shimming bmax gearbox ??

[fastinfastout]

Hi, recently built up a new bmax as per kit settings, and has only had about 20 runs on it.

I disassembled the rear g/box as it was slipping and thought I needed new diff balls, but to my surprise, the bevel gears has been sheared completely.

I pulled apart the front gearbox, which seemed to have a looser setting, and it looked new.

I dont understand. The rear didnt have any play at all, and its worn really bad.

I read up somewhere here, that to shim it so there is no play, which I built as per kit instructions. Then it should free up on on its own later on.

How do you shim the gear box correctly so that this will not happen again?

[sosidge]

First task is to get some finer shims than the ones in the kit. Yokomo do some, I used the ones from TOP. 12mm for the diff, 8mm for the input.

Then, shim out the excess play on the input gear, and line it up so that it meshes in the middle of the diff gear. Also make sure that the input gear spins freely in both gearbox halves (mouldings are not necessarily identical).

Then, work out how many shims the diff can carry before it stops spinning freely in both gearbox halves. Now adjust those shims until you get a setting with the tiniest amount of backlash. If you have no backlash, make sure the gears turn smoothly by hand, no grinding.

Once assembled, check that everything is still free running with minimal backlash (fingers on the diff outdrive and the centre driveshaft will do this), and run the car conservatively for 5 minutes to bed the gears in.

Recheck the mesh, adjust again if necessary. That should get it right.

I had big problems getting the mesh right on the car. The two biggest problems were the thick kit shims (at 0.2mm, too big for an accurate adjustment) and the unequal moulding of the top and bottom gearboxes, (which would make a mesh that seemed OK in one half be too tight in the other).

Hope that helps.

[fastinfastout]

thanks for that!

I did end up getting some finer shims after reading your post a while back, but I think my rear g/box was shimmed too tight following the manual instructions.

[KRob]

I run mine as per the kit instructuctions.
However, a friend and myself who run B-Max's have found it is possible to lunch the rear gears if the slipper is set too tight.
Loosening the slipper by a fraction meant the gearbox hasn't been touched since (5 nationals down the line) and the car is still as fast as anything off the line.

[fastinfastout]

I may have had the slipper too tight as it was a new built car and I didnt know how to adjust the slipper as I lost the manual and just wanted to run the car.

Also, I followed the kit instructions, which may have led to the rear gearbox failing.

After putting a new drive and diff gear, I noticed there was too much play as the manual states not to use the ZC-S50S 5mm shim.

I ended up putting one 5mm shim between the drive gear and bearing.

Hopefully this will prevent any premature wearing.

I ended up going in the rear box
1x 5mm shim on drive gear
2x total 0.1mm thick 12mm shim diff

see how it goes

[Gonzo]

Hi there! These posts were helpfull to me too when building my BMAX.

You were right about the molding: I had to take away (quite) some material in the gearbox top covers, especially the one in the back, to even get the diff alone run freely when the box was screwed together. It was pushing too hard on the big bearings, and also touching the inner moving part of the bearing from the outside.

I also had to put 1 extra small diameter shim between axle and outer bearing at the back, so outside the gearbox, compared to the manual, otherwise the axle (with little 45 degree gear) could move back and forward.

Before I received my new bmax, I ran a 2nd hand one for a couple of weeks, that was build by the previous owner. It was shimmed correctly, but I don't think the previous owner took any material from the gearbox covers. That car created so much drag in the gearboxes, the slipper had to be set tighter to get drive, the rear 45 degree gear suffered and it almost cooked my electronics.
The bmax I've builded now runs much smoother, and creates equal drive with a much looser slipper setting.
After running in a couple of minutes without load, I ran 1 pack with the new one today on a dirt track. The car drove and sounded real smooth, but to make sure I'll open up the gearboxes tomorow for a gear check. It looks good so far, if the gears are still look OK too, me very happy!















Like I did on my B4 too, I drilled a hole in the spur cover, and holes in the spur. That way I can lock the spur easely, handy to check if slipper slips before both diffs, and slipper setting in general for gear protection.

I use the following to "measure" the slipper resistance: with the spur locked like shown on the last picture, I pick the car's nose up 90 degrees and hold firmly both back wheels to the ground. Then I push the front of the car a bit forward, and check at what angle the front of the car falls down from it's own weight (without packs).
To get enough drive on my new bmax, the front already drops at an angle of 70 or 80 degrees, so almost immediately, and rather fast. On the 2nd hand bmax without trimmed gearboxes, it had to be 45 degrees to get enough drive, which is apparently too tight for the gears and electronics.
I'm really curious for anybody else doing this test and posting their findings!