Jump to content

Under Drive


Recommended Posts

Can anyone see a problem with the following recommodation by Dave at Ashcrofts it seems to be the answer to my problem ??

With 42 " Tyres Unimog axles & a 3 speed auto basically i need an overall gearing increase of 20% in high & low .

High is not a problem but low cannot be changed aparantly!

So Dave has sujested a Under Drive & run in high range with it engaged as it gives approx the right gearing that i wanted in low range :)

Plus obviously i gain a super low etc.

Any worries ??????????

Thanks F***ker

Link to comment
Share on other sites

Pete had mine fitted for 2-3 years used for events and daily driving,

best ££ spent on the L/R

gives wheel speed exactly the same as the winches pulling so aides momentum

very strong unit compact and easily fitted.

best of all worlds as you keep normal high/Low ranges but add another 2 sets of ratios when engaged.

finally deceents are a breeze rather than a roller coaster ride(unless that is what you want)

Link to comment
Share on other sites

20% bigger tyres?

having 35"s and a tdi mine is underpowered at times when low 1st is too low and low 2nd too high. therefore high range and underdrive engaged and then 1st is a perfect split on my std setup (LT77, LT230) and works well for hill climbs.

guess it depends what the difference is between your high and low range and whether high+underdrive works out 20% higher than normal low.

i have got it wrong though (many a time) and low + underdrive is far too much torque on std LR diffs! mog will be significantly better but thats alot of torque multiplication esp with 42s

Link to comment
Share on other sites

an alternative may be to run a 4 speed box but a three speed 1:1 transfer box- you get 7 separate gears which when used with mog axles and 40"s gives a first equal to somewhere between u/drive 1st and 2nd on a 90, and a high 4th gear just longer than third in a standard three speed rangie (if that makes sense). i have a yank 4 speed auto, a 1:1 t/box, mog axles and 38.5" boots and the gear ratio's are going to be perfect i hope (the calculations certainly suggest that they are going to be useful anyway!)

Link to comment
Share on other sites

Pete, I used to do this to get a 30% reduction on low by using high range and putting the underdrive in. I only used it for short times (some nasty hill climbs) and had no problems. Strength won't be the issue but it might get a bit warm as there's not a huge amount of oil in the underdrive and it'll be used hard. However, if that gave problems I don't think it'd be a massive problems to fit an external oil reservoir.

It'll solve your low range problems but not the high range ones. Oh, and pop it in low / low and the control down hills is unreal.

Link to comment
Share on other sites

or run an atlas 4 speed T case............?

Looked at that but decided that we be the next truck with all Yank bits this seems the easiest option at the moment <_<

Ill stick with the 3 speed fridge as its rated to 1200hp as it is .The underdrive is really a easy out solution on this truck :D

Link to comment
Share on other sites

so is the atlas, its a bolt on from memory to a TF727 like yours (if theres an adaptor kit needed its gonna be less than $400),it would give you twin stick and 4 ratios.................?

tasty, nutritious and mind numbingly bling... :)

Link to comment
Share on other sites

and faintly expensive too- i think it was quoted at about $1400 + delivery for a nice atlas case.

they would help with clearance around yank gearboxes though, which is a bit of a god send compared to the clearance from an LT230!

Link to comment
Share on other sites

wash your mouth out Mark!!!


I contacted Advantaced Adaptor last year & the seemed to know nothing about a box to fit a 3 speed :angry:

Any part No etc


Bit late for this project as the underdrive turned up today :blink:

But would be good for reference ;)

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

We use cookies to ensure you get the best experience. By using our website you agree to our Cookie Policy