Fitting an LSD to a M100 Sirion

This is more of an idle ponder than an outright question, but here goes.

I have noticed that on wet roads particularly, my M100 Sirion Rally 2 when driven enthusiastically is quite capable of spinning up an inside tyre as it scrabbles for grip. I have a full set of almost new Toyo Proxes tyres in 185/60r15 size fitted to the factory wheels.

I imagine a combination of light weight, relatively wide tyres (so weight is spread out over a larger contact patch) and open differential is conspiring to cause this behavior. Also, driving in a way not suitable for the conditions on public roads.

However, I know several members on here have said that fitting of an LSD to the gearbox of their respective cars has revolutionised the handling.

My questions are therefore:

  1. Did Daihatsu offer a factory option for an LSD for the M100 Sirion with the K3-VE2 engine?
    b. Where might I be able to find one, and how much is it likely to cost?
    iii. How does one fit an LSD to a car? Is it a self-contained unit by itself, would it be an entire gearbox & LSD unit that replaces the entire gearbox & open diff I have at present? Or is it a doohickey that I need to strip the gearbox down to component pieces and then rebuild to fit?
  2. Do I need anything else, such as different driveshafts to engage with the new diff? Would these typically come bundled in with the LSD itself?

I understand this will tame the loss of traction on the inside wheel and reduce understeer dramatically, but are there downsides? Does it put more strain on other components in the drivetrain?

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I cannot answer some of the questions, but I do know that Daihatsu never offered one from factory. But, D-Sport in japan used to (and maybe still do) make a 1.5 way LSD for the K3 Storia (Sirion).

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As b_hoves said D-Sport is an option. I thought there was a Cusco that also works (might be a Starlet option that was the same).

Yes it will put more strain on things. Also note that a front LSD does not need to be nearly as “tight” as a rear lsd to get a benefit. If it is too tight then things will go from sweet and back to understeer as the wheels fight each other. Fwd with locked diffs are not fun at all and prob only good for drag racing. I have found that when you have a fwd lsd perfect for racing it becomes a cow on the road normal speeds and a cow that will fight you at parking speeds. Why? The steering gets really heavy. A viscous LSD is a nice compromise for road only cars and would be least strain.

I’d contact Hart Performance for Daihatsu M100 gearbox and diff stuff. They have some nice bits and will be at the fore front of advice in this area of what to and how to fit.

Also to increase front traction soften the front and stiffen the rear. Weight transfers diagonally. If the front right spins, then jacking the rear left will put more weight on that tire and hence stop the spinning. Some will perhaps tire of my comments on spring rate, leave the front near standard spring rate and at least triple the rear rate. This alone will reduce wheelspin. If you can alter your damping also set the rebound to about middle and then start going faster damping front a few clicks- try it. Then go slower damping rear a few clicks - try it. Then repeat from the front.

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I never tire of the stiff rear / soft front discussion - it’s worked on my Sirion and am taking that principle to the Fiesta I have also. I feel the issue is, a lot of the “off the shelf” options have too soft rear springs as default, and peoples time and money simply isn’t there to experiment with various chocks, to get correct rate, to then just custom springs made etc.

Regarding the LSD on the street, yes the 1.5way D-Sport can become a bit of a handful for every day driving. Heavy steering in the carparks - check
Clunking now and then - check
Very sudden engagement at times based on throttle / steering angles - check, this one can take you from tracking around a slight bend in a nice arc to tracking an entirely different and tighter arc.

Driving on the track though, that’s where it the massive transformation is.
If my Sirion was a daily driver or weekender but street driven, I would honestly exhaust all other options for front grip - grippier tyres (wider, better compound/type, etc), maybe wider front track vs rear, stiffer rear, ensuring enough droop etc prior to getting the plate-style LSD.

These two MCM videos give a good understanding of the relationship of an LSD inside a manual gearbox on the Front engined, front wheel drive car like ours.
Limited Slip Diff + Clutch Install into 2Sexy [Part 1] - YouTube
Limited Slip Diff + Clutch Install into 2Sexy [Part 2] - YouTube

To answer your questions directly:
As other have said, D-Sport 1-Way or 1.5-Way is the primary options.
The LSD sits inside/within the “gearbox”. You could pull yours out and open it up to replace the open differential with the LSD. Or build up a secondary gearbox with different gearsets+LSD to then just swap one for the other (depends on the effort/expense).
Whilst you’re in there, you can change the Final Drive for one that provides more acceleration at the expense of top-speed* (noting that the gearing is very long and the theoretical maximum speed of the factor gears is basically impossible). *however, if you do a lot of highway driving this will seriously impact fuel economy.
I did not change driveshafts. Factory items are holding up apparently fine so far. It is effective a track-only car and the driving it does on the street is to and from various workshops / tracks etc.

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Thankyou for your comprehensive reply.

The comment about stiffer rear springs makes total sense and sounds like an alteration that is more within my skillset. My current dampers are not adjustable, but it is not difficult to change these if needed.

I will contact some custom spring manufacturers to enquire about prices for standard-length springs with a rate of ~400 ft/in (7.1KG/mm) - which from looking elsewhere seems to be your recommendation if I have understood correctly?

Is there any documented specification for a standard sirion/storia rear spring I could take with me, or is the best idea to remove a rear spring and toke it with me and say ‘I want a pair of units identical in height and diameter to these, only with a rate of ~400 ft/in (7.1kg/mm) please’?

I will try to change one thing at a time, so first springs and then see if I need different dampers as well. This is a road car primarily, I enjoy driving it enthusiastically but I don’t want to ruin its drivability in the real world.

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This is an example of the King Springs Special Coil Procedure - I’m sure other spring manufacturers would need similar info. Yes taking in the factory one would be the easiest option.
From what I understand though, the stiffer spring needs to be physically shorter to achieve the same ride height. The full droop of the rear beam axle (governed by the full shock extension length) will need to keep the shorter coil captive. Therefore, shorter shocks might be needed - I believe people use KYB Excel G Hyundai X2 Shocks, or Suzuki WagonR or similar to achieve this.
But with minor modification there might be more shocks with Eye top and bottom with metal bush and a 12mm hole will likely fit and have a shorter extended length.


*the data pre-filled is for my ride height etc which is already on “coilovers” (not that rear is)

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Actually the coils are often kept the same length if going for more rate at exactly the same height (bigger wire or less coils achieves this). In the image below, note how the top of the spring has some coil close together, this is a lowering spring. This is done to ensure they are kept captive. With weight on the car the top part coil binds once full weight is put on the spring. You get less bump travel but it ensures the spring won’t dance around at full droop. If you did put shorter shocks in, then let the spring manufacturer know how much less drop there is. Spring like in the picture are quite a bit heavier than really needed as a lot of steel just does nothing much more than stop travel.

image

I did fail to mention that on a FWD a limiting the drop of the wheel with shorter shocks/dampers can work really well. Remember weight that transfers diagonally. So if a inner rear wheel comes off the ground all that weight is going to the outside front wheel. That’s the one that is loaded. Only the outer rear wheel needs to be on the ground to stop the car falling over.

400-450lb/inch works on a Mira. With the trailing arm being a lever rate at the wheel ends up perhaps half that. I actually changed the seats for a coil over type 2 1/4" base (you can then use an adjustable base also - this is not just to adjust height but also for corner weighting). That way I could use a heap of off the shelf springs a mate has. Another way to help figure out rate would be to measure coil diameter, number of coils and lengths and put the data into an online rate calculator. Then start eliminating coils. I do this but putting a “packer” or “rubber” in the coil. Drive it and note the change. Recalculate based on having one less full coil and see what the rate is. Then put another in and repeat until you get near a rate you like. Here’s some examples.

The above shows packers that have eliminated three complete coils which will have bought the rate up a heap (like three times more).

image

These ones are what NASCAR uses and they can pull them out in seconds during pit stops to change the balance of the car a track, weather and the car itself changes.

Where would you get these for yours? Just some big lumps of timber that fit between the coils. With no shock in the middle of the spring just poke it right through the middle. Do drill some holes and put some big zip ties through. Be careful and aware that this is for “testing and tuning” and is not permanent. For legal reasons, I am going to say do this on a close private road.

I hope this has not confused things.

I like an LSD on the road. However, I don’t do a lot of traffic and not in suburbs much. My street is 20-30 tight corners to get up 4-5km with a gain of 300m in altitude. If it rains you can get stuck if the diff is open. Not sure about the D-sport, but with the Cusco you can alter the clutches to reduce chatter and get an action either progressive or aggressive. But it does mean pulling the diff/gearbox in and out. I wouldn’t get a 1 way. The 1.5way gives a better connection between the front wheels under braking which can reduce one wheel locking up.

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Makes perfect sense. If you want to keep the same ride height they’ll need be the appropriate length to support that ride height.

My comments about the short springs requiring shorter shocks was due to finding some ‘off-the-shelf’ stiff springs designed for other Daihatsu’s where the spring length is quite short (100-150mm). Therefore, even if it’s not the right way to go about it, if you start with the spring and make everything else fit around it, you’d need to keep in mind the spring remaining captive, possibly with a helper spring or the adjustable spring seats making up the difference. This is also made ‘worse’ in my instance as the BC Racing rear shock I have is not threaded to adjust its length the same way I see other Daihatsu BC Racing rear shocks. I presume you would have the same issue if you had short springs and OEM length shock absorbers, whereby the spring was not kept captive.

Fair comments about the LSD - it certainly would have its place on the road in some applications. That back street sounds more like a hillclimb than a suburban street :slight_smile: :slight_smile:

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