Tamiya TRF 501X One Way Diff review

TRF 501X - Optional one-way unit. Part No. 53951

36t one-way pulley - part no. 53867
501X front one way unit - part no. 53951

An optional front one-way diff' was released at the same time as the 501X, to give different drive options. This should be one of the most popular upgrades for the car.

The fixed drive (as kit) arrangement of the 501X is the easiest and most predictable setup. A one-way diff however will allow the car to turn quicker and pull itself out of corners harder. On a bumpy track the one-way diff' can be hard to drive consistently.

The one-way kit comes disassembled and includes a one-way specific version of the 35 tooth black front pulley.  So you are still left with the over driven front wheels.  There are a couple of ways to get an even power split when running the one-way – the easiest of which would be to swap the front differential to the rear, giving black 35t pulleys at either end. 
The other option is to buy the TA05 one-way pulley which is 36t like the rear.  The inner diameter of the 36t one-way pulley is slightly larger however to that of the black pulley included in the one-way kit. 

One-Way kit contents
Large heavy-duty one-way bearings
Two diifferent diameters
The actual one-way body is double-sided and can accept two different sizes of pulleys – one either side of the central flange.  Fitting the white TA05 pulley I bought wasn’t so straight forward however.  Fitting the optional 36t pulley on the opposite side of the central flange means that the one-way bearings will be the wrong way around.  These bearings are a press-fit so could be removed and re-inserted the opposite way around.  The other option is to run the white 36t pulley on the normal side with the smaller diameter, and use the screws included to centralize the pulley – not really ideal.
In the end, I decided to go the easy route and use the black 35t pulley included.
One-Way installed
Red sponge - keeps drive cups in

The pulley attaches to the blue alloy hub with three small steel screws. The one-way bearings have a thin metal shim slipped over each one, followed by the eccentric bearing sleeves. Finally the out drives are pushed in place.

Small circles of foam push inside the out drives - and are an important inclusion. The foam is pushed by the dog-bone end of the CVD shafts inside the out drives. This in-turn pushes on the out-drives gently and keeps them fully inserted. If the out-drive comes out slightly, it can cause a bent drive shaft or break a one-way bearing.

TEST: I used the one-way for the first time at my second race with the 501X. I ran the car with fixed drive in practice before swapping to the one-way for the first round.
Now, I do prefer the feel of a one-way anyway, but wanted to test the 501X as it comes before swapping to the optional one-way.

The car was good in practice (with diff)- I only had to do half a lap with the one-way before my first qualifier to realise the difference - it was immense! The car would turn so much quicker but also more controlled and predictable I felt.
The high traction corners were a case of coming off the power and letting the car hook into the corner - like it was on rails. Then powering out, up to the next corner and off the power again to hook round (no brakes needed).

For the low traction hairpin in the middle of the track, it was easy to apply some brakes and have the back end slide around smoothly - then aim for the corner and then put the power on, pulling the car straight and powering up and over the jump.

For a track like this, or any other fairly smooth track, the one-way really makes a massive difference to my driving style. On a very bumpy and unpredictable track, a centre one way would be a better option - but is not available yet.

In the end I managed to qualify 6th in the B final & finishing the day 15th out of 60 drivers - out qualifying some very good drivers in the process. I can honestly say I would have been at least one final lower without the one-way.

One-Ways are definately a matter of personal tastes - some like them, others don't. The one-way needs a smoother style of driving than a fixed drive, but the reward is a quick and fun-to-drive car. The 501X one-way is well priced and very well made, this is THE #1 upgrade for the car. Now, let's find a screw kit!

UPDATE:
We noticed when racing the car, with team oOple driver 'Stu Wood', that the red foam inserts are simply too hard for the job. Using something like these is vital to keep the out drives fully inserted - without them the suspension can lock up under power - badly effecting the handling.
But, the red foam inserts are just too hard and themselves put too much pressure on the shaft when the suspension is compressed. This can add a lot of friction to the 'CVD' style drive shaft, locking it up in some cases.
You can fix the problem easily by using a tiny off-cut of an old inner tyre foam in place of the supplied red foam. This should give enough pressure to push the outdrive in but not enough to lock up the CVD.


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