4wd rally car diffs

[Busdriver 6209]

I have been getting my Bowler bodied TT02-T up and running. Question what is the best combination for standard diffs. Should it be front locked, rear open or the opposite or both open , both closed etc etc Your thoughts would be much appreciated!!!

[hIghQ 149]

Unless you want to go drifting, do not lock the rear diff. You can either lock the front or at least make it stiffer than the rear for a more stable and predictable handling.

[InsaneJim69 1584]

Locked front produces front end push (understeer) locking the rear provides rear end push (oversteer) . You can lock the rear and then make sure you have lots of steering lock to help counter steer etc. 👍

James.

[Toad16v 282]

In the XV01 it seems to be reccomended that you use a heavier oil in the front diff than the rear. Not quite so easy with a tt02, in my tt01 I have a ball diff in the front but I wouldn't really suggest that as a great option - think the tt01 and tt02 diffs are interchangeable. Either AW grease up front or perhaps an aftermarket oil filled diff perhaps?

[Juhunio 6830]

7 minutes ago, Toad16v said:

Either AW grease up front or perhaps an aftermarket oil filled diff perhaps?

There's also a Tamiya oil filled diff for the TT02; 54875. It's gasket sealed so you could use it with heavier oil up front and lighter in the rear

Or keep the kit gear diff up front with diff putty and add an oil filled diff in the back with a lighter oil?

[hIghQ 149]

1 hour ago, InsaneJim69 said:

Locked front produces front end push (understeer) locking the rear provides rear end push (oversteer) . You can lock the rear and then make sure you have lots of steering lock to help counter steer etc. 👍

Although this is not technically wrong (in theory at least), it should be mentioned that modern high performance 4WD on-road vehicles (i.e. touring cars) all come with solid front axles (spools) and they certainly do not suffer from understeer. In the rare case where people use an optional front diff on these cars, these are set much stiffer than the rear diff (in numbers, rears usually have diff oils between 2,000 and 8,000, while the fronts use 500,000 to 2,000,000.) So much for on-road.

Off-road cars do not use locked up fronts, but it is still the norm for front diffs to be set stiffer than rears.

The only case I have ever seen, where rears are stiffer than fronts (or even rear spools being used) is for drift cars.

[Busdriver 6209]

It’s currently set up with a fairly stiff front and loose rear so I’ll leave at that for the moment. Thanks for all the responses 

[Big Jon 684]

The TT02 has so much steering scrub radius that it pushes too much with a locked front diff and won't turn in hard enough. Mine works ok with AW grease (packed). I'm going to try the sealed diff with 120k eventually.

A TT02 that works on dirt isn't that good on tarmac.

[hIghQ 149]

Don't know if it's because of the different suspension geometry of the Type S, but my TT-02 SR turns just fine (even on the race track) with a fully locked front diff...

[Big Jon 684]

2 hours ago, hIghQ said:

Don't know if it's because of the different suspension geometry of the Type S, but my TT-02 SR turns just fine (even on the race track) with a fully locked front diff...

The Type S has modern TC geometry with very little scrub radius and proper steering axis inclination. The standard TT has no SAI and a massive scrub radius. The Type S drives beautifully!

[Busdriver 6209]

15 hours ago, Big Jon said:

The Type S has modern TC geometry with very little scrub radius and proper steering axis inclination. The standard TT has no SAI and a massive scrub radius. The Type S drives beautifully!

This is a TT02-T. Neither buggy, nor car nor S, so will that make a difference! It’s running CC01 wheels and tyres and has a wider track by about 8mm than my S type?

[Jonathon Gillham 4607]

15 hours ago, Big Jon said:

The Type S has modern TC geometry with very little scrub radius and proper steering axis inclination. The standard TT has no SAI and a massive scrub radius. The Type S drives beautifully!

Can you explain SAI? I want to know more

[Big Jon 684]

4 hours ago, Jonathon Gillham said:

Can you explain SAI? I want to know more

Steering axis inclination (SAI)/kingpin inclination (KPI) is a non-adjustable suspension angle that, on full-size cars, improves centering ability, reduces steering effort, and allows for front wheel braking. 

When viewing the car from the front, imagine a line drawn through the kingpins or ball joints intersecting the ground. A car with a vertical line has no (0°) SAI, while a car with SAI will have the point that intersects the ground well outside of the lower kingpin. Ideally, the intersect will be at the exact center of the tire tread, giving zero scrub radius, meaning that the tire pivots around the center of it's contact patch. The tire isn't dragged forward and backward as it's steered. Scrub radius changes with hex width, wheel offset, and tire height.

On a car with no SAI, unless the kingpins are somehow inside the wheel at it's centerline, there's a large scrub radius, causing the tire to be dragged sideways instead of pivoting while turning. It's easy to see on a buggy like the Top Force, and it's easy to see SAI on a buggy like the new TD4.

It's not an angle that's discussed much, even though it has a major effect on handling. 

[Jonathon Gillham 4607]

11 hours ago, Big Jon said:

Steering axis inclination (SAI)/kingpin inclination (KPI) is a non-adjustable suspension angle that, on full-size cars, improves centering ability, reduces steering effort, and allows for front wheel braking. 

When viewing the car from the front, imagine a line drawn through the kingpins or ball joints intersecting the ground. A car with a vertical line has no (0°) SAI, while a car with SAI will have the point that intersects the ground well outside of the lower kingpin. Ideally, the intersect will be at the exact center of the tire tread, giving zero scrub radius, meaning that the tire pivots around the center of it's contact patch. The tire isn't dragged forward and backward as it's steered. Scrub radius changes with hex width, wheel offset, and tire height.

On a car with no SAI, unless the kingpins are somehow inside the wheel at it's centerline, there's a large scrub radius, causing the tire to be dragged sideways instead of pivoting while turning. It's easy to see on a buggy like the Top Force, and it's easy to see SAI on a buggy like the new TD4.

It's not an angle that's discussed much, even though it has a major effect on handling. 

Thanks for the explanation. So this is fixed by tue design of uprights/c-hubs and can't be changed right? But there must be some interaction with camber as that leans the hub over?

[Big Jon 684]

Yes, it's part of the front suspension design, and can't be changed without changing parts. It's measured on full size cars as a diagnostic aid to help determine if frontend parts are bent.

SAI is unaffected by camber change because the axle centerline doesn't move in relation to the imaginary SAI line no matter how much it changes in relation to true level.