Quote:
Originally Posted by robinson.s Hi all,
I have just picked up a 4WD invader. The only issue I find in this vehicle is the crawling speed is high when it comes to heavy decline stretch.
Would be great if some one can help me understand, if I could add a additional low gear? Does the transfer case will have space for that? Or the next option would be to change the existing low gear. And what would be the ratio?
Thanks & Regards |
Quote:
Originally Posted by svsantosh Sharat, I am thinking a petrol engine will retard more (loose RPM quicker) than a diesel. Why dont we bring torque into the discussion. Does the inherent nature of diesels having higher Torque makes it lazy to loose RPMs' and thus inhibit engine braking; a petrol has lot lesser torque to shed... |
Hi Robinson,
The Slowest speed possible on a vehicle with out using brakes is know as the crawl speed.
Crawl Speed = (Engine Speed at Peak Torque Rpm/ Crawl Ratio) X (Circumference of Tyre in Meters)
Gears have 2 effects on the engine and wheels.
1) Slows the engine speed to the wheels
2) Increases the engine torque to the wheels.
Crawl Ratio = 1st Gear X T-Case Ratio X Diff-Ratio
f
or an Invader with NGT520 & 3.73:1 Diff-Ratio (29" Tyres circumference = 2.31mts)
Crawl Ratio = 4.3:1 X 2.46:1 X 3.73:1 = 34:1
Crawl Speed = 1800/34 X 2.31 = 122 meters per minute or 2 meters per second.
For an MM550XD with KMT90 & 4.88:1 Diff-Ratio (31" Tyres circumference = 2.47mts)
Crawl Ratio = 3.986:1 X 2.46:1 X 4.88:1 = 47.8:1
Crawl Speed = 2000/47 X 2.47 = 105 meters per minute or 1.75 meters per second.
For my MM540XD with KMT90 & 5.38:1 Diff-Ratio (31" Tyres circumference = 2.47mts)
Crawl Ratio = 3.986:1 X 2.46:1 X 5.38:1 = 52:1
Crawl Speed = 2000/52 X 2.47 = 94.9 meters per minute or 1.58 meters per second.
For a CJ340 with KMT90 & 5.38:1 Diff-Ratio (29" Tyres circumference = 2.31mts)
Crawl Ratio = 3.986:1 X 2.46:1 X 5.38:1 = 52:1
Crawl Speed = 2000/52 X 2.31 = 88.8 meters per minute or 1.48 meters per second
For a CJ500D with KMT90 & 4.27:1 Diff-Ratio (28" Tyres Circumference = 2.23mts)
Crawl Ratio = 3.986:1 X 2.46:1 X 4.27:1 = 41.8:1
Crawl Speed = 1500/41.8 X 2.23 = 90 meters per minute or 1.33 meters per second.
Notice the difference in the last 4 examples? The effect of the Differential Ratio, Tyres, and Engine Peak Torque RPM on Crawl Speed!!!!!!
The easiest method to reduce the Crawl Ratio is to Change the Diff-Ratio.
For the Invader you can go for 47/11 or 43/10 in both cases you will have to change the differential carrier, spider gears and Bearings.
Why do some engines rev up while descending on idle, it is because the wheels load up the engine and acts like a load governor and the engine revs up to balance the load.
However sometimes its an indication of a weak clutch cover assembly/pressure plate if you are familiar with the vehicle only then can you confirm that.
If 2 engines with the same (nearly similar) displacement i.e 2.1L then the petrol will always generate more torque.
XD4.9P 2112cc = 12Kgm @ 2000rpm
F4-134 2187cc = 16Kgm @ 2000rpm
However a TCIC crde diesel with the same displacement will generate much more torque.
M-Hawk 2180cc = 26Kgm @ 1800rpm
Regards,
Arka
PS - Max Torque at the wheels is peak engine torque X Crawl Ratio.