Bridgestone South Africa

Rims, Tubes and Flaps


Flatbase type

Rims, Tubes and Flaps - 1.gifAs the name suggests this type of rim has a flat rim base (not 5" tapered). If there is a letter after the sectional width, this indicates the height and contour of the flange (dimension F), e.g. a conventional flat base rim 6.0" wide between the flanges and 20" diameter at the bead seat, having 1.50" flange height is indicated as 6.00T x 20.

Divided type

Rims, Tubes and Flaps - 2.gif(This can be in wide base or flat base configuration)

Divided type wheels are used for certain commercial vehicles and industrial trucks. It is possible for the rim contour to be of the flat base variety, but the updated design incorporates the 5" taper bead seat. Tyre fitting and removal are carried out by dividing the two halves of the wheel (which are bolted together to form a complete unit). This is limited to certain sizes and applications where tyres are used only in angle formation on each hub.

Rims, Tubes and Flaps - 3.gif

Before tyres are inflated, the clamping nuts should be tightened. When dismantling the assembly, the tyre must be completely deflated before removing The wheel and clamping nuts, otherwise the wheel halves will blow apart as the nuts are slackened.

Rim width

This measurement is taken between the vertical inside surfaces of the two flanges. Tyres must be mounted only on rims with the correct width.

Rim offset

When tyres are fitted as duals, it is vital that the correct offset wheels are used to provide an adequate gap between the inner sidewalls. With too little spacing tyres will rub, the air flow will be insufficient and consequently heat buildup will be excessive. With too much spacing, unequal load distribution on uneven road surfaces will be increased and dragging and scuffing on short turns will be exaggerated.

Rim maintenance

Bead seats that are dirty and rusty can cause bead failure as a result of excessive tyre chafing. The task of demounting a tyre then becomes difficult.

As dirt build-up interferes with the proper fit, the chances of lock rings flying off dangerously are increased.

Rims should always be cleaned with a wire brush each time they are stripped. If there is evidence of rust, it is good practice to paint the rims well after brushing. If a number of different rims are being used, make sure that each type can be clearly identified and thus avoid the great danger of mis-fitting lock rings and flanges. Tyres must be inflated in a safety cage.

Types of wheels

There are three main types of rims used when disc wheels for commercial vehicles are manufactured. Wheel types are identified from these rim descriptions, viz:

1. Wide base
2. FIat base
3. Semi-drop centre

Rims are always identified by the width between flanges and the nominal diameter of the rim which is measured at the tyre bead seat. Most commercial vehicle wheels are either of two, three- or four-piece construction, with loose flanges and lock rings on one side.

Wide base type

A great number of the wheels manufactured today are the wide base type, all incorporating 5% +1% taper bead seat. Before the description, The letter B is written, dimensioned to one decimal place, e.g. B6.0x20, B6,5x20.

Semi drop centre type

Semi-drop centre rim or S.D.C., is intermediate between the well base types and the commercial hide flat and wide base types. Tyres meant for S.D.C. rims and well base rims should not be fitted to flat base rims.

Drop centre type

This is used for tubeless truck tyres and is a one-piece wheel with a well in the centre. Low flanges make fitting and removal easier and the bead seats have a 15% taper to guarantee an airtight seal.


Tyre maintenance personnel would do well not to neglect this aspect. If not looked after, flaps can use major breakdowns. An old flap must never put in a new tyre. Flaps showing distortion shound the valve hole and along the rim slot must be scrapped. Check that there are no signs of cracking along the bead toe curve.

Use an inexpensive bridge plate, especially where excessive brake drum heat occurs. Offcuts of old flaps may be used to make bridge plates.

Besides preventing "pooching" through the valve slot in the rims, they will also serve to hold the valve stem straight and thus facilitate pressure maintenance on inner duals.


Costly lost hours resulting from 'punctures' and be failures caused by incorrect inflation and mounting, can be avoided by following these procedures:

  • Dust the inside of the tyre or the tube with French chalk or other lubricating powder.
  • Insert tube into tyre and inflate just sufficiently to shape up.
  • Rims, Tubes and Flaps - 4.gif With a swab or paint brush, apply a solution of acceptable tyre-fitting lubricant to the bead bases and the accessible part of the flap. The solution must not run onto the tyre. (See Figures below illustrating how important it is to allow the tube to move independently of tyre and flap in order to prevent stretch being localised at the tube base.) When fitting the flap, ensure that the 'wings' are neither folded nor creased. Fitting a light metal 'bridge plate' this stage will ensure well positioned valve stems.
    Pressure control is thus facilitated and the flap will not 'pooch' through the valve slot in the rim.
    Such ordinary inexpensive accessories (round plates with a diameter of about 80mm with a hole to take valve stem) are more and more easy to obtain and are strongly recommended.
  • Rims, Tubes and Flaps - 5.gif Centering the valve, mount the tyre on the rim and hold the valve firmly against the rim while flating, until beads are correctly located.
  • As soon as the beads have moved into position, allow the tube to deflate completely. This very important step will prevent the creased tubes that are commonly mistaken by operators for over-large tubes.
  • Re-inflate to the correct pressure.

Tube breaks in valve area.

Rims, Tubes and Flaps - 6.gif

If inflation procedure is not followed correctly, the result could be a circumferential crack or a tear in the valve base at the edge of the bridge washer. In figure 1 we have the position of beads, and tube at inflation pressure of 50 kPa.

Although the tube is fully rounded inside the tyre, the pressure is not enough to move the beads on wide base rims. Figure 2 shows the beads being pushed onto the tapered bead seats. To do This, between 140 and 280 kPa pressure is needed, depending on tyre size and condition of rim.

Because of the force exerted by this high pressure, the tube can stretch only in the rim area. Resistance to stretch being greatest at the valve base, the tension there is often sufficient to crack the tube at the edge of valve base.

Do's and don'ts

DO examine used tyres before re-using.

DON'T allow an unskilled person to do this as he won't recognize the signs of damage caused by heat creasing and too much wear, etc.

DON'T allow personnel to over-inflate a new tube to twice its original size to 'check for holes'. The result of this will be thin spots and creasing caused by stretching.

DO always fit new valve cores.

DON'T allow grit or dust from dirty working area to enter loose tyres, resulting in slow leaks.

DO check that the valve is clean before inflating. To do this, depress the valve core and blast out any dust which might otherwise cause a slow leak.


The danger of working on loose flange rims has resulted often in serious injury or death in the past. We all know the necessary safety precautions and it is up to each of us to make sure that these are carried out in our tyre bays.

Bridgestone South Africa

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