tractor 4wd drivetrain

[wower]

hello,

i bought myself 2 of those new tyres (from 42129) and thought they go well together with the old 42099 tyres make a tractor (something I always wanted to build) :-)

now my question is (please redirect me if necessary, but I couldn't find such a thread anywhere):

- can you think of any existing drivetrain instructions for 4wd with smaller tyres front and bigger tyres in the back? I really only ever found LEGO tractors with 4 identical wheels..

- since the big ones are roughly 33% bigger in circumference, do I need to have the front axle turn 33% faster with some extra gear combo? or just include a center diff?

 

Many thanks for all possible hints and tricks!

 

[1gor]

Most likely you needed both; 32% overspeed at the front and central differential... For some reason on my Deere 7230R I didn't need front wheel overspeed; but in my case front is 94.8mm and rear 123mm (slightly smaller difference)

https://brickshelf.com/cgi-bin/gallery.cgi?f=535335

It was an old project and I don't have belly photos anymore (didn't think that someone would be intetested in)

 

[2GodBDGlory]

I think you could just get by with a central differential, but the tractor would probably have front torque vectoring because of the greater torque the smaller wheels provide, making it act a bit more like a front-drive than a rear-drive. Like 1gor says, it is a good idea to gear the front up relative to the rear, but I don't think you'll have to be too precise about it if you have the central diff as well.

[wower]

I see.. I like the mechanical aspect, i.e. having 3 diffs and relatively proper gearing. Problem is of course in the end the tractor won't drive properly if so much as 1 wheel is in the air. :-/

[1gor]

1 minute ago, wower said:

I see.. I like the mechanical aspect, i.e. having 3 diffs and relatively proper gearing. Problem is of course in the end the tractor won't drive properly if so much as 1 wheel is in the air. :-/

I had such issue when testing front 95 and rear 107 tire (I had a dream that Lego will release 95mm tractor tire because it was my goal to make JCB 8330 Fastrac), so I ended up with 4220 Fastrac without central differential...still could not figure out the reason (and now I have no time for Lego)