Eric Worpe delivered a superb presentation at the MGCC ‘T’ Register’s ‘Rebuild’ seminar in March 2013. Eric used flip charts to aid his presentation and I have been working with him to ‘flesh out’ the flip chart notes to produce a series of articles for inclusion in TTT 2.
Eric divided up his presentation into seven headings which he termed as “Seven Deadly Sins”. We have so far covered the first four ‘Deadly Sins’ i.e.
CHASSIS – is it true? – Issue 19 (August 2013).
FRONT AXLE GEOMETRY – Issue 20 (October2013)
FRONT SPRINGS – Issue 21 (December 2013)
KING PINS – Issue 22 (February 2014)
TRACK ROD AND DRAG LINK ENDS – Issue 23 (April 2014)
We skipped an Issue, so there was nothing in Issue 24, but we return to the fray in Issue 25. In this issue we’ll look in depth at the sixth ‘Deadly Sin’: TYRES AND TRACKING. Over to Eric……………..
“With most TA/TB/TCs covering less than 2,000 miles per year, the length of time taken before the tread wears down to the legal limit probably exceeds the safe life expectancy of the tyres. How long that is depends on variables such as driving style, pressure, exposure to sunlight especially UV, temperature, road surfaces and even exposure to ozone from arc welding.
As the rubber compound hardens with age, the tyre’s adhesion qualities deteriorate, especially in the wet. So, if you ever find the rear of your car overtaking you on a roundabout, it’s time to consider new tyres. Look for fine cracks on the side walls, especially if the tyres have been run on low pressures, which can result in “fatigue” of the sidewalls, a particularly dangerous situation that can lead to “blow-outs”.
Check the date code on the side wall, which since the year 2,000 should have four digits indicating the week and year of manufacture, i.e. 2604 would be week 26 in 2004.
If you’ve ever wondered about how tyre pressures are determined, you are not alone. Minimising tread wear over the whole width of the tyre would seem to be a good starting point and one simple to examine. However, other aspects such as road grip, ride comfort and even lighter steering have potential influence.
In the case of the TA/TB/TC models, tyres make a significant contribution to absorbing road shocks given the stiff suspension set up. This feature is somewhat challenged by those who increase tyre pressure to reduce steering effort, although I’ve heard of one dealer in Excelsior tyres advising a front wheel pressure of 35 psi compared with a figure of 24 psi recommended in the “Brown Book” for the Dunlop B5s.
I would be concerned at increasing the tyre pressure as this might not only lead to premature wear of the tread’s centre section but also put additional strain on the suspension components, in particular the stub axle spindles. But then you have renewed these, haven’t you?
16 inch rear wheels were a Factory endorsed modification and gave the advantages of greater tyre cross sectional areas on stronger wheels, giving a softer ride.
There’s a surprisingly wide choice of 19 inch tyres, all at quite high prices compared with regular modern tyres. The table which follows shows some of the choices available, together with tyre dimensions and an approximate guide to cost excluding VAT.
Dunlop B5 tyres were fitted originally, although Blockley and Excelsior Competition H both have a pre-war tread pattern, which some claim improves the handling of road irregularities. Softer composition rubber gives better road holding especially in the wet, but at the expense of wear.
Avoid the cheaper plastic-like inner tubes as these rupture, good quality rubber inner-tubes are likely to be between £15 and £25 and should have an offset valve stem.
Ed’s note: The reference to Factory endorsement applies (I think) only to the TC. I’ve been unable to find any evidence of 16 inch rears being fitted in the period when the TA and TB models were in production but I stand to be corrected.
The TA ‘Cream Crackers’ trials cars were initially fitted with knobbly tyres on the rear wheels to help with grip but I think these were on 19 inch wheels not 16s. In any case, these were banned in 1937, probably due to the success of the TA in trials.
The Dunlop B5 may not have been introduced until the post-war period. The tyre on the front of the TA sales brochure doesn’t look like the B5.
Setting up a static toe-in compensates for the tendency of the front wheels to splay outwards due to the wheel being offset from the king- pin’s centre of rotation (Fig. 1).
The splaying-out forces are generated under forward movement conditions and increase due to road resistance, speed and when braking (Fig. 2).
As the front wheels splay outwards, they compress the pre- loaded springs in the track-rod ends (Fig. 3); this and various deflections in the linkages are taken into account when specifying the toe-in such that the wheels end up running in parallel, which improves straight line stability.
Fig. 1 – Wheels offset from king pins
Fig. 2 – Wheels splay outwards as vehicle moves forward
Fig. 3 – Springs in track rod ends compress and then limit amount wheels splay out
The most obvious sign of incorrect toe-in can be seen on the front tyres as they will be forced to “scrub sideways”, a misalignment of just 1/8 inch in toe-in can result in an equivalent wear due to scrubbing the tyres sideways over 17 feet for every mile travelled. (yes, I know I should get out more). The effect of scrubbing the tyre sideways is to produce a “feathered” edge to the tread blocks (fig. 4). In severe cases, one edge of the tyre can also become worn down (fig. 5). However, this effect can also be due to excessive camber angles. A modest level of tracking error can take many miles before any “feathering” indication becomes apparent, so some vigilance is needed.
“Feather” edging can also be due to the effects of rear wheel steering. Any misalignment of the back- axle sets up a tendency to “crab” which is automatically compensated for in steering the vehicle. Regular checks for loose clamp bolts between the spring plates and the back-axle bracket are well advised. Also check the distance between the spring’s front eye and axle. The locating dimples/nodules in the springs may not be positioned equally due to odd springs or the nodules may have worn down and slipped out of their locating dimples. Do consider regularly checking the various other suspension fixings such as shock absorber fixtures, back-plate mounting bolts, shackle plate bolts and anything else that gets shaken about.”