TOPIC:

I suppose I could have put this in the "Look at what i just got" section, but I do want to fiddle with it & pontificate about it

I've just sold 3 cars to clear some space ... Nature abhors a vacuum, so I guess that was my mistake - I immediately spotted & bought this on eBay:



... despite having this fixed to the side of my monitor:

It arrived late this morning, I wouldn't describe the packing as 1st class - a 2nd hand box, carrier bags, a scrunched up copy of the Sun from April, and lots of loose fasteners, but everything seems to have survived all right:



I spotted that a motor was attached - a bit of a bonus as I wasn't expecting one, so I home in on that straight away as it wasn't obvious what it was, just that it wasn't a standard silvercan ... taking off the spur gear cover revealed something interesting - the spur gear looks brand new, and it's 48DP, not 32 ...




Note the rather dog-eared "Race Prep Racing Motors" sticker, + the dreadful soldering:



Inboard end is stamped "ROAR", "0" (degrees, i guess), "RACE PREP", and two digits that look like 13, 15, 16 or 18, depending on the angle you look ... most likely 15 or 16 - turns I guesss.

I gave it a quick tweak & the magnets are pretty strong, so I popped off the 24T pinion & span it up - it sounded a bit rough at first, but after a quick lube & a couple of minutes running it settled down - it spins very fast & is evidently carrying a lot of advance on the timing - it's much faster forwards than backwards.



The car as whole is very tidy, bodywork is not boxart & doesn't stand being looked at too closely, but I'll leave it alone ... it does need a bumper though. Dampers are a little stiff & feel like the oil in them is too thick, but with that motor in it & a bit of tweaking on the dampers, it'll be faster than any Tamiya car I have (with the probable exception of the very few brushless equipped cars I have), and handle better than everthing else (although my DT-02 Grasshopper will be very close):

Going back to the box, the other items were:

The manual & exploded diagram, header card from a slipper clutch baggie, partly used star sticker sheet, inexplicable flame decals, and some bits of paperwork for an Mtroniks ESC:




Some late Meteor front wheels & tyres,rear tyres, silvercan motor & a 12T 32DP pinion, used gearbox halves, a badly gouged bumper, an MSC, and NIP "lightweight" front & rear wheels, which I'll probably use.






The MSC is bin fodder, the wiring has been very badly bodged, including this very early Acoms plug:




Lots of leftovers from a damper rebuild, the original 32DP spur gear (more on that later), fasteners (both new & used), bearings (ditto), and ...




A completely inexplicable (or at least unlabelled) Tokyo Marui bag of parts, and ...




This box spanner. I'm sure someone will immediately recognise it ...

Right, I've been thinking about gearing - the fact that the spur gear & pinion are 48 dp rather than 32 has thrown me a bit - a 24T pinion is outside what I know of as sensible pinion sizes, it doesn't sound/feel right /

I know that the stock pinion tooth count for a Cobra is 12T for 32dp gears, which is ok for a silvercan, but for a motor this naughty, I'd be looking for a pinion as small as it gets - 9T. But that's 32dp, what is it for 48DP? Divide 9 by 32 & mulitply by 48? ... with rounding that comes out as 13T or 14T ... that may well be right, but I think I need a method a bit more scientific


Ok, I found a site on modding the the Cobra - www.planet-venom.co.uk/ , which advocates using RC10 48dp spur gears, and has a chart of gear ratios. Interestingly, the site seems to be incomplete & doesn't seem to have been updated since 1993... I guess he gave up & bought an RC10

Shaded areas are spur/pinion gear combos that don't fit, BTW:



This is all very well, but that's an awful lot of gearing options & numbers in there... which don't make a lot of sense immeditately - for instance the 85T spur / 24T pinion "should be" 3.583, but is 10.119 ... that would only make sense if some other factor is involved.


From "reverse engineering" that chart, a factor of 2.8572 makes all the numbers work, so that must be the gear on the other end of the spur gear shaft, the idler gear & the outside of the diff.

That still doesn't get me anywhere though - I already know that a smaller number of teeth on a pinion "lowers" the gearing & makes the numbers bigger ...

Fortunately I have the original 32 DP spur gear so I can count the teeth - 58T ... 58 divided by the 12T stock pinion is 4.833, multiplied by the magic number, 2.8572, equals 13.810. Now I can go back to my spreadsheet ... and immediately see that most of is it useless - I need a gear ratio of 13.81 or higher for a silvercan, and for the equivalent of a 9T 32dp pinion, a ratio of 18.413+ - only a tiny bit at the top right hand corner, and possibly, the uncharted territory beyond it, offers credible gear ratios:



So, according to that, without experimenting, I need a 15T pinion. Or smaller, probably.
Looking it my spares box I see I have a 14T, 16T, and and 18T, so the 14T is going to get used. There's a chance the car might be going "NNNEEEEEEEEEEEEEEE!!!!!!!" while doing 2mph, flat out, but at least it won't be overgeared


I see that I got the same answer as my completely uninformed guess was

Very nice i saw this on ebay myself and was tempted but managed to resist the urge i need to stop adding to my cars i have nearly thirty now and am starting to think its becoming an obsession

tamiya wrote:

...i saw this on ebay myself and was tempted but managed to resist the urge i need to stop adding to my cars i have nearly thirty now and am starting to think its becoming an obsession

LOL, you're starting to think?

I shouldn't joke, but for me, yes it is ... and I kind of embrace it - there are far worse things to be obsessed by.

A bit more fiddling ... first look under the bonnet:




Nice(ish) servo:




ESC is the 12T that came out of my Wild Dagger, I must have had something in mind for the new Etronix Pulse 2,4 radio ... I'm pretty sure I wasn't going to put it in my Wild Willy or my Cheetah anyway ...




Looks a bit more balanced to me with the bumper on, I like the wheels better too.



I've given it a quick razz round the garden & I'm quite impressed - acceleration is bonkers, but I suspect the motor will support a much higher top speed - I'll try going up a couple of teeth to a 16T pinion next time. I thought the suspension was a bit too hard & heavily damped, but it coped very well on the series of bumps that flip most Tamiyas, and it didn't come close to rolling on cornering, despite how muck lock the steering goes to. The steering servo is pretty fast too.

When you're looking for "sensible gearing" it's often better to ignore number of teeth & pitch, use the ratio of the outside diameters instead. If you've got a car like this with a lot of motor adjustment, it's theoretically best if you go for the biggest pinion with the finest teeth that are strong enough - the finer the teeth, the smoother the drive & the lesser the energy losses through noise & heat. A smoother drive also helps longivity of the other transmission parts. Relative size is also important as it affects the way each tooth of the pinion engages with the counter gear - if the pinion is too small or the pitch too coarse, each tooth engages with a scraping/paring action rather than properly lining up & slotting into place smoothly. a bigger pinion will also give you more teeth engaged at any one time, meaning the loading per tooth is much reduced - again this helps counteract wear & noise.
Sub 12 tooth gears are a bad idea & should be avoided - at these sizes, the gear tooth shape accuracy becomes critical & there's much less chance of having a perfect mesh with the next gear.
That's a lovely example of the Cobra by the way Jonny!

Looking better already kicking myself now for not bidding it looks fantastic condition and you got a bag of extra's with it could this be an actual ebay bargain most of the cars i get from ebay are in a shocking state.

Thats two Mardave's now JR with the Meteor could you be converting to them?

tamiya wrote:

Looking better already kicking myself now for not bidding it looks fantastic condition and you got a bag of extra's with it could this be an actual ebay bargain most of the cars i get from ebay are in a shocking state.

Thats two Mardave's now JR with the Meteor could you be converting to them?

I don't know about a bargain, I had to think twice & evidently so did a lot of other people - it had been up for sale at that price at least twice before ... but yes, condition was excellent, I've had my share of eBay shockers

Oh, and it's not two Mardaves - it's four, currently - three Meteors & this Cobra ... + more if you count ones I've had in the past - the Cobra I sold on Sunday, two more Meteors & a 1/12 saloon stocker