In this brief article we are going to discuss the BMW Chassis Mounted Shifter, what its functions are, and how to retro-fit your BMW with a Chassis Mounted Shifter.
There are an infinite number of ways in which you could modify your BMW daily driver for a track day. Apart from essential upgrades like Engine, Tires, Suspension, Brakes that bring about noticeable differences in the way your car feels, there are still a few more seconds you could knock off your lap-time just by increasing the accuracy of your BMW’s gearshift.
Some of us like it old-school and enjoy the feel and satisfaction of a gear-shifting in a manual transmission car. Unlike the modern electronic paddle-shifting cars, a manual transmission’s gear shift accuracy largely depends on the driver’s skill. A less than accurate gear-shift during your lap could cost you a podium finish.
No matter how capable your car may be, if you are unable to extract the best performance during a competition, then it would be a tragedy. So here, we talk about how one could retrofit a BMW OEM transmission with some simple mod’s and get professional level race car-like gear shift quality.
What is a Chassis Mounted Shifter?
A Chassis mounted shifter is gear-shift linkage that is anchored to the chassis in addition to the gearbox.
OEM Gear Shift Mechanism:
OEM cars are always refined in such a way as to isolate the occupants from all engine and transmission vibrations. For this purpose, the OEM powertrain designs incorporate soft vibration-absorbing type of engine and transmission mounts. This way the engine and transmission are secured to the body, but at the same time allowed to vibrate more without transmitting the vibrations to the body.
As a result of this, the gear shift lever is attached only to the gearbox and not to the chassis, since the gearbox is allowed to vibrate with respect to the body. Consequent to this, the gear shift linkage is designed with a certain amount if flexing allowed in order to accommodate for the movement of the gearbox with respect to the body.
In a track situation, gearshifts need to be done in the quickest possible interval so that the engine can be maintained within its power band throughout the race. Given the level of competition at the track, OEM transmissions would seldom provide the level of accuracy that even an amateur race driver would expect.
Racecar Gear Shift Mechanism:
Racecar transmission designs increase gear shift accuracy by mounting the engine and transmission on to the body with almost no isolation, or in other words, ‘Rigid Mounting’. Refinement would not be a priority during a race, hence increased vibrations felt in the car resulting from rigidly connected racecar transmission are never a concern. The fastest lap is all that matters.
This would eliminate the need to allow for any play or freedom of movement in the gear shift linkages. This approach of ‘Rigid Mounting’ is also known as ‘Chassis Mounted Shifter’.
What is a DSSR?
DSSR stands for Double Shear Selector Rod. A DSSR will distribute the torsional forces on both the sides of a selector rod connection. The OEM design, on the other hand, connects from only one side and hence distributes torsional force unevenly. The effect of having a DSSR is a more precise and smooth shift every time by eliminating shift linkage play that is normally observed in an OEM design.
Which BMW’s are a good candidate for Chassis Mounted Shifter?
Amateur Racers tend to choose their racecars based on rather simple high mileage daily-driver BMW’s that come from an era when technology was relatively simpler and have a working manual gearbox. So now, since they are low-tech and easy to work on, you could really customize a lot in the car as per your liking and budget.
Potential BMW candidates would be E30, E36, E46, E90, E92 and E28. You could easily find a decent example in a reasonably good condition. You wouldn’t need to be too fussy about the car being in pristine order, since you are anyway going to throw away a lot of parts as and when you customize your car.
How to install a Chassis Mounted Shifter
In order to install a Chassis Mounted Shifter, you need to follow these steps: (Example illustrations show a BMW E30 Chassis mount shifter replacement)
- Dismounting the Transmission from the vehicle:
The gear shift linkage is located between the body and the top of the transmission. Hence it is difficult to access this mechanism from underneath.
So dismounting the Transmission from the vehicle should be the very first step.
- Replacing your Shift Linkage with a DSSR:
The OE Transmission Shifter linkage, even when new, has a lot of compliance and play. Hence if you do not upgrade this system to a DSSR, you might not get the accurate gear-shift feel that you are looking for in a race situation. As seen in the figure below, the OE system anchors the gear shift lever to the body of the Gearbox, which is a source of play in the gear shift lever.
In the DSSR system, the Shift mechanism is replaced completely and the Gear shift lever is anchored to the Chassis instead of the Gearbox body. See the figure below-
All the joints and bushing replacements for this retrofitment are available as aftermarket parts suited to your BMW model.
- Attaching the Gear shift lever to the Chassis
After replacing the OEM mechanism with DSSR, the Gear Shift lever now needs to be anchored to the body. This is done by bolting clamping plates and tightening them onto the hole provided for the gear shift.
- Gear Shift Length Adjustment:
Most of the Aftermarket parts come with a threaded length adjustment along with a grove and set-screw lock.
You can now rotate the lever to adjust length and then lock the thread using the set-screw lock.
If this adjustment is not sufficient then you may consider Gear Lever length choices available from the same aftermarket source. The important thing to remember is that gear shifting effort reduces with increase in the gear lever length. You may need to try out multiple length settings to arrive at one that suits your driver style.
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- Replacing the Engine mounts:
As explained in an earlier section, the idea behind a Chassis Mounted Shifter concept is to arrest almost all sources of play in your gear shift lever. The first and most primary source of movement is the engine. So your first task is to restrain the engine mounts. OEM engine mounts generally are made of either Rubber or a multi-layered composite of sandwiched Metal and rubber layers.
These have to be replaced with a harder material mount aftermarket solution made for your particular BMW model. The most ideal mount would be made of a material that is much harder than rubber, e.g ‘Nylon’.
- Replacing the Transmission Mounts:
As mentioned earlier, while converting the OE gear shifter linkage you need to dismount the transmission for proper access. While the transmission is dismounted, it would be a convenient time to replace the transmission mounts just before mounting back the transmission.
Similar to the approach used for replacing the engine mounts, now replace the Transmission mounts. These mounts have to be replaced with a harder material mount aftermarket solution made suited to your particular BMW model.
The OEM transmission mounts are almost always made of Rubber. The best replacement material would be Polyurethane. If Polyurethane is hard to come by for your model, you may look for any harder material that has a Shore ‘A’ Hardness 80A.
Other material choices: Hardness 95A, Delrin & Aluminium
In this brief article we have discussed the BMW Chassis Mounted Shifter, what its functions are, and how to retro-fit your BMW with a Chassis Mounted Shifter.
Frequently Asked Questions (FAQs):
Where do I find a Chassis Mount Shifter for my BMW?
You can find several aftermarket brands that provide a matching replacement for your OE gear shifter.
Some of the more well-known are: