XRAY - Model racing cars

I have been racing my 808 now for a few months but am struggling to keep bearings in the rear pinion. i dont know if it is crashing causing the rear driveshaft to try to push out or something else.

After 3 x 10 min heats and one 30 min final the pinion is slopping around in the diff case.

I bought the new V2 diff cases last week and fitted them with new Avid bearings. I run the alloy centre spacer between the bearings, but i have noticed it is not the same thickness as the pinion. (Fit the spacer and the pinion can still move fore and aft on the bearings meaning the spacers is too thick)

When i screw the 3 bearing retaining bolts down in the diff case the pinion and bearing assembly can still move in and out of the case and i think this is where the problem is coming from as it is not properly supported. The 2 x bearings and one x spacer ring height is less than the height from the screw face to front bearing face.

My solution is to accurately measure the depth of the V2 diff case and machine up a new bearing spacer that is exactly the same size as the step on the pinion. then i am going to machine a spacer up to fit in front of the front bearing that packs the two bearings out so the 3 screws solidly hold the bearing assembly in place.

I believe getting rid of all the clearance will fix the problem i currently have. At the end of a race meeting i struggle to turn the wheels over by hand as the pinion is sliding in and out of position and not meshing with the main gear anywhere near properly.

Front pinion lasts longer, but has similar issues,

Are there any thoughts from the gurus or people who have been racing the car longer than i have???

Cheers and thanks.

Darren

hey is your diff shimed properly. and i just cheked mine and there is play back and forth and i have no probs with it as iam running the same as you are v2 bulkhead and avid bearings and the spacer

Dan,

My diff is shimmed as per the instructions. Bearings with spacer in between and 3 bolts tighted onto bearing.

When i do this i can move the pinion and whole bearing assembly in and out showing there is clearance between the spacer ring and pinion gear and case.

By maching a new spacer ring for the pinion then a new shim for the front of the gears i can set the pinion gear up so it is a proper fit between the bearings, then shim that pack so it sits above the little flats that the bolt heads clamp onto the bearings so they are held in place.

once i get everything apart tonight i will measure it up and draw on CAD to show where there is clearance. once i have done that i can machine the new spacers up and build the diff so there is only 0.005~0.010mm axial clearance.

this will give a better mesh on the gears and hopefully ensure longer life of the bearings so i can get through a complete race meeting without feeling the notchiness in the bearings or totally ruined bearings once i get home!

I have the original bulkhead & avid bearings.with all assembled my pinion gear has .010-.015"(best guess from memory) for/aft movement & that goes down to about .005" when everything is bolted up ready to go.I have so far not had any pinion/diff gear problems in roughly 15 gal.I also do not use those plastic diff bearing spacers I use 8x16x5 bearings in there place & I think that takes away a possable slop point to begin with but niether do I think that is a problem as lots of people use them trouble free. When you say your diff is shimmed per instructions what do you mean? The instructions say how to close or open the mesh but not how many shims to use where,its more of a feel thing.With everything bolted together how much back lash is there when you hold the pinion & turn diff out drives?Is this your first car of have you had good luck shimming diffs in other cars?Has someone else at your track felt your backlash & said its good too? Are you sure your rear chassis brace is tight at both ends? Very few people have pinion problems especially continual problems,maybe something small being overlooked.Me & my bud out here ran several gallons each before the bearing spacer was even for sale with no probs.Where does your rear dogbone sit in the center diff out drive,middle or closer to diff? When we built our cars we ground down the dog mone end a hair & slotted our center out drives a bit but I think our cars were from a second batch that didnt really need thet though. Search this site & you'll find the old diff problems & how people fixed them complete with pictures.

Butcher,

Thanks for the reply. This is my first RC car, but i am a toolmaker by trade and have lots of experience building full size drag race car diffs and this is why i feel there is too much clearance in this setup.

I have just measured the spacer ring that goes between the two bearings and it is 0.1mm thicker than the shoulder on the pinion gear, so there is 0.1mm i can get rid of. Next to measure is the depth of the bulkhead (what i am calling the diff case) that it mounts into. i will let you know how deep it is v's the overall thickness of the bearings and pinion. once i do that i will know the clearance in there and the amount of shim i will need to remove all the clearance.

I believe the 0.1mm i have found in the spacer ring is too much and i wiant to get the whole assembly down to at least 0.01~0.02mm clearance when it is bolted into the case.

The diff centre is shimmed so there is a very very small amount of backlash when the pinion is all the way in. however with the axial movement there is a heap more backlash when the pinion is pulled out (pulling on the drive lin end)

Once i have made the new shims up i will take some pics of it.

Hi Eric,

As i said above i think the diff centre is shimmed properly for gear mesh when the pinion is all the way in. the problem as i see it is the axial clearance in the spacer ring and diff case that allows the pinion to be further in and no backlash, or float back and have a lot of backlash.

As i said this is my first RC car and maybe this is what it is supposed to be like, but i come from an industry where we work in 0.001mm and it just feels to me like there is to much clearance.

I'll have photos and more details by the end of the week to share.

Cheers and thanks.

Darren

It does push in and out from new but there will be lots more play when the drive shaft couplers and pins get some wear on them, so keep an eye on that too.

Thanks Daz. i am aware of the drive pin wear, but i am specifically looking a tthe pinion movement in the Bulkhead.

Ok so i have now measured the bearing+pinion+bearing stack height and it is 12.97mm.

The bulkhead measures 13.63mm from the bottom where the front bearing sits to face that the M3 button head screws tighten down on.

13.63-12.97=0.66mm clearance!

Sizes are good as they were measured with micrometer and CMM (Co-ordinate measuring machine)

So 0.66mm axial clearance (0.026") means the pinion can be set with good mesh when it is pushed all the way in, but pull it out and there is a heap more clearance in the mesh, and that is what is causing the notchiness when i turn the wheels over by hand. The natural thing for the pinion to do is be pushed back to maximum clearance by the main diff gear.

Easy fix. I will make a new spacer ring that is size for size to the pinion gear thickness. Then i am going to make an 0.8mm spacer to bring the bearings forward so that the 3 M3 screws tighten down on it properly and eliminate all the axial clearance in the pinion. Then i can re-set the gear back lash once and for all and know it isnt going to be an issue with gear mesh anymore.

Darren

Why not just pull the pinion out as far as it will go & shim the diff over to that?That is how we normally do it.If when shimming the diff over you end up with a little pre load as the diff is put in the bulkhead thats OK & normal for a mass produced product in the RC industry.I think your over thinking this & making too much work of it.Remember as soon as you get to the track your gonna basicly throw this car off the roof of a building 20+ times in a 5 min qual.Let me see if I can find a picture of what these cars go through in use & why some play is required.

This is what these cars do every lap that you just cant see without a high speed camera & why perfection on the level your looking for is not needed.Notice the angle of the rear shock tower & wing.When it comes back round to the pits it looks normal & why the rear dogbone has so much room to slide forward in the center diff out drive & why I recomend shimming the diff over to the pinion & seing how it works before doing all that work to the pinion bearing stack.