or Crawler Box as they are called

The crawler box was designed to give the owners of Rovers with LT230 transfer cases a very low range option for off road use, but keep your highway gears in place... basically, the best of both worlds for dual duty Rovers, and a great asset for off road hard core Rovers!

The addition of the Underdrive kit allows you to retain your stock 3.54 gearing for your Rover's use on the highway, but have at your disposal a very low range crawl ratio for your Rover. So you can now drive to the trails at a descent rpm and highway speed, then when you reach the trail, you can have a crawl ratio that is much lower than anything else offered for the Rover.
The latest generation underdrives are better than ever. The images above and below show the prototype unit on our test mule Tdi Defender 90, ECR 4. The shifter housing is now welded on, as you can see in the image above. This solves the leaking problems of the older units, and makes a stronger overall unit.

The housing is slightly larger as well. This means more oil capacity and the new gearset used is stronger and longer lasting for durability both on road and off.

The stock crawl ratio in a Defender is roughly 40:1. On the trail the 40:1 ratio means a pretty fast crawl ratio and that for some of today's trails can cause problems. Problems such as stalling, slipping the clutch, hard start offs on steep hills and the inability to tackle some obstacles. The underdrive will drop your crawl ratio to 116:1 (with 3.54 gears, lower with lower ring and pinion gears). The higher the first number the slower the truck will crawl, 40:1 is average stock crawl ratio, 116:1 is a very slow crawl rate) and that means more control on the trail, and better off road ability for your Rover. If you simply changed your ring and pinion you would cut your highway speed and raise your engine rpms on the highway and you wouldn't get anywhere near a crawl ratio of 116:1. The crawler is the best of both worlds of unbelievable crawl rations combined with unchanged highway performance.

Gear Ratios
 Stock LT230 Low Range 3.321:1
 Low Range with Underdrive 8.933:1

WARNING: Driving a Rover with an Underdrive is highly addictive! If you drive an Underdrive equipped Rover off road just once you will never want to go back to the stock set up, so be careful or you may become a victim of Underdrive withdrawals.


Fitting the Underdrive to your Rover is quite simple. Care must be taken with the pre-load for the transfer case, but the unit assembles quickly and versions are available to fit almost any LT230 application.

The image above shows an Underdrive installed on a 1.41 LT230 transfer case for a Tdi Defender 90 that ECR recently completed. The Underdrive is easily fitted to the transfer case while everything is still in place in the Rover, but this image gives you the location and size of the unit.

Once installed the unit hides nicely in space that really isn't used in the NAS Defenders under the cubby box. See the image above. You can also see the linkage system that ECR uses in the install. The Underdrive itself does not come with any shift linkage, but you know if ECR does the job the linkage and shifter will look and work perfectly and match the inside of your Rover with style.

This images shows a recent Underdrive install on a 1995 Defender 90 Station Wagon. The Underdrive lever comes out near the hand brake lever and has a small range of motion up and down and has full detents for positive shifting. We can even wire in a warning lamp into your existing dash module if you desire.

This image shows our standard linkage set up in a 1994 Defender 90 Soft Top. Notice that this D90 has a Tuffy center cubby box, and that our linkage system works with it and operates smoothly and easily. Keep in mind that the Underdrive is available for any Rover with an LT230 transfer case, not just Defenders.

From another angle you can see just how tasteful and complete the ECR Underdrive install is. The lever takes up space that is not used in the Defender and allows easy access to all your equipment. The linkage is made with the highest quality hemispherical joints for long service and smooth operation. It may not look like much, but you'd be surprised just how much that little black lever can do for your Rover in off road situations!

Here Bill Burtis climbs a near vertical rock section on the "Little Tea Pond Trail" at the Tea Pond 2000 event in July '00 with the help of his ECR installed Underdrive!

A customer who installed an Underdrive in April of 2003 had this to say:
"Well, I installed the unit myself 2 weeks ago, fabbed out a shift lever and in 2 and 1/2 hours later I found out that the device is the single most impressive bit of machinery I've ever played with... cool. Last weekend went down to Moab and did Poison spider mesa, golden spike, gold bar rim and moab rim... no problem at all with stock gears, 3 inch lift, 35 inch tires and one hell of a low range. You guys and Ashcroft are incredible..."
Leo Hardy (via email 4/23/03)

The unit is built to the highest standards and will help you to scale obstacles that you never thought possible with your old crawl rate. This unit really is the answer to the problem of keeping your Rover good on the street, but being able to tackle the heavy stuff off road.

$ Not currently available

The unit is not available for mail order sales or DIY installs through ECR, but if you are doing your own install you can purchase an Underdrive drirectly from Ashcroft Transmissions in the UK.

Demonstrations of the units are available at ECR. We have the latest generation unit going into ECR 5 (Our test mule Defender Project) and wouldn't go off roading without it!

Let us know if we can help you with any of your off road upgrade needs.

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