

  This has the net result that the strength of the real spring-

  transmitted by the rocker arms and risers to the upper beams and finally up to the seat

  remains approximately constant over the entire range of compres-

spring.

  In the forms of the prior art, it was necessary to compromise in the choice of spring to allow a relatively light occupant to fully lower the seat and overcome the spring force, even near full compression. This solution required the use of a relatively weak spring so that a person weighing for example 45 kg can fully lower the seat. This, on the other hand, poses a problem for heavier occupants, for example an individual weighing 80 kg will find an inadequate spring resistance when releasing the lock from a high position of the seat and will find that the seat falls

  quickly to its maximum lowering.

  This problem is overcome in the present invention which maintains a nearly equal force of the spring over the entire range of adjustment of the seat and thus achieves a better compromise in terms of increased spring loading strength to provide more support. adequate seat for heavier individuals while remaining usable by lighter people. Further improvements in response are achieved by arranging the rear bent levers so that the backward force, applied against the seatback, produces a force component that adds to the force of the body weight force directed to the seat. low and help

  thus to push the rear end of the mechanism of ajos-

  seat. More particularly, the lift arms extend from the rear transverse shaft in a generally backward and upward direction when at a maximum raised position. A backwards force against the seat back is used to pivot the rear lift arms downward, rotating the rear shaft that turns the rocker arm into the seat.

  direction necessary to compress the load spring-

  back and push the rear end. A lightweight seat occupant can help lower the back of the seat by pushing against the seatback. The trend towards more compact, low-suspension automobiles forces the driver and front passenger to an increasingly tilted position on the seats, with the legs extended forward, making it easier to push against the seatback. seat than to move one's own body in order to apply a force vertically

  down to lower the mechanism.

  The invention will be better understood, and other purposes, features, details and advantages thereof

  will become clearer during the description

  explanatory text which will follow with reference to the accompanying schematic drawings given solely by way of example illustrating an embodiment of the invention and in which: - Figure 1 is a view in. side elevation of the seat adjustment mechanism according to the invention; Figure 2 is a plan view of the mechanism of Figure I with medial portions of the transverse shafts removed and omitted; - Figure 3 is an extreme front view of one side of the mechanism, taken along the line 3-3 of Figure 2; FIG. 4 is an extreme rear view of one side of the mechanism, taken along line 4-4 of FIG. 2; - Figure 5 is a side view as in Figure 1, with the rear end of the mechanism in high condition; - Figure 6 is a side view as in Figure 1, with the front end of the mechanism in high condition; FIG. 7a is a vector diagram illustrating the forces acting on the rear end of the seat adjustment mechanism; - Figure 7b is a vector diagram illustrating the effect of the force applied against the seat back to help overcome the rear spring loading; and - Figure 8 shows curves of the spring of

  loading for a practical example of this invention.

  Referring to the drawings, Figs. 1 and 2 show the seat position adjustment mechanism 10 having two lower beams 12 slidable along mounting rails 14, and two parallel upper support beams 16, seat. The lower beams 12 are interconnected in fixed relation

  spaced and mutually parallel by two trans-

  connecting versaux comprising a rear shaft 42 and a front shaft 52. The rear shaft 42 is rotatably supported by the erect legs of two U-shaped rear legs 54, whose bases are bolted to the lower beams 12 by means of fixing 56, or in the alternative can be welded or riveted. The front transverse shaft 52 pivots on upright legs 58a of U-shaped front legs 58, the bottoms of which are attached to the corresponding lower beams 12. Two lift arms are radially attached to and rotatable with each of the shafts 52, 42.

  A front leg 26 and a rear leg 28 are -

  attached to each upper beam 16. Each lug is inverted L-shaped with a horizontal leg attached to the upper surface of the upper beam 16 and a hanging vertical leg pivotally connected at pivot points 30, at the outer radial end of the upper beam. a corresponding lift arm 22, 24. The front legs 26 are supported on the front lift arms 22 while the rear legs 28 are supported on the rear lift arms 24. The rear pivot points 30

  are attached to the leg 28 while the pivot points

  Before 30 are linearly movable in a slot 32 of each leg 26 to account for a linear displacement component of the upper beams during the rotation of the lift arms. Alternatively, the slot may be provided at the rear pivot points for the same effect. Each shaft 42, 52 also carries a fixed radial rocker arm 40. A linear rear lock 34 is anchored at its housing end to the lower beam 12 at the pivot 36 and is pivotally connected.

  at its opposite end of rod 38 at the radial end

  external arrangement of the rocker arm lever 40, whose inner end is fixed to the rear transverse shaft 42. In the drawing, the rocker lever 40 and the lift lever 24 are shown in the form of a unitary bent lever element, but may be separate lever members that are axially spaced along the shaft 42. The linear locking member 34 comprises a rod 4.3 axially sliding through a tubular housing 46 of the lock and a rear loading coil spring 48 is axially compressed between the housing 46 and a retaining disk 50 which is fixed near the end of the rod 43, loading

  normally the lock 34 to a telescopic position

  The pin 34 is axially withdrawn from the housing 46. The latch 34 includes a mechanism inside the housing 46 through which the rod 43 is

  normally locked to housing 46 against a trans-

  axial lation relative to him. The mechanism of

  locking can be released remotely through

  a flexible release cable (not shown) which is connected at its distal end to a remote release button or other actuating means mounted on one side of the seat, for easy access by the occupant

  from the headquarters. A complete description of the construction and

  The operation of the preferred linear locking mechanism 34 can be found in US Patent 4,577,730 of March 25, 1986 assigned to the same applicant and the lock is available to this applicant under

product number MM65 LOXX.

  FIG. 1 shows the rear end 20 of the mechanism 10 in its fully lowered condition, the

  rear locking 34 to a telescopically state.

  contracted and the rear loading spring 48 to its maximum compression. When the rear lock 34 is released at a distance from its normal locked condition,

  the rod 43 is urged towards a telescopic condition

  extended by the coil spring 48, rotating the rocker arm 40 and rotating the rear shaft 42, thereby also rotating the rear lift arms 24 counterclockwise to raise the rear ends 20 of the upper beams 16 at the high position shown in Figure 5. The rear of the mechanism 10 can be reset to its lowered condition of Figure 1 by releasing the rear locking 34 again, applying sufficient force down to the rear end 20 to compress and overcome the loading of the rear spring 48 and thus linearly shorten the lock 34

26244'51

  then lock the device 34 in the contracted state for

  keep the mechanism 10 in the lowered state.

  The front end of the seat adjustment device operates in a similar manner and includes a front lock similar to the rear lock 34 with an anchored housing 46 pivoted to the same lower beam 12 at the pivot point 60, as can best be Referring to FIGS. 2 and 3, and a locking rod 43 pivotally connected at 62 to the radially outer end of the front rocker arm

  which is radially fixed to the front transverse shaft 52.

  The telescopic extension of the front lock 35 under the bias of the front loading spring 44 causes a counter-clockwise rotation of the transverse shaft 52 of the

  Figure 1, rotating the front lift arms 22.

  The anti-clockwise rotation of the lift arms 22 raises the front ends 18 of the upper beams 16 to a raised position shown in FIG. 6. The front ends 18 can be reset to the lowered position of FIG. 1 by releasing the front lock 35, applying a sufficient downward force to overcome and compress the front loading spring 44 and rotate the front rod 43 and the lift arms 22 clockwise, thereby telescopically shorten the front lock 35, and finally re-engage the front lock. 35 to keep the mechanism at the

new position.

  It may be noted that the adjustment of the height of the front ends 18 and rear ends 20 of the upper beams 16 is independent and is obtained by selectively releasing the corresponding interlocks 34, by triggering that can be individually operated remotely. As a result of these independent height adjustments, a seat bolted to the upper beams 16 can be tilted up and down forwards and backwards, and by combining the front and rear adjustments in a rocking motion, the seat can also be raised or lowered without

  significantly affect its inclination.

  The loading springs 44, 48 are coil springs having a constant rate of elasticity and therefore offer increasing linear resistance while each is compressed during the lowering of the corresponding end of the seat support mechanism. The front and rear lift arms at the fully elevated position extend upward and

  the back of the corresponding transverse shaft 52, 42.

  The weight of the seat occupant body applies downward force to the ends of the lift arms at the pivot points 30, tending to rotate the shafts clockwise along with the corresponding rocker arms 40. To this tendency normally resists the locked condition of the front locks 35, 34 which hold the mechanism 10

  and the seat that is supported at a chosen position.

  The force vector F1 of the body weight down acts by a first arm M1 which is presented by the lift arms 22, 24 and is transmitted to the linear lock / spring assembly by a second arm M2 which is presented by the arm tumbler 40 like this is

  shown in Figures 5 and 6. Figure 7a shows graphically

  the force vector diagram, from which it can be seen that the net force F2 transmitted by the ratio of the arms M1 / M2 to the locking rods against the force of,

  spring loading F3 is a function of the angles

  tans C between the arm M1 and the vertical vector F1 and / between the arm M2 and the locking rod (which

  is substantially also the axis of the loading spring).

  The axially net compressive force F2 applied to the loading spring and opposing the force F3 of the loading spring is F2 = F1 M1 cos0 M2 cos / The loading force of the spring F3 is F3 = dL R o dL is the length the stroke of the loading spring which is compressed and R is the characteristic of elasticity rate of this loading spring. The minimum force F1 required to compress the loading spring and thereby lower the rear end of the seat adjustment mechanism 10 is given by Fl (min) = dL R M2 cos / A cosc M <The quotient in the expression ci above increases almost linearly in certain ranges angles <and / A, for example when o "is understood

between 45 and 0 and / = 90 +/- 45.

  The front and rear loading coils 44, 48 are wound to have a constant rate of elasticity which approaches the negative of the "average" slope of the function F2 in the given range, and a spring force at extended state which is equal to the force down to which it must be opposed. The increasing spring force F3 while the loading spring is compressed during the depression of the mechanism 10 is thus almost canceled out by the increasing mechanical advantage of the angled lever arrangement M1 / M2, thus a downward force. approximately constant a magnitude larger than W (min.) will totally lower the mechanism. Looking at the end of the spring, the net leverage of the bent lever decreases as the shaft rotates to the lowered condition, reducing the loading force of the spring transmitted to the seat by the lift arms while the actual force exerted the spring on the rocker arm increases, the two tendencies being approximately canceled to maintain a roughly constant force upward on the seat ends in all positions of the mechanism 10. Typically, the rear end of the seat requires a larger spring loading as it supports the largest portion of the occupant's body weight. Figure 8 illustrates the theoretical spring curves that are calculated for the seat adjustment mechanism for the front and rear ends of this mechanism, and the rear loading spring 48 is selected to oppose a weight of 45. kg and the front loading spring 44 a weight down of 14 kg. To these theoretical curves fit possible spring curves that is to say feasible in practice and the loading springs 48, 44 are wound to correspond to these possible curves. In FIG. 8, the stroke of the loading spring is indicated on the abscissa, the forced downward on the lift arm on the ordinate, a designates the theoretical curve of the rear spring to compensate for a downward force of 45 kg, the curve b the possible curve of the spring, c the theoretical curve of the front spring to compensate a force towards

  14 kg low and spring peat possible.

  In previously known mechanisms, the increase

  The result was a compromise in the choice of loading spring: if a lighter individual had to overcome the maximum spring force required to fully lower the seat, the spring force at maximum elevation the seat had to be reduced, resulting in inadequate support for heavier individuals

  tended to fall with the seat during the liberation

  In optimizing the geometry of rocker arm angles and lift arms as described herein, it is therefore possible to use a heavier loading spring 48 than was possible before, and to offer thus better support for heavier individuals when the rear lock 34 is released, without making it impossible for the lighter persons to lower the seat adjustment mechanism 10. The same optimized geometry can be achieved with results as well. improved at the front end of the mechanism 10 to obtain a substantially equal spring force opposing the weight of the occupant over the entire

  front adjustment range of the mechanism.

  A further additional feature of the new seat adjustment mechanism 10O is in the arrangement of the rear lift arms so that the seat occupant can help lower the rear end by pushing back against the seat back which is easy to get into a vehicle by sinking into the seat and pushing by the feet against the floorboard or firewall of the passenger compartment. Considerable backward force can be applied in this manner, even by a light person, whose effect is a tendency to pivot the seat around the pivoting front points 30 and to lower the rear end 20 of the mechanism 10. Figure 7b shows a diagram of force vectors illustrating a vector F4 applied to the seat back which, in automobiles, is typically at an angle of 25 °.

  compared to the vertical, resulting in a composition

  F5 which acts on the end of the rear lift arms 24 represented by the arm M1 in Figure 7b. The vector F5 is added to the force vector F1 in FIG. 7a, that is to say to the weight of the seat occupant, thereby increasing the force F2 opposing the spring loading F3 and facilitating the compression of the seat. rear spring 48. The possibility of

  providing such assistance is made possible by

  rearwardly raising the arms rearwardly upwardly from the rear shaft 42 so that the assisting force is added to the weight vector and the two vectors cooperate to rotate the

  rear rocker arm 40 against spring loading.

  This additive effect is not possible if, for example, the rear bent lever is reversed and the lift arms extend upwardly and forwardly of the rear shafts 42 and the bent lever rotates anti-clockwise. from its elevated position to its lowered position in the views of Figures 5 and 6. It should be noted that the latter arrangement would be fully operative for the depression of the rear end 20 in response to the weight of the occupant but not would not respond to the above-described assistance force applied against the file. Nevertheless, such an inverted backward lever arrangement will benefit from the optimized geometry described with reference to FIG. 7a to obtain an almost constant effective spring force transmitted to the rear end 20, and this optimization of the arm geometry is an improvement independent of rearward orientation of the rear lift arm and clockwise rotation of the rear rocker arm 40 during the lowering of the seat to add an active or assist force produced by the seat occupant to the weight of the occupant. The useful application of the occupant assistance further enhances the ability of a lightweight individual to overcome the loading of the rear spring 48 and therefore allows the use of even heavier loading springs to better compensate for weight. heavy people without

  significant disadvantage for lighter individuals.

  The mechanism 10 will normally be provided with a suitable lock or lock for adjustably locking the lower beams 12 on the mounting rails 14 to allow adjustment of the seat position forwards or backwards. along the mounting rails, for example to adjust the space between a vehicle seat and the steering wheel. A wide variety of stop mechanisms suitable for this purpose are known and should not be described here in detail.

Claims (19)

R E V E N D I C A T I 0 N S   1.- adjustment mechanism of a six-way seat, characterized in that it comprises: a base (12); lower support means (14) linearly sliding on said base; upper support means (16) having a front end and a rear end; first axially interlockable locking means (34) actively connected between the front end and the lower support, said first interlocking means being remotely released to adjust the height of the front end relative to the base; second axially interlocking locking means (35) actively connected between the rear end and the lower support, said second locking means being remotely free to adjust the height of the rear end relative to the base; and -20 a third actively locking means   connected between the first ground and the base, the said third   means being releasable to relocate, by sliding, the lower and upper support means relative to the base; thus a seat on said upper support means   can be adjusted in height, inclination   forward and backward relative to the base.   2.- Mechanism according to claim 1, characterized in that each locking means (34, 35) .is loaded by spring, by a constant rate spring means to an extended position, and further comprising lever means between the locking means and at least the rear end, said lever means being configured to transmit a roughly constant spring force to the upper support means, on   the entire compression range of said spring means.   3. Mechanism according to claim 2, characterized in that the lever means pivot on the lower support (16) at a pivot point, the lever means comprising first and second arms extending at an angle to the other radially from the pivot point, the first arm being actively connected to the second locking means, the second arm being connected to the rear end and arranged so that the combined net mechanical advantage of the first and second arms on their range of motion cancels the spring force which increases linearly while said spring is compressed so that said second arm transmits a roughly constant spring force at said rear end.   4.- mechanism according to claim 3, characterized in that the second arm (24) extends rearwardly and upwardly from the pivot point to a high position of the rear end and pivots downward in response to the rearward force applied to the back of a seat supported on the upper support means, so the rearward force is at least partially added to the weight of a seat occupant to facilitate the driving of the seat. rear end of the upper support means while the second locking means is in one condition released.   5.- Mechanism according to claim 2, characterized in that each locking means comprises a rod (43) which is fixed at one end to the base or to the lower support means, the other end of said rod being axially movable to through a housing (46), said spring means being compressed between said shaft and said housing, said housing being attached to the other of the base or lower support means, and remotely releasable means normally locking said shaft against a axial displacement to thereby secure the base relative to the lower and upper support means. 6. Mechanism according to claim 1, characterized in that it further comprises lever means connected between each of the first and second means (34,35) of   locking for a translation of an independent movement   interlocking first and second locking means in height change between each of the   front and back ends respectively and the base.   7. Mechanism according to claim 1, characterized in that the lower support means comprises two rails   parallel (14) which are connected by two trans-   parallel versaux (42, 52) journalled to the rails; rocker arm means (40) and lift arm means (24) attached to each of the shafts; each of the first and second locks being connected between the rocker arm means on one of the corresponding shafts and the lower support means; the upper support means (16) being supported between the shafts on the elevator arm means; thus a telescopic extension or contraction of the first or second locking means is transformed by rotation of a corresponding shaft in elevation or lowering of a corresponding end of the means of superior support.   8.- adjustment mechanism of a six-way seat, characterized in that it comprises: a pair of parallel rails (12) for fixing   a mounting surface of a seat; -   a pair of lower beams (14) slidable longitudinally along the rails;   a pair of parallel shafts (42, 52) perpendicular   to and interconnecting the lower beams, the shafts being journalled for rotation on the lower beams; a rocker arm (40) and a pair of axially spaced lift arms (24) extending radially from each shaft; a nestable member (34,35) connected between an outer end of each rocker arm and one of the lower beams; and seat support means supported between the outer ends of said lift arms; thus the extension or the telescopic withdrawal of   each of the nestable elements raises or lowers   one end of the support means to   swing a seat that is supported.   9. Mechanism according to claim 8, characterized in that it further comprises a removable locking means connected between at least one of said lower beams and one of said rails for positioning   selectively longitudinally said lower beams   said seat support means along said mounting rails.   10.- Mechanism according to claim 8, characterized in that the telescopable elements are loaded to an extended position by means of constant rate springs, the rocker arm and the lift arms on each shaft being dimensioned and. arranged to transmit a roughly constant spring force to the corresponding ends of the seat support means over the entire compression range of said means forming spring (44,48).   11. A mechanism according to claim 10, characterized in that the rocker arm on the rear shaft is oriented so that the rearward force applied against the back of a seat supported on said support means of the seat-cooperates with the weight of a seat occupant to overcome the spring loading for   lowering said rear end (20).   12. A mechanism according to claim 11, characterized in that the rear rocker arm is oriented towards the rear and upwards of the rear shaft   in high condition of the rear end (20).   13.- Mechanism according to claim 12, characterized in that the lift arms on the rear shaft are oriented in a direction backwards and downwards in the high condition of the end. rear (20).   14.- mechanism for adjusting a seat, characterized in that it comprises: a lower support means (14) adapted to be fixed to a mounting surface; - upper support means (16) adapted to carry a seat, having a front end and a rear end; axially telescoping locking means (34, 35) actively connected between the rear end and the lower support means, said locking means being remotely releasable to adjust the height of the rear end relative to the lower support means; and constant rate spring means loading the locking means to an extended position, and further comprising lever means between the locking means and the trailing end,   said lever means being configured to transmit   put a nearly constant force of spring to-   the rear end over the entire compression range of the spring means.   15. Mechanism according to claim 14, characterized in that the washing means (24,40) are further arranged and configured so that the rearward force applied against the back of a seat supported on the means of upper support cooperates with the weight of a seat occupant to overcome the loading by   spring to lower the rear end.   16. A mechanism according to claim 14, characterized in that the lever means pivot on the lower support means at a pivot point, the lever means comprising first and second   second arm extending at an angle to each other radially   relative to the pivot point, the first arm being actively connected to the locking means, the second arm being connected to the rear end and arranged so that the combined net mechanical advantage of the first and second arms changes between high positions. and flanged from the rear end to cancel the linearly increasing spring force while said spring is compressed so that said second arm transmits a substantially constant spring force. at said rear end (20).   17. A mechanism according to claim 14, characterized in that the second arm extends rearwardly and upwardly from the pivot point to a high position of the rear end and pivots downwardly in response to a force. rearwardly applied to the backrest of a seat supported on the upper support means, said rearward force at least partially adding to the weight of a seat occupant to facilitate the depression of the rear end of the seat. upper support means while the locking means is in released condition.   18. A mechanism for adjusting a seat, characterized in that it comprises: a lower support means (14) adapted to be fixed to a mounting surface an upper support means (16) having a front end and a rear end and capable of carrying a seat attached to it; locking means (34, 35) actively connected between the rear end and the lower support means, said locking means being remotely releasable to adjust the rear end between a raised position and a lowered position relative to said support means lower, and constant rate spring means loading the locking means to the high position, and further comprising lever means between the locking means and the rear end, said lever means being configured and arranged so that the combined net mechanical advantage of the lever means continuously changes between the raised and lowered positions to compensate for the linearly increasing spring force while said spring is compressed, and thereby to transmit a nearly constant force of spring at the rear end.   19. Mechanism according to claim 18, characterized in that the lever means are further arranged and configured so that a rearward force applied against the back of a seat supported on said upper support means cooperates with the weight of a seat occupant to overcome said nearly constant spring force to lower said rear end.