Torque Reaction Links - Suspending The Driven Axel

User avatar
Will L
Posts: 1805
Joined: Sun Jul 20, 2008 3:54 pm

Torque Reaction Links - Suspending The Driven Axel

Postby Will L » Tue Mar 30, 2010 10:03 pm

On another thread ( link here) there is a description of a “torque reaction tab” which given my (possibly questionable) understanding may not do quite what is intended. So as not to hide what may turn into an informative topic at the end of a non obvious thread I’ve started a new one.

The purpose of a reaction tab as described by Morgan Gilbert, or reaction link as I know them, is to allow the driven axle to be compensated or sprung in the chassis. The problem being that the torque reaction will cause the motor/gearbox to try and revolve around the driven axle unless constrained in some way. Easy with a fixed axle, just rigidly attach the motor to the chassis, but not when you want the axle to be free to move relative to the chassis.

As I understand it, the forces produced when constraining the motor/gearbox need to be arranged so they act horizontally along the chassis or they will affect the performance of the suspension on the driving axle. The “classic” reaction link is configured like this.

reaction link.jpg
The link is horizontal and free to pivot at both ends, one of which should be directly above the axle, and the other one far enough away to ensure that the movement of the motor/gearbox is effectively vertical for the limited amount of up and down movement available to the axle.
reaction link.jpg (47.23 KiB) Viewed 4577 times

The torque reaction tab used by Morgan Gilbert would seem to me to ensure the torque reaction acts vertically in the chassis and will thus significantly effect the action of the suspension. Or am I wrong?

I have been puzzling over how I can justify why this is true, but my schoolboy physics/applied maths aren’t up to the task. However I’m sure there are others on here who could oblige. Russ?


User avatar
Posts: 482
Joined: Thu Oct 08, 2009 10:05 pm

Re: Torque Reaction Links - Suspending The Driven Axel

Postby LesGros » Wed Mar 31, 2010 8:32 am

Hi Will,

The best way to understand it, is to consider all of the movements involved. Then think about which movements are required or acceptable.

You are correct in saying that with a fixed axle, the gearbox/motor assembly can be fixed to frame. and with a compensated or a sprung suspension some provision for movement is required.

Starting with the axle; the axle can move up and down in the horn guides. It can also rock laterally depending upon how sloppily the guides fit. This means that the gearbox assembly also needs to be able to rock from side to side, and to move up and down; but we must restrain it from rotation. If unrestrained, torque reaction would cause the assembly to rotate about the axle until something stopped it; a frame spacer or another axle. If you want to see it in action, hold the assembly by a wheel, apply power, and watch the gearbox and motor rotate about the axle. ( but mind your fingers!)

Now to the Tab which is a sliding fit in the slot. The tab is free to move vertically. There is room for movement side to side which means that the GB assembly can rock laterally as the wheels follow any unevenness of the track. It is the sides of the slot which restrain the assembly from rotation.

Hope that helps


The man who never made a mistake
never made anything useful

User avatar
Posts: 198
Joined: Wed Jul 23, 2008 6:00 pm

Re: Torque Reaction Links - Suspending The Driven Axel

Postby Penrhos1920 » Wed Mar 31, 2010 12:04 pm

My preference is to fit one on the motor shift furtherest away from the driven axle. Here is one on a High level pannier chassis, the little bit of bent strip with a hole and a red dot to highlight it:

P6100424a.JPG (431.8 KiB) Viewed 4532 times

Here is another with the motor out of the chassis, the TRL is the circled bit:
No14-1.jpg (15.14 KiB) Viewed 4532 times

By putting the TRL in this position you reduce the movement of the motor inside the body and it can be a tight fit. With Will's method the motor moves a bit more and so needs more space around it. Hope these help

User avatar
Russ Elliott
Posts: 930
Joined: Thu Jun 02, 2011 6:38 pm

Re: Torque Reaction Links - Suspending The Driven Axel

Postby Russ Elliott » Wed Mar 31, 2010 6:46 pm

Will's OP describes the classical torque reaction link/arm, where T = Fy :

01.png (2.35 KiB) Viewed 4488 times

Although the geometry allowing vertical hornblock movement is optimised if the torque reaction arm attachment point on the motor is directly above the axle, I do not think it matters much if there is a displacement of that attachment point if the displacement is small:

02.png (1.87 KiB) Viewed 4488 times

Given that some albeit minimal slop is necessary in the arm attachment points, as y gets smaller, the ability of the arm to counter the axle torque will become less effective, and in the limiting case where y = 0, the ability of the arm to counter the axle torque will be minimal:

08.png (1.95 KiB) Viewed 4488 times

In Morgan's case, y is small. It will probably work ok though because his arm doesn't have any slop at the motor/gearbox end and is "a good sliding fit in the longitudinal direction" in the frame spacer slot:

09.png (968 Bytes) Viewed 4488 times

The torque reaction arm doesn't need to be an 'arm' at all of course - this sketch follows the same principle as Penhros has in his High Level pannier:

07.png (3.87 KiB) Viewed 4485 times

User avatar
Russ Elliott
Posts: 930
Joined: Thu Jun 02, 2011 6:38 pm

Re: Torque Reaction Links - Suspending The Driven Axel

Postby Russ Elliott » Wed Mar 31, 2010 7:47 pm

Leaving axle rotational torque reactions aside, most motor/gearbox mountings are likely to be asymmetric, and the mass of the motor/gearbox will produce a static horizontal force on one side of the hornguide:

03.png (2.09 KiB) Viewed 4469 times

The magnitude of this static horizontal force is usually small, particularly compared to the horizontal hornguide force when the loco is under load. The static horizontal force will approach zero as the c of g of the motor/gearbox gets closer to being directly above the axle, but it becomes a lot less sensible in terms of space usage:

11.png (3.44 KiB) Viewed 4469 times

Return to “Chassis and Suspensions”

Who is online

Users browsing this forum: No registered users and 2 guests