Datsun rear squat question

[pumpngo]

Hi I have a 180b sss datsun rally car .I have decided to start modifying the suspension.
I have found a lot of backyard rally guys drive their cars and don't bother changing much. I on the other hand would like to play with the suspension and make as much adjustable as possible. Right or wrong I figure I will learn something along the way.
The datsun squats a huge amount under acceleration. Therfore it must squat under brakes.
I would like to limit this as toe and camber change is a problem.
The centre rear wheel to front mount is 45mm lower at the front. I was thinking of pushing the dif mount down to compensate , which will twist the alignment of the system.
What do you guys think?
Is this angle the bigest factor in squat?
(I have asked this in a rally forum and got responses like put longer springs in)
Thanks

[Al Weyman]

Anti-Dive measures required on front, its only squat at the rear I would have thought. There is quite a lot on this in Steve Smiths Autobooks as I recollect, a lot of it to do with getting a bit of bind into the suspension, not sure it was a long time ago since I dug out the books.

[zac510]

Being a SSS model it should have the Datsun semi-trailing arm suspension?

I understand that the squat is not so bad, it's the camber and toe change that occurs with the squat that stuffs up your handling. The cheap trick is to slot the trailing arm mounts and then once it is all aligned tack weld the washers on. There are more advanced techniques where you can fit on adjustable threads, rose joints, etc to the arm but this mightn't be legal.

I don't think pushing the diff mount down will affect it, but modifying the crossmember bushings would, and I have heard of this done to restore roll centre and increase the potential travel, though mostly on lowered road cars.

Searching for Datsun 510/1600 information should be applicable to your car.

You're in Australia I imagine? Should be tons of help there to set you in the right direction!

[pumpngo]

Yes its a semi trailing suspension. The suspension can be aligned as you stated, but that does not address the problem . No matter where you start the alignment settings, the toe and camber change hugely with travel. Most people just run very hard rear suspension to limit travel. This is a double edged sword as it keeps the toe camber change under control ,but the heavy springs are not what is needed for a rally car on a rough road.
To my understanding the front suspension can be affected using anti dive so much as to have no dive.
With that in mind the rear should be affected just as much with anti squat geometry.
As far as I know the rear is affected under brakes the same as the front, so if you have anti squat in acceleration you have lift under brakes.

[zac510]

hmm, I'm familiar with those concepts but never heard of somebody trying so hard to set them into an old datto. I'd think the same problem would occur, that there is not enough room to move within the regulations and physical boundaries of the car. Best of luck though and keep us informed

[CNHSS1]

my two seater hillclimber has a very similar semi trailing arm rear susp and suffered huge squat on hard launches. the semi T design also indexes the camber change through travel too. when a car is lowered the front mounting point for the trailing arm often becomes lower than the wheel axle centre line (pro squat). essentially it needs to be at least level (zero squat angle) or slightly above (anti squat). by raising the mounting point height, the car will accelerate better and have more traction (rather than the less youd expect due to less weight transfer) as the tyres footprint is kept flatter rather than going into camber change as it would in roll induced compression.
i read up on as much as i could but no 'magic' numbers for the angles to try came up, so i made my chassis mountings adjustable for height. by playing with the heights against the 64ft times (launch distance timing beam on hillclimbs) and datalogger data, it seems that its not too critical but approx 10deg above horizontal works well.
bear in mind that your rally car will ahve more travel than a tarmac only sports car, but you get the idea.
HTH
CNH
EDIT:- Alan Staniforth and Herb Adams books touch on the subject, although its not covered in too much depth

[Zico]

Quite expensive.. but If it was possible, I would bin the trailing arms and fit a baby or full Atlas axle with a leaf spring set-up as per Group 4 mk1-2 escorts.

Would it work?

[pumpngo]

"Hooray"
I have finaly found someone that has been there done that.
I really apreciate the post as I was starting to think it was a stupid idea. Everyone on another rally forum just said forget it and drive the car as it is. Your observations at least make me want to try it as I really dont want to go too hard in the springs.
As far as changing the whole suspension; the rules wont allow that. Actually to the letter of the law , what I propose may not be legal. But under that letter almost all backyard rally cars would fail. legaly you cant slot the crossmember for toe and camber either , but most rally datsuns out there do it so no one gets pulled up.
By the law I can change the diff mount but it is not allowed to change the rotational axis of the rear suspension. It may be spliting hairs but lowering the diff to change the squat will not change the rotational axis any more than having lower or higher springs, and that is allowed.

[CNHSS1]

no worries, i was in the same boat, very little info, so took the bull by the horns and attacked the car with the welder and tried it! on the subject of springs, ive been able to drop the rear spring rates by 20% now, as the rears were over-stiff to try and stop the car bouncing off the bump stops on a decent launch or power-on understeering out of a corner. chassis is much more stable in a corner, and the balance suffers much, much less from sudden throttle changes, on or off when the driver gets it wrong ;-)
Cheers CNH

[boyracer]

I also own a 180B, though not the SSS, just a bog standard sedan (for the moment) but still has IRS.
A book I read on vehicle dynamics seemed to be of the opinion that Anti-squat in a semi-trailing arm design was limited to 25%. Doesn't leave a lot of hope. Ideally you'll need to figure out the CoG location and height, work out the alignment of the rear arms at your desired ride height and see how far off you are. My recommendations would be to 'solid' mount the corssmember to stop it from twisting under power and move the mounts up as high as you dare. I read a similar question on another Australian forum (www.bmsc.com.au) where the rules are discussed in more depth, though I was under the impression that PRC regs allowed you to relocate the suspension points by up to 50 or 100mm from their original location, perhaps this was only for Grp A ?