Actual Crawl ratios from tyre height and vehicle speed

[Wooky]

Been playing with this to measure actual crawl ratios.... thought it might be interesting to others.

In the zip file attached is an excel spreadsheet.
Enter actual tyre height measured once deflated to your "off road" pressure.
Enter actual vehicle speed at 1000 rpm in low range 1st. Preferably measured with a gps and on level ground.

I think I have the math correct,if not let me know

Cheers

W

[david bfreesani]

Ask Apocalypse to check it for you. He had a formulation on the other forum when we did this exercise.

My Sani is already calculated on my own spreadsheet. 26 meter per minute, or I think it was 1.6km/h if I remember correctly. Already worked out on the 35" tyres.

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[Wooky]

Ask Apocalypse to check it for you. He had a formulation on the other forum when we did this exercise.

Partially why I posted here.... sure Apoc will be along

[Wooky]

My Sani is already calculated on my own spreadsheet. 26 meter per minute, or I think it was 1.6km/h if I remember correctly. Already worked out on the 35" tyres.

Nice. That's where I would like to get to when I am done.

[david bfreesani]

I just realised a flaw in my calculations when you mentioned deflated tyre size. I needed to work out road speeds, so it was not too much of an issue, and then started fidling around to see what my crawl speed would be, but never dropped the tyre size to compensate for deflating the tyres. Should I just calculate on a 33" tyre for the 35s. Most tyres are anyway slightly smaller than the advertised/stated size.

My spreadsheet even takes crank torque and works out what it would be at various places along the drivetrain so that I could see what type of forces I can expect on propshafts and side shafts. I did not bother factoring in power losses of gearboxes though, it was just so that I had a vague idea of what to expect where brackets and crossmembers needed to be built.

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[Wooky]

And not just the difference in "spec" tyre size against measured but also by measuring to take sidewall flex into account.

[david bfreesani]

You got me curious, and I just had to go have a look at my spreadsheet. I was talking a total bunch of rubbish, sorry. My calculations are at 800rpm (idle). And still my figures don't come to what I quoted in my first post. Don't know what I was smoking.
But while we are on the subject, I am attaching my spreadsheet in case somebody can make use of it. It incorporates input of all your gearbox gear ratios per gear, and 2 Transfer cases. Some guys will laugh at my methods, as some of the calcs show directly in cells, like to convert the tyre height to circumference. So be careful when you are inputting that you don't mess with the formulas.

Gear ratio calcs Rev2.zip

(13.01 KiB) Downloaded 135 times

[Wooky]

.... all good I have those moments as well.

Thanks will have look

[Apocalypse]

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.

[Apocalypse]

also, if it's an auto at idle, you won't get a true ratio as the torque convertor clutch won't be locked.

[david bfreesani]

Wooky what you drive? Is that a Rang Rover or an SJ?

[david bfreesani]

Ja, also thinking of a few things. You make a good point to see what the actual rolling distance is per wheel. I however had mine standing on jack stands, so all theoretical. Would it not be better to take the radias of the wheel between the floor and the center of the axle, instead of the diameter/height of the tyre. This would take into account the height lost due to the bulge of the tyre. Or does it not work like that.

[Apocalypse]

Ja, also thinking of a few things. You make a good point to see what the actual rolling distance is per wheel. I however had mine standing on jack stands, so all theoretical. Would it not be better to take the radias of the wheel between the floor and the center of the axle, instead of the diameter/height of the tyre. This would take into account the height lost due to the bulge of the tyre. Or does it not work like that.

doesn't work like that!

the squirm in the tread when heavily deflated tends to increase the rolling radius to about the same as normally inflated - over inflated tends to push it up even higher.

[david bfreesani]

Thanks, I understand what you are saying. Makes sense.
Lets see if anybody picks up any flaws in my spreadsheet. I may have been working things out wrong all this time. But I only built it to get a guideline of forces and speeds for my project.

[Apocalypse]

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!)