Synchro-tilt mechanism for task chairs: back-to-seat ratio, recline angles, tension range and tilt lock in the control under the seat
The control bolted under a task-chair seat · synchro ratios of 2:1 and 3:1, back and seat recline angles, manual or self-weighing tension for 50–155 kg users, tilt-lock positions and forward tilt, and the 300,000-cycle tilt test it is built to pass
The control under a task-chair seat sits between the gas lift and the seat pan and decides how the chair moves when the user leans back. In a synchro-tilt control the backrest and seat recline together at a fixed ratio, the back always opening further than the seat. Chair makers buy it as a component and specify it by that ratio, the recline angles, how tension is set for the user's weight, the lock positions, the seat fixing pattern and the durability tests it has passed.
Synchro-tilt chair controls (synchronised mechanisms) supplied as components for office and task chairs: back-to-seat ratio, backrest and seat recline angles, forward tilt, manual and weight-activated tension, user-weight range, tilt-lock positions, seat-depth sliders, seat fixing pattern, and the ANSI/BIFMA tilt-mechanism and backrest durability tests. Complete task chairs with their seat height, depth and recline requirements are covered on the Ergonomic office chair (task chair) page; gas springs on the Gas lift cylinder page; lumbar pads on the Adjustable lumbar support module page; recliner backrest struts on the Damping cylinder for reclining seat and backrest linkages page; recliner locks on the Brake mechanism for reclining seats page; and machine operator seats on the Operator seat page.
CNFX editorial research · Sources checked October 13, 2026 · 4 sources
2:1 or 3:1 synchro ratio: how far the back and the seat recline, forward tilt and lock positions
The ratio is the first decision because it sets how much the seat moves for each degree of back recline. Leggett & Platt's LP-11 and LP-11 HD are 2:1 mechanisms with 23° of backrest and 11° of seat recline, while its weight-activated LP-24 is 3:1 with 20° and 6.5°, which the maker says minimises uncomfortable knee rise. [1][2][3]
Editorial calculation: 23° / 11° is a ratio of about 2.1 and 20° / 6.5° about 3.1, so the published angles match the stated 2:1 and 3:1 ratios; at full recline the 2:1 seat drops 4.5° more than the 3:1 seat while the back opens 3° more.
All three mechanisms share a 200 × 153 mm seat fixing pattern, so one seat pan can sit on any of them; Leggett & Platt lists cable or handle activation for each, and bolt-on seat-depth adjustment so a separate sliding-seat model is not needed. [1][2][3]
Ask for the ratio, both recline angles, the lock positions and whether forward tilt is integral or an option; check that the seat pan's fixing holes match the mechanism plate.
02
Tension set by knob or by the user's weight, the weight range it covers, and the BIFMA tilt and backrest cycles
Leggett & Platt sets tension two ways. The LP-11 has a side knob that covers its full tension range in less than one turn and is described as holding constant tension through the travel, for users of 50 to 120 kg; the LP-11 HD uses the same knob with a higher range for 75 to 155 kg. [1][2] The LP-24 is self-weighing, adjusting to each user without manual tension, for 50 to 110 kg, with a factory spring setting matched to chair size and an optional fine-tuning knob. [3] All three are listed as tested to ANSI/BIFMA X5.1 and EN 1335. [1][2][3]
The webcast by Haworth and Herman Miller engineers that summarises these tests explains that X5.1 load levels were based on a 95th-percentile male of 253 lb with a 10-year life at single-shift use, that the then-coming revision would use 275 lb, and that the large-occupant standard X5.11 uses a 400 lb male and applies to seats 22 inches wide or more. [4] Editorial calculation: the LP-11's 120 kg upper limit is about 265 lb and the LP-11 HD's 155 kg about 342 lb, so the HD version covers users above the X5.1 basis but stays below the 400 lb X5.11 basis; a control for a large-occupant chair should be asked for X5.11 results.
ANSI/BIFMA X5.1 test relevant to the control
Cycles
Load shown
Tilt mechanism: fatigue and wear from repeated tilting
Request the test report rather than a compliance line: standard edition, which tests the mechanism was fitted for, the tilt-cycle load and count, and the seat and back used on the test chair.
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Data for a synchro-tilt chair mechanism
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Movement: [synchro ratio; backrest and seat recline angles; forward tilt]
Tension: [knob or weight-activated; user weight range in kg]
Locking: [number of lock positions; anti-kickback]
Seat: [fixing pattern; seat-depth slider travel]
Controls: [handle or cable; custom levers and covers]
Interfaces: [gas-lift taper; back attachment]
Testing: [ANSI/BIFMA X5.1 or X5.11; EN 1335; test reports]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
What does a 2:1 synchro ratio mean on a chair mechanism?
The backrest reclines about twice as far as the seat: Leggett & Platt's 2:1 LP-11 gives 23° at the back and 11° at the seat, while its 3:1 LP-24 gives 20° and 6.5°. [1][3]
How is tilt tension matched to the user's weight?
Either with a knob, which on the LP-11 covers 50 to 120 kg in under one turn, or with a weight-activated mechanism such as the LP-24 that adjusts itself for users of 50 to 110 kg. [1][3]
What durability test does a chair tilt mechanism have to pass?
The ANSI/BIFMA X5.1 tilt mechanism test runs 300,000 tilting cycles with a 109 kg (240 lb) load, alongside a 120,000-cycle backrest test. [4]
Which mechanism suits heavier users?
Leggett & Platt's LP-11 HD keeps the 2:1 movement with a higher tension range for 75 to 155 kg; chairs for occupants up to 400 lb are tested to ANSI/BIFMA X5.11, which needs a seat at least 22 inches wide. [2][4]
Sources and what still needs a direct check
Sources checked 2026-10-13. Three Leggett & Platt mechanism specification pages (LP-11, LP-11 HD and LP-24) and an IISE webcast on office seating standards given by engineers from Haworth and Herman Miller.
