Two speed rear axles

Anonymous · Post by **Anonymous** » Tue Mar 29, 05 2:24 pm

I wanted to ask a question about 2-speed rear axles. Who has one (I think Dave Robson has), and what are the advantages / disadvantages of them? Do they help with long distance mpg? Can they take a good hammering?

Holly · Post by **Holly** » Tue Mar 29, 05 2:56 pm

Dave has an overdrive, which goes behind the gearbox. So far he's yet to say a bad word about it, and he runs 11s to I guess it must be up to the power!

I think he posted about fitting it on the old massageboard, in which case the info will still be there

Dave-R · Post by **Dave-R** » Tue Mar 29, 05 6:18 pm

I love it! I was their second UK sale.

Not as easy to fit as they claim and I found it damaged my trans a bit too but as long as you talk to someone like me that has been through it and are prepared to take your trans apart to double check it will not damage yours then it is not a hard job at all.

Thousand quid to buy one though.

Best mileage is more to do with driving at the rpm your engine makes peak torque than it is low rpms per mph. But that really depends on the engine combo.

Anonymous · Post by **Anonymous** » Thu Mar 31, 05 8:07 pm

George Chiles ran 10's with the same overdrive unit as dave, so they will definatley stand a lot of power

Anonymous · Post by **Anonymous** » Fri Apr 01, 05 8:10 am

So how does it actually work? do you have a second gearing in the rear axle? How do you drive it, do you shift 1-3 in the lower gear and then shift up the axle gear and then go 1-3 in the higher gears?

Im really interested in how these work. I bet it does make a difference to your MPG though, I mean you have a real long way to travel when you come down from Newcastle Dave. What made you buy it in the first place?

Dave-R · Post by **Dave-R** » Fri Apr 01, 05 9:41 am

I bought it so I could use 4.30:1 rear gears and still drive down from Newcastle.

It is not part of the axle. It is an overdrive fitted using a special tailshaft to the end of the trans. See photo below.
It engages and disengages either manually whenever you want with a foot switch or automatically with car speed.

Auto mode it comes on by itself at speeds over 50mph and drops out when speed drops below 27mph. This is so that the car always pulls away in bottom gear.

In manual mode you can engage it at any speed over 27mph.

So normally it goes 1-2-3-over. But once in overdrive the other gears act normally. So if you were driving along at 60mph in 3rd-over and wanted to overtake a car the trans will still kickdown a gear (to 2nd-over) for overtaking.

You can also use it in manual mode to split each gear.
Pull away in 1st.
Switch on the overdrive to get 1st-over.
Shift up and switch off the over drive to give you 2nd.
Switch OD back on for 2nd-over.
Shift and switch off for 3rd.
Back on again for 3rd over.

Total of six gears. But 1st-over is not normally used during hard acceleration because you would only be in that gear for a second or two.

I found the best thing about it is being able to downshift with a footswitch during braking going into corners. Right foot braking hard and left foot kicking the OD off. Car slows down a treat and then you wizz around the roundabout or whatever and kick the OD back on as you come out of the bend. Great fun. And all with both hands on the wheel.

Dave-R · Post by **Dave-R** » Fri Apr 01, 05 9:44 am

If you want to see the effects on mph/rpm it has use this Excel spreadsheet I made.

http://www.challenger440.pwp.blueyonder ... Echart.xls

Just put in your rear tyre size and the axle gear you have.

Dave-R · Post by **Dave-R** » Fri Apr 01, 05 9:48 am

Oh and the overdrive is made by Gear Vendors. Look them up.

Dave-R · Post by **Dave-R** » Fri Apr 01, 05 2:27 pm

Finally. If you really want to do this read all about my experiences fitting it first. (you will have to sign up to my messsage board to read it)

http://challenger.mpoli.fi/forum/viewtopic.php?t=2652

It might put you off!

Anonymous · Post by **Anonymous** » Thu Apr 07, 05 2:24 pm

Thanks Dave, I just noticed your info here and lost track of this post. I tried your excel sheet and i have 10.1 gears in my rear end apparently, if i put this in im doing about 7000 rpm and 60 mph. Now I know thats not right and im sure I have put in the wrong value, they are called 10.1 gears so what is this exactly.

I did have membership of your message board but it must have expired i havent visited it for over a year. Not to worry though

Dave-R · Post by **Dave-R** » Thu Apr 07, 05 2:49 pm

You have 4.10:1 gears in your axle Markie. So you enter 4.1 in the spreadsheet where is asks for rear end ratio.

The only other thing you have to enter is the tyre size in the three boxes along the top line.