This article attempts to explain the most common method of mounting rubber tyres on radio controlled model cars.
The information in this article is based upon my knowledge of the large scale wheels & tyres and the images used throughout the first part of this article are ones based on large scale wheels and tyres, but they are identical in their basic design principles to most of the other scales’ wheels & tyres.
Remote controlled model car wheels and tyres come in many shapes and sizes, but the majority of moulded rubber tyres use a wheel similar to figure 2 that has a moulded channel in it to accept a tyre that has a lip section moulded onto the bead area. Obviously there are differences in the proportions of the wheels and tyres, but they all share the same basic design principle in the mounting of the wheel.
Although the tyre can be mounted onto the wheel by sliding and/or stretching just the tyre over the wheel and then gluing the lip on the tyre into the channel in the wheel, usually a foam liner is used on the on-road cars.
As model cars don’t currently use true pneumatic1 tyres, foam liners are used in between the wheels and tyres to simulate the stiffness that air pressure gives in their full size counterparts. Liners are usually available in different hardnesses, (which can be quoted with a Shore rating which allows you to alter the tyres handling characteristics. They can be either glued to the wheel or the tyre and are supplied in two different formats which are discussed below.
The tyre liner can be supplied in the form of a ring of foam (figure 3), but are sometimes referred to as donuts.
Ring type liners have the advantage of being cut to size and therefore are easy to fit. The disadvantage of this type of liner is that the corners (both inside and out) are not shaped and will usually require trimming so that the tyre is not deformed and/or the hardness of the foam does not increase as it is forced into the corners.
Alternatively liners are supplied as a strip of foam (figure 4) and have the advantage of being machined to fit the outer profile of the wheel and the inner profile of the tyre.
The big disadvantage the strip type foam liner has is that you have to join the foam into a ring, which can create problems due to the type of adhesive used to join the foam.
The ideal adhesive to use to joint the foam is one that does not alter the density of the foam. If an adhesive such as cyanoacrylate is used then the foam in that area becomes almost solid. The rotating tyre will then have a hard spot, which will make the wheel/tyre bounce, as well as creating heat build up in that area of the tyre. In extreme circumstances this heat build up can lead to the failure of the tyre. Also inexperienced users sometimes have problems cutting the liner to size and can finish up with a pair of tyres where one is effectively harder than the other, as the foam has been cut to different lengths in each tyre.
You can clearly see the benefits of a properly fitting liner in the cross-section of the wheel and tyre assembly in figure 5.
This is especially true when a particularly hard grade of foam is used to make the liners.
Plastic wheels, like other parts of a model car, are injection moulded and certain chemicals are used as release agents to help in removing the moulded wheel from the die that is used to form the shape of the wheel. As these chemicals are designed to stop the plastic sticking to the die, they will also to a certain extent, stop the tyre from sticking to the wheel.
Therefore either a solvent or an abrasive is is required to remove these chemicals so that the glue can perform as expected.
Tyres can normally be fitted with your bare hands, especially if some form of lubricant is used to ease the tyre over the foam. The large scale tyres are somewhat stiffer and can more easily be fitted using a dedicated machine.
The fitting machine shown in figure 6 was made by KP Designs, who were the first to design and manufacture this type of machine for the large scale cars.
The KP tyre fitting machine is easy to use and involves placing the wheel (with foam glued in place) on the main post and then stretching the tyre over the top of the the wheel and the outside of the metal spike. The tyre is then rotated as it is pressed down to ease the tyre fully over the foam liner and into place. The tyre can then be glued up ready for use.
1 This would be more of a sales gimmick because although pneumatic tyres offer the advantage of consistent hardness around the circumference, there is much variation in air pressure due to temperature changes. With full-size real vehicles this is not ordinarily a problem, but with racing cars it is — the tyres could explode in competition use. There are also the problems caused by punctures and slow leaks.
First written around 2001.
Added note about pneumatic tyres 4th of February 2009.