Improving wet weather traction rwd.
Discussion
Hi guys,
I keep reading contradicting information about lsds in improving wet weather traction for a rwd car.
Do they help get the power down better in the wet, both in a straight line and in corners?
Im not looking to go sideways and drift tbh, im looking to not lose the back end too much but also not have the traction control completely cut the power, basically just get power to the road better in anything other than dry conditions. Ive heard an lsd can actually make the rear more dangerous as a daily driver in certain cases? It all seems to contradict itself to be honest.
The car in question is a bmw m140i. I want to remain safe but ideally get more power to the road.
Cheers
I keep reading contradicting information about lsds in improving wet weather traction for a rwd car.
Do they help get the power down better in the wet, both in a straight line and in corners?
Im not looking to go sideways and drift tbh, im looking to not lose the back end too much but also not have the traction control completely cut the power, basically just get power to the road better in anything other than dry conditions. Ive heard an lsd can actually make the rear more dangerous as a daily driver in certain cases? It all seems to contradict itself to be honest.
The car in question is a bmw m140i. I want to remain safe but ideally get more power to the road.
Cheers
It will help traction by transferring power to the other wheel once one starts to spin (or rather slip). What normally happens in a RWD car is the inside wheel breaks traction first in a turn, and spins. The outside wheel will normally have more traction and could use the power that is being wasted spinning the inside wheel.
Of course the outside wheel only has a so much grip and will brake traction eventually. However the ensuing slide is easier to control with an LSD fitted. I think because both wheels are spinning at a similar rate.
Good tyres are an obvious place to start too.
Of course the outside wheel only has a so much grip and will brake traction eventually. However the ensuing slide is easier to control with an LSD fitted. I think because both wheels are spinning at a similar rate.
Good tyres are an obvious place to start too.
No idea on the LSD front but I will say tyres can make an almighty difference.
In the rain with me in Impreza on sporty road tyres could easily catch and pass my mates M3 on sporty road tyres. When he put his track wets on the difference was unreal, not talking slightly quicker, he could literally casually drive round the outside of me.
In the rain with me in Impreza on sporty road tyres could easily catch and pass my mates M3 on sporty road tyres. When he put his track wets on the difference was unreal, not talking slightly quicker, he could literally casually drive round the outside of me.
To understand what an LSD does, and why it is needed on a RWD Car, you must understand the two conflicting issues affecting diffs, one, to send the power equally to the wheels, and secondly to allow the car to corner with two different wheel speeds as the outside wheel must travel further than the inside wheel.
With an open diff, if one wheel spins, the diff will send all the power to that spinning wheel.
If you are cornering, and apply power until traction breaks ( usually the inside wheel which is unloaded ) and the wheel spins, you will lose almost all cornering traction on the spinning wheel, and all power will be diverted away from the other wheel. This means that the car will drift sideways, but will lose power and speed as the inside wheel spins and spins, and therefore has some self correcting effect on the drift. In many RWD cars with an open diff it is impossible to enter an extended drift or donut due to this effect. This is safe, but limits the power to the grip limit of the inside rear wheel.
With an limited slip, or torque sensing diff, the spinning wheel does not get all the power, the amount of slip is limited.
The safety aspect is with enough power from the engine it is possible to break both of the tyres free at the same time and spin them under power, which will reduce cornering forces available considerably, and result the rear of the car will break free and if uncorrected by the driver spin the car off the road. However, managed properly by a trained driver or sophisticated electronics, the magic 8-16% slip angle it which maximum traction is produced at can be maintained balanced across both tyres and this lends considerable advantage to an LSD equipped car. From the drivers seat, the car is eaiser to drift and balance in the corner as the rear wheel will scrub and start to lose traction gradually, and it is only when being hamfisted or when grip conditions change suddenly that there will be a spin.
Why limited, why not just lock the diff ? In a locked diff, the wheels cannot move at different speeds, and most cornering must be done with a deliberately initiated skid, like a cart or a quad bike. This is due to the conflicting requirement of the diff to have the wheels moving at different speeds as the car corners. In the open diff, there is no friction and this causes no problem until traction is lost. This is why, as most road cars are not designed to be driven at the limits of traction, the open diff is most common.
The limited slip and torque sensing and various other designs are an attempt to balance the two requirements, by allowing two different wheel speeds but not allowing one wheel to spin away the power. There are also electronic systems that brake the slipping wheel and sort of act like a limited slip diff. Most of the mechanical systems try to be open under braking and closed under power but they are not perfect, and especially at low speeds and sharp corners like car parks, they introduce an element of understeer, as the rear wheels don't want to enter the corner and push the front wide.
With an open diff, if one wheel spins, the diff will send all the power to that spinning wheel.
If you are cornering, and apply power until traction breaks ( usually the inside wheel which is unloaded ) and the wheel spins, you will lose almost all cornering traction on the spinning wheel, and all power will be diverted away from the other wheel. This means that the car will drift sideways, but will lose power and speed as the inside wheel spins and spins, and therefore has some self correcting effect on the drift. In many RWD cars with an open diff it is impossible to enter an extended drift or donut due to this effect. This is safe, but limits the power to the grip limit of the inside rear wheel.
With an limited slip, or torque sensing diff, the spinning wheel does not get all the power, the amount of slip is limited.
The safety aspect is with enough power from the engine it is possible to break both of the tyres free at the same time and spin them under power, which will reduce cornering forces available considerably, and result the rear of the car will break free and if uncorrected by the driver spin the car off the road. However, managed properly by a trained driver or sophisticated electronics, the magic 8-16% slip angle it which maximum traction is produced at can be maintained balanced across both tyres and this lends considerable advantage to an LSD equipped car. From the drivers seat, the car is eaiser to drift and balance in the corner as the rear wheel will scrub and start to lose traction gradually, and it is only when being hamfisted or when grip conditions change suddenly that there will be a spin.
Why limited, why not just lock the diff ? In a locked diff, the wheels cannot move at different speeds, and most cornering must be done with a deliberately initiated skid, like a cart or a quad bike. This is due to the conflicting requirement of the diff to have the wheels moving at different speeds as the car corners. In the open diff, there is no friction and this causes no problem until traction is lost. This is why, as most road cars are not designed to be driven at the limits of traction, the open diff is most common.
The limited slip and torque sensing and various other designs are an attempt to balance the two requirements, by allowing two different wheel speeds but not allowing one wheel to spin away the power. There are also electronic systems that brake the slipping wheel and sort of act like a limited slip diff. Most of the mechanical systems try to be open under braking and closed under power but they are not perfect, and especially at low speeds and sharp corners like car parks, they introduce an element of understeer, as the rear wheels don't want to enter the corner and push the front wide.
Edited by ExPat2B on Wednesday 1st July 21:46
Some tyres are better than others - a BMW forum will probably be able to give you some pointers on what's best for that model.
To be honest though, traction will always be an issue in the wet for a powerful front engine RWD car. That's the price you pay for the wonderful feeling of uncorrupted steering and balance that you get.
If you want better traction then get an S3. Traction will be far superior in the wet but that will be offset by a drive that is less challenging and fun.
To be honest though, traction will always be an issue in the wet for a powerful front engine RWD car. That's the price you pay for the wonderful feeling of uncorrupted steering and balance that you get.
If you want better traction then get an S3. Traction will be far superior in the wet but that will be offset by a drive that is less challenging and fun.
I also drive an M140i and run MPSS tyres. Im always confused by reports that these tyres are lethal in the rain. Once worn close to the limit they're a bit sketchy but whilst they've got decent tread I find they grip pretty well. I make pretty good progress in the car and don't find traction an issue unless you want to get the back out. I can't answer your question regarding an LSD but IME your right foot should be all the help you need.
richs2891 said:
Haltamer said:
I would've thought the M140 surely comes with an LSD from factory?
NO LSD as standard on a M140The biggest offender where wet traction is concerned in the M140i is the aggressive throttle mapping in Sport / Sport+ which delivers a big slug of torque in the first inch or so of pedal travel. I found it much easier to accurately meter the power and feel where the traction limit was in Comfort mode, or even the ‘all off’ Traction mode which both had significantly more linear throttle mapping. It allows you to apply power much more progressively, and I found it really surprising how hard the car could accelerate in wet conditions as long as you could avoid that initial surge that would always overcome the grip of the rear tyres.
There is no doubt the car would be much improved with an LSD though, as much for consistency of behaviour as anything else.
There is no doubt the car would be much improved with an LSD though, as much for consistency of behaviour as anything else.
bigenginesmallcar said:
Hi guys,
I keep reading contradicting information about lsds in improving wet weather traction for a rwd car.
Do they help get the power down better in the wet, both in a straight line and in corners?
Its all about torqueI keep reading contradicting information about lsds in improving wet weather traction for a rwd car.
Do they help get the power down better in the wet, both in a straight line and in corners?
A tyre will transmit so much force until it breaks traction. An open diff transmits equal torque to both wheels regardless of speed. If the friction between both wheels and the tarmac is perfectly the same, an LSD wont in theory make any difference, BUT the friction is never the same. In cornering, the loaded outside wheel can take more torque before slipping than the inside loaded wheel, and even in a straight line, one wheel will slip first and then tend to loose even more friction as its smokes, limiting the torque you can apply to the one with friction.
Various types of LSD
A torsen transfers generally about 5x the torque of the faster spinning wheel to the other wheel - problem with this is, if the spinning wheel is on ice etc, it has virtually no torque applied so 5x of no torque is applied to the wheel with grip so no acceleration !
A plated diff uses ramps to apply pressure to friction plates as the speed difference increases to transfer much more (even upto 100%), but can be a bit uncomfortable on a front diff with the steering on full lock as it tries to transfer torque to bring the speeds more equal when you dont really need it to.
A locking diff applies equal speed to each wheel regardless of torque and torque varies with grip (imagine a wheel in the air, its driveshaft would have no real turning force on it at all)
Mr Tidy said:
Well at least in wet weather your RWD car will accelerate instead of struggling for grip like anything with FWD spinning wheels that bounce up and down while going nowhere!
Although an LSD would be a definite improvement, albeit at a cost.
You just cant help but have a little dig at FWD cars. If you can't put the power down in fwd or Rwd it's because you're driving like an Ape an expecting far too much from your tyres.Although an LSD would be a definite improvement, albeit at a cost.
Gassing Station | General Gassing | Top of Page | What's New | My Stuff