Anti-dive w/ strut?

[wrheadle]

Guys-

I think I know the answer, but am looking for confirmation. On a typical strut front sspension (MacPherson), is there a possibility for anti-dive characteristics (under braking)?

Thanks

Bill

 

[patprimmer]

Yes.

Regards

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[wrheadle]

OK. So I don't know the answer. Isn't the instant center of he side view swing arm on the axis of the strut, and the intersection with the pivot axis of the lower control arm?

Thanks

Bill

 

[TMcRally]

If you raise the height of the rear pivot of the front lower control arm you will reduce the amount of dive under brakes and reduce the amount of lift in the front end when accelerating.

 

[wrheadle]

I understand how to come up with % anti-dive of an SLA. How does you graphically construct the equivalent for the strut?

Thanks
Bill

 

[NormPeterson]

Assume a fictitious "upper arm" that is always perpendicular to the strut axis at the strut's upper attachment. In both front and side views.


Norm

 

[GregLocock]

By the way, if that sounds a bit odd, it is. In practice your antidive/rollcentre behaviour is modified by several other effects, such as compliance.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[TravWood]

TMcRally
"If you raise the height of the rear pivot of the front lower control arm you will reduce the amount of dive under brakes and reduce the amount of lift in the front end when accelerating"
How does it do both anti-dive and anti-lift with the same change? Especially with AWD/FWD configuration, I was of the understanding that you gained one and lost the other.

 

[TMcRally]

Hi TravWood

Hopefully someone will dive in here if I'm wrong, but my understanding of the theory is the IC when viewed from the side will move forward and up, closer to the Cog height which will reduce the loading and unloading of the front, to in turn reduce dive under brakes and reduce the lift on acceleration.

Dave

 

[NormPeterson]

Anti-dive and anti-lift are anti-motion effects, not anti-force mechanisms. Fore/aft load transfer is essentially fixed for any given acceleration or deceleration, so the greater the amount of the associated load transfer that is carried through the suspension linkage the less is "left over" to go through the elastic path (springs and dampers), hence suspension compression/extension is reduced.

The anti effect is consistent with the direction in which the acceleration is being considered, not a "see-saw" kind of thing where the effect associated with one acceleration direction are traded off against the effect for the other. This does not necessarily imply that the effects are equal, accel vs decel.


Norm

 

[GregLocock]

The reason they aren't equal is that the braking force is effectively applied at the contact patch if you have outboard brakes, whereas tractive force is applied at the wheel centre. This means you will get different amounts of antisquat and antidive from a given geometry. Tricky hey?

Cheers

Greg Locock

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[TravWood]

from experience most subarus (from about '93 at least) will benefit from lowering the rear of the front lower control arm, which reduces lift on accel but tends to increase the dive under brakes. Does this make sense Greg?

 

[TMcRally]

Hi TravWood

I know you have directed your question at Greg and I hope he replies but the ALK (Anti Lift Kit supplied by a number of manufacturers) for the Subaru's are not described accurately by them. The ALK in fact counteracts the anti lift and anti dive that is built in to the cars.

So the kits actually increase lift and dive. Which IMHO is a bit of a smoke and mirrors sell to people. It reduces the understeer in the front on corner exit where the street driver feels an issue with the set up of Subaru's but in fact a well set up car will not benefit from the kits.

Dave

 

[GregLocock]

In what way do you mean 'benefit'? Antidive and antisquat aren't good things in themselves, you add them to get specific outcomes.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[TMcRally]

Hi Greg

The standard Subaru cars have a strong tendency to understeer, it depends on which car you start with but the road going WRX is unmanageable and the competition spec base cars from Subaru are not as bad and every specification in between is a gradual improvement.

With better diffs, good dampers and good spring rates you can get the cars to be reasonably close to neutral in balance throughout the corner.

We tested the ALK's on a well sorted Subaru tarmac rally car. In practice we found the effect to be the same/similar to reducing spring rates. We had less traction in a straight line on starts and on the tighter turns. The car tended to pitch over the front outside wheel into corners and we had an increase in bump steer.

Dave

 

[TravWood]

So if I have the rest of the suspension sorted I should not need any ALK?
For mainly tarmac events starting with a 95 WRX, I was going to be:
increasing spring and ARB rates
improving dampers (ideally independent adjustment of bump and rebound)
Adjustable (lengthen) lower rear arms
Widen front track (later model)
Move strut tops inboard for more camber gain with bump travel
More front caster
Lower the car but also regain some roll centre height back (front and rear)
Reduce bushing compliance in suspension and subframe mounting
Improve all three diffs (suretrac, DCCD, plated)
Increase chassis stiffness with braces (not just typical strut tops bars)
remove some weight from the car (preferable front)

I would also like to improve the geometry of the rear strut to reduce some of the angle (when viewed from the side) but this will be pretty major surgery and more likely involve making new rear hubs.

Have I missed anything? Any of these a bad idea?

 

[GregLocock]

What's ALK?

If you think about what an anti does, it promotes an increase in transient weight transfer during a manouevre.

That can be good or bad, but sooner or later there will be a corresponding decrease in weight transfer as the body levels out. You don't get something for nothing. If you can phase those two effects to provide an increase in overall performance, great. But by themselves there is no 'average' benefit and a simple analysis says that on average they should cost you grip.

As to the rest of your suggestions, you don't mention tires, brakes or aero, which is where I'd be starting (maybe not aero depending on speed). So I'll take those as read.

I've never thought about rally cars here's my comments

increasing spring and ARB rates

why, are you running out of travel?

improving dampers (ideally independent adjustment of bump and rebound)

OK

Adjustable (lengthen) lower rear arms

For more static camber?

Widen front track (later model)

OK

Move strut tops inboard for more camber gain with bump travel

OK

More front caster

why? Some sort of fake Ackerman?

Lower the car but also regain some roll centre height back (front and rear)

OK, but what you gain in ultimate grip you'll trade off against higher spring rates, but may gain back in less ARB.

Reduce bushing compliance in suspension and subframe mounting
OK

Improve all three diffs (suretrac, DCCD, plated)

Definitely

Increase chassis stiffness with braces (not just typical strut tops bars)

Hard to do but yes.

remove some weight from the car (preferable front)

Hard to do but yes.

That is a lot of work, why not buy a better car (seriously)?

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[TravWood]

Ahh, yes sorry brakes are upgraded partially already and tyres will be the stickiest street legal rubber i can get to start with and once I progress, I hope to go to full slicks as wide as I can fit in the guards without adding problems.

Aiming to lower the car to also lower the COG, then to correct the roll centre change this gives I aim to raise the roll centres back slightly, more for better geometry than anything else (get some outside wheel camber gain will roll rather than loose it) . Lower the car will also reduce bump travel hence firmer springs, also firmer springs and ARB to increase roll stiffness again more for keeping good geometry and contact patch through corners and therefore not needing so much static camber (static camber generally only reduces straight line braking capability - as I understand?)
Extra front castor - ackerman?? I understand ackerman but I am thinking the increase in caster will help the camber with steering lock so again, less static camber needed, am I right or missed the point?
Ackerman I am pretty stuck with, as the front hubs will stay and the rack position is difficult to change, although I do intend on making a new crossmember as some point so I will keep this in mind.

Buy a better car??
To buy a better car would require a substantial bigger dollar outlay in the beginning which I don't have (otherwise I would be asking how to make a Porsche GT3 go better or buy an F3) hence I can have something that has 4 doors and can get me a short trip to work (bad public transport) and can give me satisfaction building and improving it for car club level racing, I have no plans (or probably enough skill) to make a career out of it.
This car has cost me $7k and buy the time I am done will possibly spend another $20-30k with modifications, so for the similar price of a current model WRX (ugly) I will have better than an STI performance vehicle and will be able to do it over a period of time. Yes I know its crazy to spend such money on an old car, but hey, its fun, I can keep my family and house and play with it over time.

Aero, will be something I give thought to but will be limited to the basics as I see it at present:
reduce air getting under the car in the first place
get rid of bonnet scoop (adding air under car) by using an intercooler at the front
reduce air getting under car after front wheels
extract air exiting under car smoothly
try and combine air smoothly from above car and under car without creating lift (ideally without a ridiculously big wing)
if any of these aero items can be made to create positive downforce then even better.

Thanks for your comments on my list, it is reassuring I am heading in the right direction with most of it.

 

[GregLocock]

The best thing you can do is to buy something unlovely to use as your commuter car, then you can tear this thing apart.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[TravWood]

True Greg, but then I need a trailer, tow vehicle, space to house it all!! And that will also mean not being able to do some tarmac rally/road events here in Australia due to registration requirements!
The car will progress to be a track only car and I will look to get another commute vehicle but that will be a while off. Unfortunately I am like a lot of people = rich man's tastes/hobbies without the corresponding income!
Like all things there will be compromises.