Friday, 23 July 2010

Gear ratios

Yours truly has been doing some thinking about gear ratios for Bitsa in the longer term.

Currently, Bitsa has 4.7 diffs, no overdrive, a normal transfer-box, a series main gearbox, and a 200di engine which gives an estimated 85bhp. The gearing feels decidedly short, with 2nd being suitable for pulling away most of the time, but 70mph (fairly easily achievable) pushing the engine onto the red line for continuous running.

Thanks to a rather handy site at, I've runs a selection of gearing scenarios, to try and think about what will be a good idea in the longer term.

First off, here is a graph showing the ratios in the various gears as she is currently setup.

Now, the one thing which I'm determined to do which will affect the gearing is swapping to 9.00 tyres, when I rebuild onto a new chassis with extended spring outriggers. There are lots of reasons for this, and not just that 9.00's are cool - I want more ground clearance when out laning for instance.

This doesn't look loads better, but it does get 70mph the right side of the red line, at 3733Rpm rather than 4095Rpm. Granted that's not going to revolutionise anything, but its a step in the right direction.

Next question is - what about 3.5 diffs?

This is starting to get to the sort of territory that I'm after - with the turbocharger, there should be no problem pulling this sort of overall ratio, but it gets the engine down to 2780Rpm at 70mph. I had contemplated an overdrive as well, but I think this might be over-egging the pudding...

This puts a 70mph cruse as having the engine at just 2168Rpm. It might hack that, but I'm not sure if it will do the gearbox or overdrive any favours having to deal with an engine constantly developing near peak torque in order to maintain speed.

One of the interesting side effects of this lot is the apparently incredible speed range low box is developing - 3805Rpm in low ratio overdrive 4th is 60mph. Its worth bearing in mind that the current gearing has the engine running at 3510Rpm to achieve 60mph. This could be solved by fitting an Ashcroft hi-ratio transfer box, rather than 3.54 diffs, but then that means no overdrive (its not physically possible to fit one onto the Ashcroft box)...

With the engine at 2829Rpm at 70mph, this is lower geared than 3.54 diffs, but it does still have large gaps between the gears, which an overdrive would have helped with.

A final option I considered was an LT77 mated to a series transferbox. I'm not really very keen (after all, the Series gearbox is responsible for a good deal of the series driving experience, and its also a lot of work to fit an LT77).

Frankly, this isn't high enough geared, with 70mph in 5th at 3104Rpm, when it really needs to be below the 3000rpm mark. It is however really a bit too high to seriously consider in tandem with 3.54 diffs, and I don't think it is possible to include an overdrive in the package either.

So, where do I go from here? Short term, I've got a 3.54 rear diff to fit, as soon as I manage to get round to it. Then I can turbocharge, and see what happens (its going to have to be pretty grim with the 3.54 diff and no turbo). If that seems OK, particularly when loaded (I may fill it with a load of scrap and go for a blast up the M67 hill to make sure), then I'll swap in the front 3.54 diff. Then its a case of waiting for the rechassising session, and looking for a set of 9.00's that are feeling lonely, and seeing how she goes...

No comments: