Well, the equation is a bit scrappy in my opinion, but it gives the correct answer given your value of PI.
(you can just enter the word 'PI' in the equation and it will give you the actual correct answer of 69,56)
HOWEVER - using tyre height * PI as the rolling circumference will not give you a true reflection.
you need to measure the distance traveled per revolution of the tyre - and yes it varies with deflation and vehicle weight.
The only way to do that is to mark the position of the car by hanging a plumb bob from a bumper and marking it, marking the position of both tyres on one axle, ensure the centre diff is locked, and rolling forward until both tyres achieve a complete revolution and then measure the actual distance travelled.
Opps.. sorry missed a 1 in PI
I did the rolling measurement thing an came out fairly close actually.
That's why I just went with the calculated circumference to keep it simple.
In the end it will more than likely get rounded back to no decimal so even my error in PI is somewhat insignificant.
Crawl ratio is pretty easy - it's reduction 1 x reduction 2 x reduction 3 etc.
e.g. first gear x low range 1 x low range 2 x diff ratio.
you should be easily able to find out the T/Case Ratio even if you can't find out the first and diff ratios
you'd be far better off taking speed reading at a higher speed, so stick it in first high and multiply in the T/Case ratio (remembering that a Defender does not have a 1:1 high range ratio!)
Agreed... but I ended up in a situation where I was able to actually crawl at a slower speed than I had calculated using quoted gear ratios like above.
If i used quoted ratios I should have a crawl ratio of 63:1 and at 1000rpm I should be at 2.4km/h with my current size tyres.
I can crawl at 2.0km/h at 1000rpm.
I was trying to work out what my actual; crawl ratio was so that I can work back through each of the geared sections and find where the "error" is.
Wooky what you drive? Is that a Rang Rover or an SJ?
SJ410
do you have 2 transfer cases on it?
No just a single Tcase but with crawler gears.
While I had the case out I marked the input and out put shafts and get 4.25:1 but there is a bit of play in the gear sets.... but that is a whole different story!
Wooky, there is a mod you can do to your TC. You need to get a SJ413 TC, and build portion of it over into your casing. Not sure exactly how it works, but the guys on our SJ whatsapp group was chatting about it. I think there was some pics explaining it. I'll go have a look, and port it for you.
David vd Merwe
1997 2.7TD Nissan Sani SFA
150mm Lift, 33" rubber, dual transfer cases
Wooky, there is a mod you can do to your TC. You need to get a SJ413 TC, and build portion of it over into your casing. Not sure exactly how it works, but the guys on our SJ whatsapp group was chatting about it. I think there was some pics explaining it. I'll go have a look, and port it for you.
Its called a rock lobster. That is currently what I have but from an engineering point of view it is a real bastardised job.
You end up with a cut and welded middle gear in case.
I had ordered and paid for a complete new gear set to be fitted by a reputable seller / 4x4 fitment place. What I got back was an horrific abortion.
Stuck with it for now but looking at getting a new gear set when the funds allow.
It should be 4.16:1 but when I counted on mine it is 4.25 for some reason.
I forgot to count the teeth on the actual gears when I had the case out recently to sort it out so not 100% sure what gears have been used.
I did not realize that stuff had to be cut to fit. The description given on our group makes it sound like you just swop some of the gears over, and it's job done.
David vd Merwe
1997 2.7TD Nissan Sani SFA
150mm Lift, 33" rubber, dual transfer cases
