Class 08 wheel diameter

[norbitonflyer]

I was at the Spa Valley Railway yesterday where thay had a double header of a Class 08 and class 09. Something puzzled me. The side rods of the two locos were visibly moving up and down at different rates, going in and out of phase over a distance of a few hundred yards. This struck me as odd. The rate at which the rods move depends only on the rotation speed of the wheels, which is proportional to the road speed and the size of the wheels, both of which should be the same for both locos.

As the diameter of the wheels is specified as 4'6" the wheels should rotate once for every 4.5xpi = 14.2 feet it travels. I didn't manage to count how many rotations it actually took to get back in phase - the syncopated motion is quite mesmerising! - but in 250 yards they should rotate 750/14.2= or about 53 times. If one is rotating 53 times and the other 54 times in the same road distance one must conclude either that one of them was undergoing significant wheelslip, or the wheels differed in diameter by about 1 part in 54, which is one inch. Can the wheels really differ in diameter by that much?

NB I am aware that Class 08s and class 09s have different gearing, but that does not affect the calulations - as the two locos were coupled together and running at the same road speed, it just means the electric motors driving the wheels were turning at different speeds.

As far as I can determine, all the BR-design "350" shunters have/had 4'6" wheels, as did the SR-design class 12s, but the earlier LMS, LNER and GWR designs (including class 11) had smaller wheels, usually 4' 0.5", so there is about an 11% difference in wheel size and a class 11 and a class 08 coupled together should go in and out of phase about every nine rotations.

 

[Dunfanaghy Rd]

I would be astonished if the wheels of both locos were the same diameter (unless both were brand spanking new). The diameter varies over time, as the wheels are re-profiled due to wear. I'd guess at a potential difference of 2-3 inches.
Pat

 

[Big Jumby 74]

As you rightly say the class 11's were/are the odd men out at 4' 0.5". Your ref to '08' I presume means D3489? which is a class 10, but that's piffling detail, as you say they also had 4'6 wheels! Unless Spa Valley has acquired an 11 of late (I'm well out of touch), although they're easily identifiable by the shallower cab roof profile as well.

 

[Cowley]

I’ve seen this before as well but I just assumed it was down to worn tyres on one of the locos as @Dunfanaghy Rd suggests?

 

[Clarence Yard]

Getting out my old (1990’s) copy of MT 288, the diameter new for a 08 (and 09) was 1372mm, limit for scrap was 1287mm. Tyre thickness was 76mm new and limit for scrap was 33.5mm.

That would give you a lot of scope for the rods of those two locos moving in and out of sync.

 

[Taunton]

Getting out my old (1990’s) copy of MT 288, the diameter new for a 08 (and 09) was 1372mm, limit for scrap was 1287mm. Tyre thickness was 76mm new and limit for scrap was 33.5mm.

That would give you a lot of scope for the rods of those two locos moving in and out of sync.

One wonders how a coupling rod loco keeps all wheels wearing down the same. If doing a lot of sharp curve work the outers will likely wear more than the centre ones. I am aware that a regular workshop issue for 08s was "shifted cranks".

 

[Big Jumby 74]

One wonders how a coupling rod loco keeps all wheels wearing down the same

They wouldn't is the short answer. Quite possible that in extreme cases, if left unchecked, other related damage may be the result?

== Doublepost prevention - post automatically merged: 9 Aug 2025 ==

Getting out my old (1990’s) copy of MT 288, the diameter new for a 08 (and 09) was 1372mm, limit for scrap was 1287mm. Tyre thickness was 76mm new and limit for scrap was 33.5mm.

A good 5 inches in old money, both taken to extremes, quite a difference.

 

[Clarence Yard]

One wonders how a coupling rod loco keeps all wheels wearing down the same. If doing a lot of sharp curve work the outers will likely wear more than the centre ones. I am aware that a regular workshop issue for 08s was "shifted cranks".

They are usually on different profiles - the outer are both P1 and the middle one should be P9.

The biggest cause of shifted cranks on BR was over speeding. You also had to keep the rods set up correctly, without letting them have too much play.

 

[norbitonflyer]

Thanks for the explanations - I had no idea wheels were allowed to wear that much.

As you rightly say the class 11's were/are the odd men out at 4' 0.5". Your ref to '08' I presume means D3489? which is a class 10, but that's piffling detail,

D3489 moved to Helston in Cornwall two years ago.

The 08 was D4157 (08922), with visiting 09004 (D3668)

(Resident 09026 was in the shed and obviously not in operational condition, and class 12 15224 was on display at Tunbridge Wells West)