Many of us know the feeling: you’re driving on a cold day, or after your latest power mod, and you roll into the accelerator in 4th or 5th at around 2000 RPM. The tach needle climbs, but the speedometer needle doesn’t. That’s clutch slip.
Or, you start your TDI and while it’s idling you hear a faint thumping from the front of the car. When you open the hood the sound is more obvious. Your dual mass flywheel (DMF) has decided it’s done.
Many TDIs are approaching or have already crossed the 150,000 mile, or even 200,000 mile mark. If yours has a manual transmission, the flywheel and clutch assembly may be on its way out. Clutch life is highly variable, depending on driving conditions, and more important, driving style. Some clutches don’t last 100K miles. Others go 250K or more. Like brakes, it depends on how often and how hard it’s used. In this article we will overview popular options and their respective benefits and drawbacks…
A flywheel and clutch assembly is made up of four components: the flywheel, pressure plate, clutch disk and throwout bearing. The flywheel is bolted directly to the crankshaft of the engine, and the pressure plate is bolted to the flywheel. Both components always turn at engine speed. The pressure plate is the clamp that pushes the clutch disk and flywheel together. When the pressure plate starts squeezing the clutch disk against the flywheel, the clutch disk starts spinning. The inner hub of the disk is attached to the input shaft of the transmission. When the pressure plate clamps down on the disk, the disk spins at engine speed, the clutch is engaged, and the car moves.
A clutch can slip for several reasons:
- The friction material on the disk is worn, and even when the pressure plate clamps down on it, it doesn’t create enough friction to prevent the disk from slowing down when engine torque is applied.
- The pressure plate has lost some of its clamping force through wear or overheating cycles, and cannot clamp on the disk hard enough to prevent slippage
- The flywheel or pressure plate surface has been compromised, either by overheating (usually from abuse) or contamination (from transmission or engine oil, or antifreeze) and does not allow the friction material on the disk to work properly
- You’re putting more power through the clutch assembly and overpowering the clutch.
This is common in TDIs as the clutch capacity was not much greater than the car’s stock power, and TDIs are easily modified for more power. A chip tune, re-flash, and larger injectors, or a tune alone in Pumpe Deuse cars, is usually enough to overpower the stock assembly.
Other things can go wrong, too, such as springs failing in the flywheel, the throwout bearing failing, or the lever that pushes the bearing against the pressure plate. These problems can cause the car to:
- Not move because the clutch slips or fails to engage, or because a part of the clutch is broken and loose in the bell housing, preventing the clutch from operating
- Make odd noises (thumping, squealing, rattling)
- Not stop without stalling because the clutch will not disengage
- Be difficult to drive (hard shifting, abrupt engagement, not full disengagement).
When these things happen it’s probably time for a new clutch.
What clutch is in my car?
VW has used several clutch types in TDIs, mostly made by Sachs or LUK. Early TDIs (A3 Jetta and B4 Passat) used a single-mass flywheel and a Sachs clutch. This is not, contrary to what some say, the same clutch as used on VR6 cars from that era, although many suppliers sell VR6 clutches are replacements for the stock setup.
From 1999.5 onwards VW TDIs were equipped with a dual-mass flywheel design. Basically, the inner piece that gets power from the engine is separate from the outer piece that pushes against the clutch disk, and the two halves are attached using heavy-duty springs. This dual mass setup counter-acts vibrations from the engine and reduces vibration at idle and resonances at speed.
Early A4 TDIs (99.5 and very early MY2000 cars) had LUK DMF and clutch setups. Many drivers favor the LUK because it can hold more power (approx 50 ft/lbs more) than the Sachs setup, has a lighter pedal and very smooth engagement, and seems to be very durable. During the 2000 model year VW switched to DMF and clutch made by Sachs. Although it drives similarly to the LUK (slightly heavier pedal, not quite as smooth) it is weaker than the LUK, and slipping is likely once a TDI is modified.
Although they can last as much as 200K+ before failure, the dual mass flywheel design is inherently less durable than a single mass flywheel, or SMF. The springs in the flywheel can fail, causing the wheel to rattle or thump. In some cases the two halves of the flywheel can come apart and can cause severe damage to the transmission or engine. Rattling out of gear, noises or shaking when going in and out of gear are signs of a failing DMF that should be replaced immediately.
Single Mass or Dual Mass?
An SMF, made out of a single cast piece of steel, has no moving pieces and cannot break or fail. SMFs are also less expensive than DMFs. So for reasons of durability and cost, many TDI owners choose to swap to a single mass flywheel and matching clutch kit to avoid problems in the future.
They are referred to as “G60″ or “VR6″ flywheel kits because the flywheel is the same size as what came on the G60 Corrado. As an added bonus, the Sachs kit has a much higher power-rating than stock.
SMF setups do have downsides, however. First, many owners comment on the “rattle” a SMF setup in a TDI makes at idle when in neutral with the clutch engaged. Although this sound is not very loud (you’ll have to have a window down and all accessories off in the car to hear it clearly), some drivers don’t like it. Experts say the noise is actually the output shaft of the transmission rattling, a vibration dampened by the stock DMF. It does no harm. The rattle is louder with lightened flywheels.
Second, the stock DMF does dampen some harmonics caused by the TDI, and owners with a single-mass setup do comment that they can hear noise and vibration at different engine speeds not present with the DMF. This is very minor, however, and not noticed by most drivers.
For these reasons some owners wish to keep the smooth engagement and feel of the dual mass over the simpler single mass design, and upgraded clutch kits for dual-mass flywheels are available. Some Dual-Mass Flywheel kits such as the LUK kit below come fully assembled with pressure plate and flywheel screws already inserted and ready to install.
What’s the best replacement setup?
There are lots of clutch options for TDIs including stock replacements, stock single-mass setups, and upgraded kits. The table below shows some of the options available, flywheel type used, and power-handling ability. Although South Bend Clutches (SBC) are listed here as upgrades, there are many other upgrade manufacturers out there including ACT, SPEC, and DC.
|Flywheel Type||Torque Capacity||Pedal Feel||
|Valeo Replacement Kit||Single Mass||230 ft/lbs. (5 speed)||Stock||Kits are available for all TDIs, including 6 speeds. 6 speed kit handles more torque.|
|SACHS Clutch Kit for DMF||Dual Mass (uses OE)||250 ft/lbs||Stock||Stock replacement for 2000 and later TDIs|
|SACHS G60/VR6 Kit||Single Mass||300 ft/lbs.||Stock||Great for moderate power upgrades, 14, 17, and 22 lb. flywheels available|
|LUK OEM Clutch Kit||Dual Mass||Approx 300 ft/lbs.||Stock or lighter||This was original equipment on ’99.5 A4 chassis TDIs. Stronger than later clutch setups.|
|South Bend Stage 2 Daily for SMF||Single Mass||325 ft/lbs.||Heavier than stock||Organic material on both disk surfaces. Stock engagement feel.|
|South Bend Stage 2 Endurance for SMF||Single Mass||425 ft/lbs.||Heavier than stock||Uses Feramic (sintered iron) on the flywheel side of the disk. Stock engagement feel|
|South Bend Stage 2 Endurance for DMF||Dual Mass (Sachs)||380 ft/lbs.||Heavier than stock||Same as SMF Stage 2 Endurance kit, but uses OE flywheel from MKIV cars.|
When choosing a clutch, first priority is picking one that will handle any current and planned power modifications. Clutches are expensive and require significant labor to install, so it’s best to pick a setup that has enough headroom for future modifications. Keep in mind that manufacturers measure torque handling capability at the crankshaft. Wheel torque ratings will be approximately 15% less than crankshaft ratings.
Sintered Iron? Kevlar? Organic/Metallic? Clamping force? What’s all that about?
When aftermarket clutch companies make upgraded clutches, they usually start with stock components (SACHs pressure plate and disk, for example) and modify them. They have two ways to increasing the clutch’s holding power:
- Higher clamping force
- Better friction materials (better=more grip)
Deciding how to meet torque handling goals is a balancing act between pedal weight (how hard the clutch is to push), engagement characteristics (gradual or abrupt), and wear characteristics.
|Stock: low metal content organic||Like Stock (because it is)||Lowest||Materials are chosen for driveability and long life as long as they meet torque handling criteria|
|Higher metal content organic||Little different from stock||Greater than stock||Higher metal content organic materials provide better grip with little downside compared to stock|
|Kevlar||Abrupt||Higher than organic||Kevlar is favored in competition use because of its quick engagement. This same characteristic can make it a challenge to drive smoothly on the street.|
|Sintered Iron||Little different from stock||Higher than Kevlar||Sintered iron is usually used in combination with organic material (organic on pressure plate side of the disk, sintered iron on the flywheel side). It’s more expensive than Kevlar, but engages more like organic material and seems to last longer.|
Clutch makers also increase clamping force by modifying the spring mechanism in the pressure plate. However, increasing the clamping force makes for a heavier pedal, so some makers limit increases to 20% of so to maintain stock-like driveability. Too high a clamping force can also damage crankshaft bearings.
All companies will be able to tell you what materials they use and how much they increase clamping force on their clutch setups.
What weight flywheel?
It’s very common to use lightened single mass flywheels when seeking best performance in modified cars. A lightened flywheel decreases rotating mass in the drivetrain, allowing the engine to speed up and slow down faster. The engine be more inclined to stall on takeoff, but will also rev more easily and potentially perform better. Gasoline engines can support lightened flywheels will few drawbacks. However, because of the TDI’s combustion event’s violence, a lightened flywheel will transmit vibration to the cabin. Stock TDI flywheels are 22 lbs. Lightened ones can be 17, 14, even 11 lbs. Although there are performance advantages, you may want to try a lighter flywheel in a TDI before you buy.
Sorting it all out
Some closing comments:
- Although dual-mass flywheels are more fragile than single-mass ones, DMFs do last a long time and offer some (admittedly minor to many) driveability advantages
- The strongest power-handling clutch setups use single-mass flywheels
- LUK OEM (VW packaged, not the aftermarket replacement) clutches handle more power than the SACHs equivalent setup.
- Pick a clutch with enough power-handling ability for current and future upgrades
- Make sure you’re not choosing something that you will not enjoy driving because of abrupt engagement or a heavy pedal.
There are lots of upgrade options for TDIs. Sort the data and shop with your goals in mind and you’ll end up with a setup that will be a pleasure to drive, handle your power, and last a long time.