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Difficulty
Credit
Blackdog/Getrag
Mod ID
304
Cost
For
450 Fortwo 600cc
450 Fortwo 700cc
451 Fortwo 999cc
452 Roadster 700cc
454 Forfour
450 Fortwo 700cc
451 Fortwo 999cc
452 Roadster 700cc
454 Forfour
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Fortwo 451
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Roadster
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Crossblade
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Forfour 5 Speed Manual
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5.021 |
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12.545 |
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3.308 |
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1.913 |
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3.357 |
Final Drive On The Five Speed 451 Gearbox
Final drive is the fixed ratio that sits between the gearbox and the drive shafts.
All of the figures above have been calculated with final drive already.
For example, first gear of the Brabus 451 is 3.308, multiply this by the final drive ratio of 4.529.
3.308 x 4.529 = 14.981932 which is rounded up to 14.982
Therefore if you take the above precalculated figures and divide them by the final
drive ratio you will arrive at the true gear ratio for each gear in the transmission.
Final Drive On The Six Speed Gearbox
The Fortwo and Roadster gearbox is unlike many other gearboxes as it had two final drives.
Although it is a six speed gearbox, it only has three gears in it. This is doubled by the two final drives.
Gear 1 = Gear 1 + FD1
Gear 2 = Gear 2 + FD1
Gear 3 = Gear 3 + FD1
Gear 4 = Gear 1 + FD2
Gear 5 = Gear 2 + FD2
Gear 6 = Gear 3 + FD2
This made the gearbox smaller but slowed it down as it had to change more cogs.
Reverse Gear
Reverse uses two gears and final drive 1. The engine obviously doesn't spin backward
in reverse which is why it uses two cogs before the final drive to reverse the rotation.
The reverse gears are the same in all six speed gearboxes, from 599cc fortwo to Brabus Roadster.
1.5625 (25/16) x 1.960 (49/25) x FD1.
Brabus Gearbox In A Non Brabus Smart
It still isn't widely known but you can't just put a Brabus gearbox in a non Brabus smart.
What happens is the car is usually happy to drive in gear 1, 2 and 3 but as it switches over to final drive 2,
the gearbox speed is wrong so the ECU calls it up as a shifting fault and shows 3 bars on the speedo.
You can't just mix and match smart gearboxes. The gearbox ratios have to match what is coded in the ECU.
Obviously a 451 gearbox won't fit on a Roadster or 450 fortwo. The fitment is different.
A 450 gear box will fit a Roadster etc but DON'T DO IT.
Just because all Suprex engine gearboxes will fit all Suprex engines, it won't work.
The problem is that the ECU is expecting a certain speed coming out of the gearbox depending on the speed going in.
If the speeds are different from expected (wrong ratios), you will get a transmission error.
Usually it will fail as you shift between gear 3 and gear 4.
Is The 451 Gearbox Faster To Change?
The biggest complaint on the original car (450) was the slow gearchange.
The 6 gears was reduced to 5 so a single final drive could be used and speed up the change.
The old system was controlled by the ECU, the new gearbox has a dedicated controller.
Couple that with the new software and you have a gearchange that is faster than the original.
Keith contacted me with the following information about the 451 gearbox.
The gear change is faster due to it having two motors involved in the changing gear operation.
One disengages while the other engages. This is the same set up as the smart forfour vehicle.
Plus they took a sensor out due to a improvement in gear change.
No, unfortunately not. My remapped 450 changed gear faster than my remapped 451.
So what smart have actually done is removed a gear which affect MPG when cruising at
high speeds but haven't improved the speed of the gear change to a point that can be felt.
A bit of a let down then?
Yeah, it's a backward's step abortion of a gearbox compared to the 6 speed on the 450 and 452.
