BJ4 non-WE to BJ2 conversion - a few Qs

[colmo]

I'm planning on a BJ2 conversion using an original BJ4 chassis, GT2 front sub-chassis and B4 front end.

My original plan was to modify the chassis by simply extending the taper at the front, slicing off the extraneous bits, finishing in a point.
To this, I'd attach the GT2 piece.

However, this is going to result in a long wheelbase - about an inch more than a B4. Could this work? It appears to be well within the legal limit BRCA allow (460mm total length).

The original chassis can't be shortened much due to battery slots. I could cut a G10 chassis myself, or have a CF or alloy one cut.

[Shifta]

I know this is a saddle pack configuration but it doesn't look like this one has been shortened much..
You could always turn the rear wishbones around to shorten to wheelbase a bit!

http://rc10b4.blogspot.com/2008/10/rc10-b42.html

http://rsb.se/arena/showthread.php?t=39428

http://www.oople.com/forums/showthre...t=41353&page=3
look down the page.

Good luck with the build.

[colmo]

Nice idea with the wishbones - that would place weight further back in the car, so might handle quite well when it's slippy. A tuning option, anyway.

I could try the long wheelbase first (that Swedish effort looks very similar), then cut a notch into the chassis to recess the front sub-chassis without getting too close to the battery slots.

My plan is to seat saddles, placed sideways, one in front of the other, next to the motor (an original BJ4, remember). Without the centre shaft, they'll just fit. That means no need for shafts or any of the old top deck bar the really rearward part. How I stop the whole thing flexing front-to-back is another thing, but may feature a carbon fibre tube roll cage!

[peetbee]

also this one done by MrSpeedy
http://www.oople.com/forums/showthread.php?t=31166

[mrspeedy]

Good luck with build up mate, its an interesting concept, and quite challenging to get it right, I didn't

I only ever ran mine indoors, I reckon it'll work better outdoors tho.

I ended up with a chassis with the motor mounted on the left side to improve weight distribution, and with a thunder tiger geared diff in the stock BJ4 gearbox, that worked well, and I still have it if you interested

I also have a custom bodyshell a bit like this

[peetbee]

Originally Posted by mrspeedy

I only ever ran mine indoors, I reckon it'll work better outdoors tho.

I thought you ran it outdoors last month Si?

[sparrow.2]

Originally Posted by mrspeedy

... and with a thunder tiger geared diff in the stock BJ4 gearbox, that worked well, and I still have it if you interested

Which car did the gear diff come from and what did you have to do to make it fit?

[mrspeedy]

Different car Pete

The parts were from the thunder tiger sparrowhawk saloon, these have a smaller crown wheel than the buggy/truck version.

The crown wheel will need turning down in size to fit inside the tc3 diff case, the pinion gear will also need reducing to fit, and the hole in it made to fit the AE input shaft. The gearbox case will need to be clearenced to get the diff to fit, and adaptors will need to be made to be able to use the metric diff in the Imperial sized gearcase bearings, copper tube was a perfect fit ....

It's not any easy job, but it will fit, and the TT diff is top quality and tough as old boots.....

The end result is that it allows you to use a fairly light diff oil to get steering, but the oil in the diff stops it unloading too much and maintains traction ...

Also the diff is quite heavy and puts that weight just where you want it

Like I said, I still have the diff and case here if anyones interested ?

[peetbee]

Originally Posted by mrspeedy

Different car Pete

Really? I didn't realise that!

[Thomas P]

Originally Posted by mrspeedy

Good luck with build up mate, its an interesting concept, and quite challenging to get it right, I didn't

I only ever ran mine indoors, I reckon it'll work better outdoors tho.

I ended up with a chassis with the motor mounted on the left side to improve weight distribution, and with a thunder tiger geared diff in the stock BJ4 gearbox, that worked well, and I still have it if you interested

I also have a custom bodyshell a bit like this

Ohh, si Me like this car..i did a BJ2 on a BJ4WE car..love this one, what a lid !

[mrspeedy]

Cheers Thomas, I only built my BJ2 cos I saw your video nice .....

It now has an X6 gearbox tho

[colmo]

This is the basic layout I'm planning for the BJ₂O (yup, named it already):



The front sub-chassis could be moved rearwards if the long wheelbase (which requires minimal cutting of the chassis) proves unsuccessful.

A quick, unscientific test of balance showed it was about right - motor + mount balances approx 2/3 of the lipos, so 200g v 200g roughly.

Receiver would go in front of the motor (I use Assan receivers, which are very lightweight) and the ESC would go up front and centre, behind where the servo would be (which might require the wheelbase to stay long!). It can be subtly shifted left or right to fine-tune balance.

Bracing the chassis might simply require long rods from the sub-chassis to the motor mount. Going down the sides is made tricky by the lipo position. I'd also consider making a removable piece for bracing the motor mount to the rear gearbox, to allow easier slipper access. I can cannibalise a BJ4-WE rear top-deck for those bits.

Alternatively, I can simply employ a BJ4 original top deck - the front central hole on the sub-chassis lines up with the front centre hole on that topdeck.

My major drawbacks are lack of machining skills or tools, and time...

This layout could work on a Kyosho ZX-5 (not FS) and a few other shaft-driven, non-mid motor 4wd buggies.

[Garry Driffill]

It looks awsome but just one flaw here, when the cars in the air with the motor sat that way round when under throttle or brakes to alter pitch in the air it wont be as stable as the conventional type of layout...

[mrspeedy]

When I had my BJ2 I never found it to be an issue in the air, just as long as you don't nail it !!! and deffo easier to control than my B4 ever was ....

[Garry Driffill]

Maybe so but to me it just doesnt sound/look merchanically correct or fluent as a regular placed motor would be

[nicefrog]

Most of the steering in the air comes from the tyres rotating anyway

[colmo]

I'm looking at the front end atm. The GT2 front subchassis doesn't marry to the B4 top plate (posts are positioned wider on the GT2) - I'm looking at the GT2 equivalent part #7893 (which would require GT2 steering parts too); I'm not sure if the B4 shock tower will fit it though? (Edit: it does fit - see below for proof!)

[colmo]

After a frenetic day of blood, sweat, snot and loads of cursing, I built this:

[colmo]

Here's a gallery of my build. Nothing much to explain, just sawed off the chassis to a point and drilled, attached the GT2/B4 front end, and reversed the back-end so the wishbones are swept forward.

Still to do: Run rods from the rear top plate to the GT2 sub-chassis, drill and attach servo mounts, and sort out a means of retaining the saddles next to the motor - atm, I'm thinking of heat-moulding some lexan (a bottle) into a battery tray, and reinforcing with fibre tape.

Concerns: The rear of the chassis could bottom out on a very nose-up landing, and there's quite a bit of side-to-side flex (may or may not be a good thing).

[colmo]

I'm finishing the build for the debut test run tonight, and need some help with shocks - I'll either go 35wt or 40wt front and 30wt rear (Schumacher oils, it's all I have), but any suggestions on pistons? I have a mish-mash of Losi and AE ones to use.