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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.

It’s taken a few more evenings but I have finally got this buggy built and ready for it’s shakedown run.

I’ll run through the last few manual stages and mention a few of the tweaks I did at final assembly, because a few things I was happy with on the initial build were not quite right once everything else had been fitted.

Step 17

Very straightforward, the straps seem to hold the cells in well (one of the benefits of the moulded parts compared to flat carbon straps). The cell trays sit loose in the chassis, if they fall out I might dab them in place with a little shoo-goo.

Step 18

Time to start getting the electrics in. This always seems to be very time consuming because you want to make the wiring as neat as possible without making it so short that you can’t get the cells in and out or change the motor position, and without getting in the way of the body either. Fortunately the B-MAX4 body is pretty roomy - more on that later.

First to go in is the motor. The motor mount is the cam type (as used on the Associated TC3 and also on the Yokomo CGM touring car), this makes installation easy because you can access the motor screws from the side (other shaft cars have the screws on the front of the motor). Unfortunately on the B-MAX4, the cam has so many nobbles on it that it is actually quite difficult to turn the motor smoothly and set the gear mesh. I had to resort to putting one of the parts bags between the spur and pinion to get it right.

One quick tip is to not fully compress the spring, if it is coil bound it won’t hold the clamp tight.

On top of the gear mesh fiddle, the pinion cover is awkward to put on too.

Step 19

Time for the rest of the electrics and the wiring. Although there is a good amount of room in the B-MAX4, this took me a while. Reason being that I use a Keyence Rapida Pro brushed speed controller, which has the setting buttons on the side, making access difficult. Eventually I settled on the layout below, with the speedo beside the spur gear and the motor wires in the gap. I then ran seperate battery wires behind the spur (normally I combine the motor and battery positive but it didn’t really work on this car).

B-MAX4 wiring layout - complete with brushed motor, NiMH cells and FM receiver. It's like a museum in there!

The Futaba 133F receiver and AMBrc personal transponder went in easily, however I was not so happy with the servo mounting. For some reason Yokomo have the holes in the chassis 2-3mm wider apart than the other cars I own, and there is no scope for adjustment, so my Futaba S9451 servo had to be clamped on the outside edge of the ears. I dug a couple of cone washers out of my pitbox to spread the pressure on the mountings. By the way, you don’t need the small servo mount spacers with the S9451.

Cone washer used on servo mounting

Step 20

I’m not using the B-MAX4 wheels, I will be using the more common “Losi”-type front wheels with the Champions Pack hexes. To be precise, I will be using Tamiya wheels for the time being because I have several of those from my DB-01. These fit just fine, if a little snug on the front hex. Unsurprisingly the Yokomo hexes are wide like those on the Associated B44, so B44 wheels are the natural choice, and are what the team drivers seem to be using (Yokomo have a long-standing relationship with AE). I doubt the kit tyres would be much use on UK tracks, but they are something to use on the street.

Step 21

Fit the batteries. You need a 100mm link wire on the saddles when you build them up.

Step 22, 23 & 24

Now when I first saw pictures of the car I thought “nice concept - shame about the shell”. Well, the good news is that the shell looks a lot better with your own paint on it than it does with Yokomo’s stickers. Biggest problem is the windows. The front screen runs up to the top of the canopy, making the car look like a slap-head. But, paint your own with a lower front screen and the car is transformed (I first saw this on Naoto Matsukura’s Japanese championship car). It actually has something of the original Yokomo YZ-870c about it.

All done!

Since the car will be mostly run indoors I decided to make the “hot weather” cuts, opening up the vents at the front and cutting away the body at the back. Further trimming is needed around the front of the body to give clearance for the steering as the standard cut lines foul at full lock.

Step 25

Helpfully Yokomo finish the manual with a couple of tips on how to adjust the diffs and slipper. The diffs can be adjusted externally with an allen key through the side of the outdrive (always a useful feature, and surprising how many manufacturers don’t include it). The slipper adjustment is also unique as far as I know, you insert an allen key through the access hatch on the top deck to hold the notched slipper nut in place, then turn both rear wheels to tighten or loosen it as required.

Final fit

Having put the car together and set up the electrics there will still a few things I wasn’t happy with.

First of all was the steering. I noticed a tight spot in the last few degrees of movement, which was making the servo strain a little and stopping the steering from “snapping” to full lock. Investigation showed that the hub carrier was the problem. There is a small part of the mould at the rear edge which fouls the steering block at full lock. Solution was simply to trim away the edge of the moulding with a knife.

Hub carrier modification for easy full lock

Second issue was the play in the suspension arms. It was just too much for my liking, so I have put some shims on now. You need 3mm inside diameter shims, I used the ones made by TOP (PA-100305). It needed about 0.5mm on the front pins and about 1mm on the rear.

Shimming the B-MAX4 suspension pins

The final issue was (and still is) the drivetrain. It’s pretty rough to start with. I ran the car on low throttle for a few minutes, this seemed to help a little, but it is still rough. I’m hoping that a few solid races will bed the drivetrain in nicely (rather than chew up the gears which is what I fear). I already have the shims on the looser suggested setting and am wary of going any further at this stage in case the gears strip.

Unfortunately I think the moulded gearbox and top deck is to blame. These are never as accurate as machined parts, and on the B-MAX4, the gearboxes and spur gear sit in plastic parts. With the tops off, the gearboxes are pretty smooth. Fit the tops and tighten them (sensibly) and they get a bit tighter.

Conclusion

Apart from the smoothness of the drivetrain (which I haven’t come to a conclusion on yet), I am overall very impressed with this car. Parts quality and fit is up to the Japanese manufacturer’s usual high standards, the spec (even without the Champions Pack) is right where it should be, and although superficially it looks like a lot of the other cars on the market, Yokomo have put their little own design touches on it, particularly around the centre drivetrain and the front suspension and steering.

Of course - building it is one thing - racing it is the real test!

Returned to the workbench again today and got a few more bits onto the buggy

Steps 10, 11 & 12

Time to assemble and mount the suspension arms and hubs. Once again this car roughly follows the “Losissociated” style that has become the norm, although Yokomo have put their own twist on it. The top of the front steering knuckles hang below the caster block, giving the car a deceptively different look from the top, and the rear shocks are mounted behind the arms and feature kinked top links. Parts quality is again very good.

Assembling the front hubs is another opportunity to break into the Champions Pack, this time for the wheel hex adapters (for “Losi”-type fronts) and the optional aluminium kingpins. The wheel adapters are an easy fit, although I used some of the supplied black grease to hold the pins in instead of the suggested rubber cement (which might be a bit too permanent for easy maintenance). The king pins are unusual in that there are two ways of fitting them, which either lowers or raises the steering knuckle. I fitted them in the “up” position, mainly because it is the same as the kit pins and also because it is a bit easier to line up! Experience will show what effect this has on handling (in theory, the “up” position will create a higher roll centre at a given ride height, which will take away a little overall grip but increase responsiveness at the same time).

I did notice that the king pins actually have quite a lot of free play on them anyway, so I will consider a shim or two after the shakedown run.

Close-up of the B-MAX4 front hub

The rear hubs accept the near universal “Losi”-type pin drive. I’m not a huge fan of this to be honest (nor of the small front hex), but it has become the de facto standard. If only Tamiya’s Top Force or Kyosho’s Lazer had been more successful, then we would have had a sensible 12mm hex like the touring cars do…

On the topic of pins, Yokomo supply a split pin for the drive - and as much as I hate losing standard pins, these will be impossible to get out in a hurry, because they were almost impossible to fit! Look carefully at the driveshaft and one side of the hole has a slight chamfer round the edge which makes it easier to seat the pin before you squeeze it in with pliers.

Close-up of the rear hub

The suspension blocks that the arms hang from are adjustable - they can be turned upside down to alter the front kick-up and the rear anti-squat. I should have mentioned earlier that there are a couple of team driver setups at the back of the manual (including Neil Cragg’s Euro setup), and if you want to build your car with one of these you need to decide before you start, because the rear suspension mount that was fitted in Step 1 is also adjustable, and is well buried if you decide to change it now! I’m building mine to more or less kit settings.

The front arms are a very good fit but curiously the rear arms have a good millimetre of free play. Almost as if Yokomo forgot to include some spacers (there are plenty for the hubs). It’s another part I will look at shimming after the shakedown.

That’s enough words for now - here’s a nice picture!

Overview of the car after Step 12

Steps 13 & 14

Time now for one of those time consuming jobs in every car build - assembling the turnbuckles. Once again I am turning to the Champions Pack to pull out a set of seven titanium versions (although I suspect that the kit steel ones would be absolutely fine too, they are an almost identical thickness). The ball cups are a new design for this car, chunky and with an access hole in the top for quick unscrewing of the ball studs beneath. This should decrease wear from popping off the ball ends themselves.

Once assembled and set to the correct length (easier to do the final adjustment on the car rather than by hand), I noticed that the front upper links were rubbing against the shock tower, causing a tight spot as the suspension dropped. A quick examination suggested that the rearward sweep of the link was causing the edge of the ball end to rub, so out came the craft knife and a small chamfer was made. Result - smooth suspension travel again.

Trim the ball end like this to stop it rubbing on the front shock tower

View from the front of the B-MAX4 with turnbuckles now fitted

Steps 15 & 16

Now we come to the biggest reason people will have chosen the Champions Pack - the alloy shock absorbers. These have come straight from the BX (the previous Yokomo buggy) and are superb quality. Apart from threaded alloy bodies, they also include the famous Yokomo bleed caps, titanium nitride shafts and machined pistons.

They are easy to assemble (due in no small part to the quality of the parts), but I have a couple of tips.

First, use a little shock oil to lubricate all the o-ring seals as you assemble them.

Second tip is more personal preference than anything. I don’t compress the shocks fully to bleed them because what happens then is you cause a vacuum in the shock which actually pulls the shaft back in when you try to extend it. I don’t like this at all, so I set the shocks to rebound without pulling back in from full extension. To do this, push the shaft in only about 1/2 way before bleeding, the check that the shaft does not pull in at full extension. Also check that the shaft can be fully compressed without locking up from hydraulic pressure. Generally, this style of shock will squeak a tiny bit at full compression because it needs air bubbles to work, so don’t worry about it. In fact, I would rather hear the squeak than none at all!

The shocks are then fitted with the kit supplied black springs and spring bottoms, and mounted onto the car with some more newly designed parts. Thankfully these don’t seem as easy to crush as the old Yokomo ones (I’ve seen many a Yoke with dodgy handling because the shock mounting balls were crushed and causing major friction).

Alloy shock in place

It’s getting late again - more updates soon!

I had a crazy idea in my head that I could get this built on a Wednesday evening ready for racing on Thursday night… well that was never going to happen with two small children around the house. But, about three hours of work tonight has got a pretty recognisable car chassis assembled.

The box

Quite big, with most parts bagged up in one large sack, and the chassis, body, wheels and paperwork seperate. Also, since I paid a little extra, the all important “Champions Pack” has been dropped in there too.

"Enjoy this product knowing that you are driving one of the best handling RC cars ever designed."

Inside the B-MAX4 box - "Champions Pack" is bottom right

Step 1

Is fitting the suspension mounts to the main chassis. Over in a flash - impressed already with the parts fit.

Step 1 complete

Step 2

Time now to attach the lower halves of the gearboxes, motor mount and centre transmission. Front gearbox needed a little time to get in place without overtightening the screws, it will sit flush when fitted correctly. Don’t forget to read the manual addendum!

Step 2 out of the way

Step 3

This is the most complicated yet - the servo saver and steering rack. But the parts are so good that it really is a doddle to do. The B-MAX4 has a bellcrank servo saver, same concept as the older Yokomo tourers but a more compact design. It’s mounted on bearings which is a nice touch for a supposedly “budget” car (although this car is well specced even without the Champions Pack, it has full bearings, full UJs and a slipper as standard).

Another TOP tool I have acquired for this build is their 3mm hand tapper (PT-081030). Although there aren’t many machine threads into plastic on the B-MAX4 (just the shock mounting screws and the ball studs), tapping the holes in advance makes them so much easier to fit and with no risk or damage to the plastic. Highly recommended.

The TOP thread tapping tool

Step 3 complete - tap used on steering ball studs.

Step 4&5

Step 4 is absurdly easy - the diffs come pre-assembled, so all you do is slip some bearings and shims on the outdrives! I gave the diffs a twist and had a look for signs of lubrication and they seemed alright, so I will give them a go as they are. I will probably give the car a quick rebuild after its first run anyway to see what has bedded in and what needs adjusting. A good feature of these is that they can be adjusted externally by putting an allen key through the side of the outdrive - no need to pop out the driveshafts. Step 5 is also pretty straightforward, since the centre driveshafts are also pre-assembled. I prefer the alternative shim setting at this stage as the standard one seems too tight.

Close-up of the B-MAX4 gearbox

Step 6

Step 6 sees us cover up the gearboxes, and also fit the anti-roll bar holders - one of which will have to come off again later when the Champions Pack is opened.

Drivetrain sealing begins

Step 7

This is a fiddle. The slipper clutch seems a solid design - but lining up all the parts while trying to keep the spring compressed needs a few attempts to get right. Be careful with this because I have a feeling that if you tightened it up in the wrong position you might cause some serious damage to either the drive pin or the slipper plates. On top of that, the slipper nut is awkward to screw on (read the addendum), especially with the threadlock paint that Yokomo put on the drive cup. The Champions Pack has an optional 81T spur but I chose to stick with the standard 87T (I’ll be using a 10×2 brushed, probably on 19/87 which is 10.76:1).

A view of the slipper before it gets covered up

Step 8

Fitting the top deck is straightforward again, quite a few screws but it all lines up well.A nice feature is the access cover for slipper adjustments.

Starting to take shape

Step 9

Time to put the hardware in the shock towers now. Both seem chunky and will hopefully prove to be durable. Not a huge number of mounting positions, three for the shock, two for the camber link, but should be plenty for most people.

A look at the rear tower with hefty wing mounts...

...and now for a look at the front!

OK then, that is all the building for today. Will update the blog when I get some more under my belt.

My original plan was to get some socket head machine screws to build this car with, replacing the cross-head self-tappers that come in the kit.

A bit of a hunt around on the internet revealed that nobody seemed to be doing a kit and it would be difficult to try and put together one from a single supplier due to a few unusual sizes.

During my hunt I came across this Wikipedia article which revealed that Japanese cross-head screws have a slightly different head to the Phillips screws they look a lot like. According to Wikipedia (if you have faith in - which on this occasion I have no reason not to), a Phillips head is designed to “cam out” if too much torque is applied, making the tool jump out, whereas the JIS (Japanese Industrial Standard) is not. Tellingly, the JIS screws are often marked with a little dot on the head - which you see quite often on the Japanese car kits. This must explain why so many of us strip the heads on our Tamiya cars while building them - we are all using the wrong kind of screwdriver!

So, I have decided to take a gamble on using the kit screws - and getting some new screwdrivers as well.

I have chosen two TOP screwdrivers - the 5.8mm for the M3 screws (PT-03105) and the 4mm for the M2.6 screws (PT-031040). These are distributed in the UK by Much-More Racing. These are a flawless fit in the screw heads - the picture below should show how much more defined the tip is than my old (and admittedly worn) Stanley Phillips #2…

JIS vs Phillips

Another quick “tip” (geddit?) about the self-tapping screws, same as would apply to any screw really, is to be careful not to overtighten them - the plastics on the B-MAX4 seem a little softer than other competition cars and might prove easy to strip - and to tighten them in a star sequence to even out the pressure on the components.

Time to get on with the interesting bit - the build!

Here are some pics of my TOP Scythe. For those that don’t know this is a 1/10th electric touring car, combining the best of HPI, Yokomo and TOP’s own parts to make a very competitive chassis.

I just took these pics quickly as it is a sunny day, the car was run at an outdoor meeting a couple of days ago hence the tyre dust all over it. Sorry if it offends your eyes.

Somewhat battered shell now, having had the car 9 months or so. It is a Protoform Mazdaspeed 6 if you were wondering.

Under the shell. The car is quite close to standard, there are a few bling parts on there like the red nuts, but I still run the plastic bulkheads because for me the car is a lot easier to drive like that. And yes, I know I am using stone age brushed/NiMH/40MHz technology - but it works!

Couple of quick comments about this side - Keyence speedo is far and away the best brushed speedo I have ever used - totally programmable on-board and no schottky or power cap needed. I am using a low-profile servo (Futaba S9550) which does not overhang the chassis. And my lead ballast (50g of it to make 1500g safely) is in the centre of the chassis, just in front of the motor mount and beside the ESC. At some point I might experiment and move some of it behind the motor mount for a more rearward CG which should add stability when the grip is low.

Shot of the front showing the TOP steel spool - a very worthwhile addition on carpet and tarmac IMHO. Also, a tip on shock mounting, get some longer screws (M3×16mm) and fit backwards with a locknut so you can remove them more easily for maintenance and not crush the plastic mounts.

Another shot here showing the optional split suspension mounts - I use them because I run a raised front roll centre (2mm spacers under the mounts) and this gives more belt clearance.

Final shot of the car, the back end. Not much to say except that the TOP diff protection seals keep everything a lot cleaner and consistent, well worth the small outlay, and another tip about building the Yokomo-based shocks - cut down the triangle spacer for extra travel, you need it to get the droop right.

Any questions? Just leave a comment and I will reply.

My fault really, tried to clear a large tabletop in my final that I knew I couldn’t. Landed awkwardly, flipped on the roof, cracked the rear tower off at the wing mounts.

Rather worryingly this is the exact same breakage I have seen posted on another DB01 on the net. Hoping it doesn’t happen again.

To be fair to the car I had already crashed it heavily many times during the day without anything else breaking.

Been a while since I last posted anything!

Just picked up the new Tamiya DB01 Durga buggy from the local Tamiya specialist Cleveland Models. Essentially a TRF501X but with a plastic chassis and downgraded components. Reminds me a lot of the TA05.

Here are a couple of pics before it gets wrecked at tomorrows NE regional at Teesside!

OK, here is my final version of the static strap.

Now using eyelets which are crimped and soldered onto the wire. Makes the setup very easy to fit and remove, also the assembly has no effect on the tweak or integrity of the chassis.

See previous posts on this blog here and here.

…these are a few pictures.