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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Control arm</span></span>
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</style><div role="note" class="hatnote navigation-not-searchable">For the control arm in a <a href="Clinical_trial" title="Clinical trial">clinical trial</a> or other scientific experiment, see <a href="Scientific_control" title="Scientific control">Scientific control</a>.</div>
<p>In <a href="Suspension_(vehicle)" class="mw-redirect" title="Suspension (vehicle)">automotive suspension</a>, a <b>control arm</b>, also known as an <b>A-arm</b>, is a hinged <a href="Suspension_link" title="Suspension link">suspension link</a> between the <a href="Chassis" title="Chassis">chassis</a> and the suspension upright or <a href="Wheel_hub_assembly" title="Wheel hub assembly">hub</a> that carries the wheel. In simple terms, it governs a wheel's vertical travel, allowing it to move up or down when driving over bumps, into potholes, or otherwise reacting to the irregularities of a road surface. Most control arms form the lower link of a suspension. Control arms play a crucial role in the suspension system of a vehicle. They help to keep the wheels aligned and maintain proper tire contact with the road, which is essential for safety and stability.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>
</p><p>The inboard (chassis) end of a control arm is attached by a single pivot, usually a rubber <a href="Bushing_(isolator)" title="Bushing (isolator)">bushing</a>. It can thus control the position of the outboard end in only a single <a href="Degrees_of_freedom_(mechanics)" title="Degrees of freedom (mechanics)">degree of freedom</a>, maintaining the radial distance from the inboard mount. Although not deliberately free to move, the single bushing does not control the arm from moving back and forth; this motion is constrained by a separate link or <a href="Radius_rod" title="Radius rod">radius rod</a>.<sup id="cite_ref-Hillier,_IFS_2-0" class="reference"><a href="#cite_note-Hillier,_IFS-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>
</p><p>This is in contrast to the <a href="Double_wishbone_suspension" title="Double wishbone suspension">wishbone</a>, which are triangular and have two widely spaced inboard bearings. These constrain the outboard end of the wishbone from moving back and forth, controlling two degrees of freedom, and without requiring additional links. Certain vehicles — notably, many <a href="Honda" title="Honda">Honda</a> products from the 1990s -- feature what's known as a <a href="Double_wishbone_suspension" title="Double wishbone suspension">double wishbone suspension</a>. A double wishbone design features both upper and lower control arms that work in tandem with each other to properly locate the wheel. The additional radius rod is then attached to the upper arm.
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<div class="mw-heading mw-heading2"><h2 id="MacPherson_strut">MacPherson strut</h2></div>
<div role="note" class="hatnote navigation-not-searchable">Further information: <a href="MacPherson_strut" title="MacPherson strut">MacPherson strut</a></div>
<p>Control arms are most commonly encountered as part of the MacPherson strut <a href="Independent_front_suspension" class="mw-redirect" title="Independent front suspension">independent front suspension</a>. The control arms are <a href="Perpendicular" title="Perpendicular">perpendicular</a> to the axis of the vehicle and are termed <i><a href="Axle_track" title="Axle track">track</a> control arms</i>. A diagonal <i>radius rod</i> constrains the strut from moving forward and back.<sup id="cite_ref-Hillier,_IFS_2-1" class="reference"><a href="#cite_note-Hillier,_IFS-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Hillier,_Strut_IRS_3-0" class="reference"><a href="#cite_note-Hillier,_Strut_IRS-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>
</p><p>In MacPherson's original design, an <a href="Anti-roll_bar" title="Anti-roll bar">anti-roll bar</a> also acted as the radius rod. This requires the bar to be attached through a ball joint, so as to also provide longitudinal control. In most contemporary designs, still commonly termed MacPherson struts, the radius rod and anti-roll bar are now separate, with the anti-roll bar mounted in a sliding bush.
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<div class="mw-heading mw-heading2"><h2 id="Spring_attachment">Spring attachment</h2></div>
<p>A control arm may be used to carry the suspension load and transmit them to the spring or <a href="Shock_absorber" title="Shock absorber">shock absorber</a>.<sup id="cite_ref-Hillier,_IRS_4-0" class="reference"><a href="#cite_note-Hillier,_IRS-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> <a href="Torsion_bar_suspension" title="Torsion bar suspension">Torsion bar suspension</a> commonly does this, with the outboard end of the <a href="Torsion_bar" class="mw-redirect" title="Torsion bar">torsion bar</a> attached to the inboard bearing of the control arm.
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<div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div>
<ul><li><a href="Double_wishbone_suspension" title="Double wishbone suspension">Double wishbone suspension</a></li>
<li><a href="List_of_auto_parts" title="List of auto parts">List of auto parts</a></li></ul>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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<li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">
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</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.mtcparts.com/what-does-a-control-arm-do/">"What Does a Control Arm Do?"</a>. <i>MTC Parts</i>. 12 January 2023<span class="reference-accessdate">. Retrieved <span class="nowrap">2024-09-23</span></span>.</cite></span>
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<li id="cite_note-Hillier,_IFS-2"><span class="mw-cite-backlink">^ <a href="#cite_ref-Hillier,_IFS_2-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Hillier,_IFS_2-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><cite id="CITEREFHillier1991" class="citation book cs1">V.A.W., Hillier (1991). "60: Independent Front Suspension". <i>Fundamentals of Motor Vehicle Technology</i> (4th ed.). Stanley Thornes. pp. <span class="nowrap">368–</span>369. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>0-7487-05317</bdi>.</cite></span>
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<li id="cite_note-Hillier,_Strut_IRS-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-Hillier,_Strut_IRS_3-0">^</a></b></span> <span class="reference-text"><a href="#CITEREFHillier1991">Hillier 1991</a>, pp. 373–374</span>
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<li id="cite_note-Hillier,_IRS-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-Hillier,_IRS_4-0">^</a></b></span> <span class="reference-text"><a href="#CITEREFHillier1991">Hillier 1991</a>, pp. 371–372</span>
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