Sunday, 22 June 2014

BT-7M Tires

The slanted tires weren't the first attempt to help with the BT-7's tire problem. Turns out, when a tank goes really fast in really hot weather, its road wheels aren't going to last long. One of the comparative trials between the BT-7 and BT-7M involved measuring how the tires heat up and how long it takes before they are destroyed.

Fig. #18: Overall view of the A-7M from the right with tracks.
Fig. #19: Overall view of the A-7M from the left with tracks.

"A-7M and A-7 trials.
  1. Trials goal: to discover the reliability of tires when the vehicle travels on wheels.
  2. Date and place: June 22nd, 1939. June 23rd, 1939, June 27th, 1939. A stretch of highway between Kharkov and Belgorod.
  3. Weather conditions: Dry, sunny. Air temperature ranged from 28 to 32 degrees.
  4. Trial conditions: the trials were held on a concrete and road tar or gravel covered highway with the speed matching the average speed in the A-7 manual. For the first 80 km, the tank was stopped every 20 km to check the temperature of the tires.
    After 80 km, the wheels were cooled to the ambient temperature with buckets of water and the tank began a nonstop march, first until the rubber started to melt, and then until it was destroyed completely."
Here are the results of the trials.


Vehicle type and number
Tire type
Vehicle mass in kg
Side
Load on the wheels, kg
Distance before the tire started showing damage
Notes
Drive
Support
Drive
Support
Inner
Outer
Inner
Outer
A-7
610-42
NK, 110x830 production
13950
Left
2160
2045
246
215
-
-
No further trials of this vehicle were peformed
Right
2126
2018
-
-
-
-
A-7M
0890-3
NK
110x830
production
14360
Left
2205
2145
246
246
-
-
-//-
Right
2092
2143
246
211
-
-
A-7M
0993-5
NK
110x830
with drilled openings
14425
Left
2270
2250
-
191
-
-
-//-
Right
2300
2265
-
172
-
-
A-7M
442-55
NK
110x830
production
14680
Left
2255
2145
160
160
-
160

Right
2266
2143
158
160
-
160
A-7M
442-55
NK
110x830
Filed down according to sketch #2
14680
Left
2255
2145
522
522
1265
1265
The left support wheel travelled 560 km on tracks before the trial

It's a lot of numbers, but you can see that the wheels in the last test (BT-7M, filed down tires) performed leaps and bounds better than the rest of them. Here's how it looked.

Fig. #56: Sketch of the drilled rubber tire.

Fig. #57: Sketch of the filed down rubber tire.

"Conclusions:
  1. Inner regular production tires on A-7 and A-7M vehicles begin to melt after 65 km of nonstop motion at a speed of 38-40 kph on a gravel of tarred concrete highway. This is explained by the inner tires being under more load than outer ones even with horizontal axles.
  2. Production tires with drilled openings (fig. #56) also begin melting after 65 km. This is explained by the inner tires still being under more pressure than outer. Additionally, the ventilation holes for improved cooling led to an increase of pressure on the foundation of the rubber, which did not give a reliability increase compared to regular rubber.
  3. Production tires filed down according to fig. #57 showed satisfactory performance. Melting began only after 500 km, and the destruction of inner and outer tires happened at the same time. Production tires on the same vehicles began melting at 158 km.
    Trials of these tires with tracks on and off-road also showed satisfactory results.
Conclusions: tires filed according to fig. #57 are much more reliable than other tires subjected to the same conditions, and mass production of this type of tire is reasonable."

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