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Polyurethane Differential Bushing (Differential Mount) by PolyPro

Polyurethane Differential Bushing (Differential Mount)

A differential bushing is the elastic support through which the final drive housing or the rear axle beam is attached to the body, the subframe or the arms. It keeps the unit in its designed position, takes the shock loads that come with every change of drive and stops driveline noise from reaching the cabin. PolyPro casts these mounts from polyurethane in its own moulds; the other mounts and bushings are listed in the catalogue.

 

What a Differential Bushing Is and How It Works

The final drive takes torque from the propshaft and splits it between the halfshafts. The reaction to that torque tries to rotate the housing, so the differential is never bolted rigidly to the body or subframe but carried on elastic mounts. An independent rear suspension normally uses two or three of them: a front nose bushing and the rear fixings; a live axle carries the mount where the beam or the trailing link is attached.

The mount does two jobs at once. First, it limits rotation and fore-aft movement of the housing so that the angles of the universal joints and the driveshafts stay within range, which is what decides whether vibration appears at speed. Second, it absorbs the whine of the hypoid gear set and the shock that arrives when you lift off and get back on the throttle, as the driveline slack is taken up.

 

Signs of a Worn Rear Differential Mount

A differential mount is shock-loaded, so it does not wear evenly but in steps — after towing, hard launches or a stretch of gravel road. The first symptoms are easy to confuse with play in the propshaft or a worn centre support bearing.

  • a dull thump from the rear when pulling away and when lifting off
  • driveline whine and vibration that build up with speed
  • a shake that appears at a certain speed and disappears when coasting
  • knocking over bumps and when the rear axle droops
  • visible displacement of the differential housing relative to the bracket
  • a torn elastic body, cracks and traces of oil on the mount

Checking is simple: put the car on a lift and load the housing briefly with a pry bar, lengthways and sideways. If the unit moves noticeably and the metal pulls out of the elastic body, the mount has to be replaced.

 

Polyurethane or Rubber for a Differential Mount

A rubber differential mount damps noise well, but it is the first thing to give up under shock loading: the body tears along the bond line with the metal shell and the housing starts to move in its bracket. Oil makes it worse — a weeping pinion seal destroys rubber faster than mileage ever does.

Polyurethane copes better with shock and cyclic loads, is not attacked by gear oil or road salt and barely sags, so propshaft and driveshaft angles stay within their design range. Axle noise does become slightly more audible than with fresh rubber, which is the price of holding the unit more firmly. Polyurethane also copes more calmly with the heat of an exhaust line, which often runs right beside the mount. The Shore hardness of each grade is stated on the product page.

 

How to Choose a Differential Bushing

Differential mounts differ in seat, direction of load and hardness. The model name alone is not enough: check the dimensions, the mounting type and the conditions the rear end actually works in.

By Seat and Mounting Type

First decide which mount you need: the front nose bushing of the housing, the rear differential fixings or the axle beam mount. Then measure the old part — outer diameter of the shell, height, inner sleeve bore for the bolt, and whether it has a flange or a shoulder. Some mounts are pressed into the bracket, others are clamped by a bolt between plates, and the two are not interchangeable. If the old part has disintegrated, work from the bracket seat and the length of the fixing bolt instead. Inspect the bracket itself as well: a worn seat or elongated holes will not let the new mount sit square, and its life will drop from the very first drive.

By Durometer

Hardness is chosen to match how the rear end is worked. For ordinary passenger use a grade close to stock is enough: it damps the whine and keeps the driveline quiet. For a heavy vehicle, trailer towing, regular gravel roads or an engine with high torque, a firmer polyurethane makes more sense — the housing rotates less and the shock on every change of drive is weaker. The other side of a firm grade is a slightly louder driveline at low speed, which is why a purely urban car is not pushed into extra hardness. The Shore value is given on the page of each product.

By Operating Conditions

A city car on asphalt and a vehicle that regularly runs on gravel or carries loads work the differential mount in completely different ways. The second case adds impacts when the axle droops and running at large propshaft angles, so the life of a soft mount is noticeably shorter. Check the neighbouring parts as well: a worn centre support bearing, a tired universal joint or play in the pinion will destroy a new mount within a short time. If the old mount failed together with a weeping seal, cure the leak first: polyurethane tolerates oil, but the unit itself gains nothing from working in it.

Why Buy a Differential Bushing from PolyPro — Retail and Wholesale

PolyPro is a Ukrainian manufacturer of polyurethane car parts. We cast differential mounts in our own moulds, check the dimensions of every batch and sell without middlemen, so the price carries no resale margin and technical questions are answered by the people who made the part. Retail is sold per piece — take a single bushing or a set for the whole unit. If the position is not in the catalogue, we make a custom one from a drawing, a sample or the worn part, starting from a single piece.

Wholesale runs directly from production to workshops, parts shops, fleets and resellers: the partner programme gives bonuses for turnover or for the volume of a single order, access to prices and stock levels, and shipping under the partner's own brand. The products are covered by a warranty, and international shipping is available.

 

Frequently Asked Questions

How does a differential bushing differ from a transmission mount? 

A gearbox mount works mostly in torsion under the reaction torque of the engine, while a differential mount takes shock loads from the driveline and vibration from the rear axle, so it has a different profile and is usually firmer. The parts are not interchangeable. Our transmission mounts are a separate product line.

The driveline whines — is it the differential or the mount? 

A worn mount does not create the whine itself, but it stops damping it, so the noise of the hypoid gear set reaches the cabin. Telling them apart is easy: whine from the differential changes with load and coasting and usually has a metallic edge, while a failed mount shows itself on a lift, with the housing moving visibly in its bracket.

Can only one of several mounts be replaced? 

Technically yes, but the remaining mounts have covered the same mileage under the same loads. If one differential bushing has failed, the neighbouring ones almost always show cracks or permanent deformation, and a new part next to a worn one takes more of the shock. In practice it pays to replace all mounts of the unit at once.

Which Shore hardness should I choose? 

For ordinary passenger use take a grade close to stock: it damps axle noise and keeps the driveline quiet. For a heavy vehicle, towing, gravel roads or increased torque, choose a firmer polyurethane. The exact Shore value is stated on the page of each product.

Can a worn differential mount cause propshaft vibration? 

Yes. When the mount sags or tears, the housing shifts, the angle between the differential flange and the propshaft changes and a shake appears at a certain speed. Check the universal joints and the centre support bearing together with the mount: these parts load one another and tend to fail in pairs.

How long does a polyurethane differential mount last? 

Polyurethane does not age from ozone and does not delaminate when gear oil reaches it, so it lasts longer than rubber under the same conditions. The exact life depends on the load: towing, gravel roads and axle droop shorten it. Life also drops if slack is left elsewhere in the driveline, because the new mount takes those impacts.

Can I buy a single differential bushing at retail? 

Yes, retail is per piece: order one mount or a full set for the unit, with no minimum batch. Catalogue positions ship from stock, custom parts ship once they are made from a drawing, a sample or the worn part. International shipping is available and the products are covered by a warranty.

What are the terms for workshops, shops and fleets? 

Workshops, parts shops, fleets and resellers are served by wholesale directly from production. The partner programme includes bonuses for turnover or for the volume of a single order, access to current prices and stock levels, and shipping under the partner's own brand. Range and volumes are agreed individually.