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Inner CV boot in polyurethane by PolyPro

Inner CV boot in polyurethane

An inner CV boot is the bellows cover that seals the tripod joint on the gearbox side of the driveshaft and holds its grease in place. It is larger in diameter than the outer boot, carries more grease and works at a small angle, but it has to absorb the plunging movement of the shaft as the suspension travels. PolyPro moulds its polyurethane boots in-house; related groups are listed in the catalogue.

 

What an inner CV boot is and how it works

The inner joint is a tripod: a three-legged spider with rollers that slides inside a tulip housing. This design transmits drive and at the same time lets the shaft move in and out of the housing, taking up the change in distance between the gearbox and the wheel as the suspension moves. The boot closes the open end of the tulip and travels with the shaft.

That dictates its shape: a large diameter on the housing, a small one on the shaft, a longer convoluted section and noticeably more grease inside than an outer boot holds. The working angle here is smaller, so the material bends less sharply and an inner boot splits less often than an outer one. On the other hand it sits close to the engine and the exhaust, so it ages from heat, and it also suffers from oil leaking past the driveshaft seal.

 

Signs of inner CV boot wear

The inner boot is harder to see than the outer one, hidden behind the subframe and the undertray. Look for the symptoms around the joint rather than on the boot itself: grease flung outwards by centrifugal force onto the gearbox casing, the subframe and the shield.

  • a fan of grease on the subframe, gearbox casing and engine undertray
  • cracks or a split in the folds on the gearbox side of the shaft
  • a loose or broken clamp on the large sealing band of the boot
  • vibration under acceleration that disappears when you lift off
  • shudder or a pulsing felt through the body at steady speed
  • traces of engine oil on the boot next to the driveshaft seal

Clicking on a turn points to the outer joint; the inner tripod answers with vibration under load. Once that vibration is there, a new boot alone is not enough: the joint is dismantled, the rollers and tulip tracks inspected and the joint replaced if worn.

 

Polyurethane or rubber

A rubber boot is softer, but in the inner joint position it works next to a hot exhaust and in constant contact with road chemicals. Heat and ozone harden the rubber and the folds crack along their tops, and near the driveshaft seal oil gets onto it as well. Polyurethane stands up better to oils, salt and abrasion, so it keeps the geometry of the folds for longer.

For an inner boot the sealing bands are the critical part: the diameter is larger, so the clamp load is higher. Polyurethane holds the shape of the band and stops it working loose in its groove. The material is denser, so the size has to be matched exactly and the clamps tightened without overtightening. The Shore hardness of every item is given on its product page.

 

How to choose an inner CV boot

Selection goes by the dimensions of the seats and the type of joint, not by the car badge. You will need the removed driveshaft or the old boot with its grooves measured.

By Joint Type

First check what is actually fitted on the gearbox side. The common design is a tripod in a tulip housing, and the boot then has a large cylindrical band that seats on the end of that housing. A closed-type inner ball joint is less common; its seat is different and the boot is shorter. The shaft may be retained by a circlip or by a press fit, which decides how the assembly comes apart. The outer joint has its own geometry, so an outer CV boot will not fit here. Check as well whether the shaft carries a dust shield or an intermediate bearing support, because both limit the length available for the boot on the gearbox side.

By Dimensions

Next take three measurements: the large band diameter on the joint housing, the small band diameter on the shaft, and the length of the boot in the working position. Inner boots are longer than outer ones, and it is that length which decides whether there is enough travel when the suspension drops into rebound. Measure the old part at its grooves with a caliper instead of estimating from the housing. If the boot is torn beyond recognition, send dimensions and photos through the boot selection form. The small band often seats not on a plain part of the shaft but on splines with a groove for the clamp, so measure that diameter separately.

By Durometer

An inner boot works at a small angle but with long plunging travel and a larger mass of grease inside it. A softer material folds more easily and loads the convolutions less when the shaft plunges out. A harder material resists rubbing against the undertray and the heat coming off the exhaust better, but it needs a more accurate fit of the sealing band. The Shore value of each item is stated on its product page, so choose by that figure and by the conditions: city, highway, unpaved roads or daily work under load. On a large diameter a boot that is too soft can balloon under grease at high revs, while too hard a grade folds poorly in the cold.

By Kit Content And Fitting

Replacing an inner boot almost always means removing the driveshaft: the shaft comes out, the joint is dismantled, the rollers and tracks are washed and fresh grease is packed in. So check first whether the item includes clamps and a portion of grease — the kit content is stated on the product page. A split boot is used when the shaft cannot be removed, but its seam needs a careful clamp. The tripod circlip and the driveshaft seal are checked at the same stage, since there is no sense in stripping the assembly twice, and clamps are always fitted new. Other sizes and groups are collected in the dust boots section.

Why buy an inner CV boot from PolyPro — retail and wholesale

PolyPro is the manufacturer: our own tooling, our own moulding and dimensional control of the sealing bands, which is what decides whether a clamp holds the large diameter of an inner boot. We sell without intermediaries and retail by the piece, so you can order one boot for one driveshaft. If a size is not in the production range, we make the boot to a drawing, a sample or your worn part, again starting from a single piece.

Wholesale customers are workshops, auto parts shops, fleets and resellers. The partner programme gives bonuses for turnover or for the size of a single order, access to prices and stock levels, and shipping under the partner's own brand. Our products carry a warranty, and we ship within Ukraine and internationally.

 

Frequently asked questions

Why does an inner boot split less often than an outer one? 

The inner tripod works at a much smaller angle: it mostly takes up the plunging movement of the shaft rather than the steering angle of the wheel. The folds bend more gently, so the material fatigues more slowly. An outer boot folds almost double at full lock and also catches stones thrown up by the wheel.

How do I tell an inner joint fault from an outer one? 

An outer joint clicks when the car is driven in a circle at full steering lock. An inner tripod produces vibration or shudder under acceleration in a straight line that disappears as soon as you lift off the throttle. A lift gives the answer: look for thrown grease and check each joint separately for play.

Can an inner boot be replaced without removing the driveshaft? 

Not a one-piece boot: it has to be slid over the shaft, and on the gearbox side the joint housing is in the way. The shaft is taken out, the joint dismantled and washed. Only a split universal boot goes on without removal, and that is a temporary measure whose life depends on the quality of the seam.

How much grease does an inner joint need? 

An inner joint takes more grease than an outer one because of the volume of the tulip housing. Use the portion supplied with the item or the driveshaft manufacturer's figure. The grease must be a CV joint grease, and the old grease is washed out completely together with the grit, otherwise the new one starts work already contaminated.

Why is there engine oil on the boot? 

Usually the driveshaft seal in the gearbox is leaking: oil reaches the large band of the boot and spreads over the folds. Replacing the boot alone will not help, the seal has to be renewed first. Polyurethane resists oils better than rubber, but prolonged contact with oil shortens the life of any boot.

Will a universal split boot fit an inner joint? 

It will, provided the band diameters and the working length match. Keep in mind that an inner boot is larger and holds more grease, so the seam is loaded more heavily than on an outer boot. It has to be clamped evenly and without distortion, with both sealing bands sitting exactly in their grooves.

Can I buy a single inner CV boot? 

Yes, retail is by the piece: order one boot for one side of the car, with no compulsory axle set. If your size is not in the production range, we will make the boot to a drawing, a sample or your worn part, again from a single piece, and ship it within Ukraine or abroad.

What are the terms for workshops and parts shops? 

Wholesale buyers get access to current prices and stock levels, regular shipments and the option of supply under their own brand. Bonuses are credited for turnover or for the volume of a single order — the conditions are described in the partner programme section.