A belated update on what I have been up to with the B-MAX4 for the last couple of weeks…
Thursday 27th November – Longbenton club meeting
This was the shakedown. Longbenton is mostly a touring car club so the track was flat as a pancake and slippy (it’s a wood floor in a school hall). But I managed to get five 5-minute runs under the cars belt (or should that be shaft?) with no issues, which was what I was hoping for. Grip was terrible (using worn Schumacher minipins) so it was difficult to make any real judgements on the handling but the car did seem to have better turn-in than my Durga would have managed.
Now although motor temperature didn’t seem to be an issue and everything accelerated and braked as it should have, I still wasn’t happy with the drivetrain. It still seemed tight and made some pretty crunchy noises under acceleration. Cue a bit of late-night wrenching…
My foolish transmission modifications
Disclaimer: In no way am I suggesting that you do this. I’m just blogging it as a record of what I did, some of it has already proved to be a mistake, hopefully you won’t make the same ones!
With file and Dremel in hand I launched my onslaught on the centre transmission. The aim was to get the spur gear to spin freely when the top cap was on. First of all I removed some material around the edge of the bearing housings on the top deck…
A little bit off here...
…then I removed some more material from the top cap itself…
...and a little bit more off here!
Now after these mods the spur spins freely with the cap tightened down – although in truth I removed a little bit too much and there is a small bit of unwanted vertical movement on the spur now as well. Thankfully there hasn’t been any damage (yet).
Next, I tackled the shimming in the front and rear gearboxes. On my on-road cars, and also with my Lazer ZX-5, I had set the bevel gears to have a small amount of backlash, this gave a super-free transmission and never caused any damage. So I did the same with the Yokomo.
One thing to take note of here is that the moulded gearbox halves are not identical, so a setting that may feel right with the top off will feel wrong once assembled.
Another thing to note is that Yokomo only supply relatively thick shims to set the mesh (around 0.2mm), whereas most other manufacturers give you some 0.1mm shims instead. So, when I set the gears to have a little backlash, the rear gearbox in particular was a bit too loose. I was a bit worried about whether the gears would last.
My fears would prove well founded…
Sunday 30th November – South Shields NE indoor regional
First impressions of the track for this meeting were not good for me – a couple of (excessively) narrow sections and a tall double-level tabletop are not the kind of thing I race well on (I prefer wide tracks and no “big air”), although there were some nice gym-mat ripples that rewarded a good line and a bit of throttle control. First practice revealed a lip on the down-ramp of the tabletop that was kicking the Yokomo up very badly regardless of how I took it. More often than not the car would end up on it’s roof. Fortunately the host club made a change to the tabletop and the lip would not cause me any problems during the racing proper.
I did have a mechanical failure in practice – I tried the “big air” route over the tabletop, landed off the track, and busted the sacrificial gear in the servo. Fortunately I have a couple of spares. The car itself was undamaged and I decided not to risk the “big air” again (shame that wasn’t the case for the other people in my heat who would take the big air and land on top of me from time to time…).
First round was spent getting a feel for the car on kit settings, managed 17th-ish in round from a field of about 45, roughly what I normally manage at these meetings, and quite pleasing considering I was on well worn tyres. There was a lot of carpet on the track and the car seemed to dig in a little on turn-in, wagging the inside rear wheel through the last corner, and getting hung up on the approach to the big tabletop. This was costing me a lot of time – partly my confidence in the trajectory of the car, partly my confidence over big jumps. It took me until the fourth and final round of qualifying to get a good line over this obstacle (gently up the first part of the up-ramp, throttle up the last third, land in the middle of the lower second section, gently down the down-ramp and kiss the apex of the left-hander at the bottom) but I was still a bit slower than most.
In the second round of qualifying I took off the rear anti-roll bar, this improved the balance overall and made the car a little more confident over the bumps (the car was very good over the bumps already, very settled), however the front end was still digging in.
For the third round I tried to stiffen up the front end with some heavier oil (up to #450 from #400). Unfortunately part of the track had got knocked out of place and the bumpy section had become viscious, so I made a lot more mistakes. I still managed to set a quicker time but everyone else was going quicker too and I was only 24th-ish in the round.
In the 4th and final round of qualifying I went back to the #400 oil in the front and managed to get a 14-lapper. Unfortunately the slack rear gearbox mesh decided to call it a day and the once-silent car ground it’s way around the last few laps. And despite going nearly a whole lap faster than I had gone at first, everyone else had also gone faster and I was still only 18th-ish in the round.
Thankfully, the damage to the gears was not terminal, and I re-shimmed the gearbox for a nice tight mesh again. I also noticed that a balljoint on the rear suspension had almost completely unscrewed itself so tightened it up.
Lining up 3rd on the grid for the C-final, I made a terrible start, completely missing the apex of the first corner. Luckily that gave me the inside line at the next corner while others tussled and I was momentarily up to 2nd again. Then as I rounded the right hander after the second set of mats, the back of the car looped around and I ended up off the track and last. The sudden lack of rear grip caught me out a couple more times early in the race, but I slowly managed to work my way back up through the field to finish fourth, just outside the trophies (again!).
The loose rear end was a bit of a mystery to me – it was either the tyres finally going past the point of no return, or the re-shimmed rear gearbox, or the tightened suspension link. Whatever the cause, it meant that the meeting ended in a rather disappointing way.
Revisiting the gearboxes
I have since re-shimmed the gearboxes – this time I have got hold of some fine 0.1mm shims with 8mm and 12mm inside diameter (I have used the ones from TOP Racing), cleaned out all the chipped teeth, and set the mesh to have no backlash and still turn smoothly. Fingers crossed these will hold out at the regional this weekend because I didn’t order any spares!
I’ve also got a couple of setup ideas in my head, main one is to try the front shocks on the outer hole on the wishbone to stop the front end digging in. According the the spring chart at Petit RC the Yokomo black is a similar rate to the Associated silver – and most people in the UK run the stiffer Associated blue on their B44′s and 501X’s.