Anti-dive basics?! 4

[colinmseries]

Forgive me if this is already explained elsewhere, I've looked and can't recognise an answer.
I have a car with horizontal, parallel, twin wishbone front suspension (a TVR) and the front is to be lowered by 38mm, which over the wheelbase equates to approx. 1deg slope to the front. The rear is to remain standard.
I notice that that this also inclines the chassis pick-ups by the same angle and wondered what effect this would have with regard to anti/pro-dive. All I've been able to teach myself so far refers to wishbones where one is inclined relative to the other, here mine will still be parallel but sloping down to the front. What have I introduced into the suspension?
I have moved the upright's top and bottom pivots down by the same 38mm and so assume I have not changed the front rollcentre?!
Thanks in anticipation, Colin.

 

[GregLocock]

As I understand it you've effectively fitted shorter road springs?

The roll centre will rise, with typical geometry, by about the same amount as the wheels go up. That is the roll centre height gain is typically around 100%

As to braking, you've reduced the CG height by 19mm so you'll get less load transfer, which is good, but I find it hard to believe that you've changed the anti's all that much.



Cheers

Greg Locock

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

 

[NormPeterson]

I'm picturing knuckles with the stub axles raised 38mm relative to the balljoints, leaving the front view inclinations of the arms essentially the same as before.

The RC will drop somewhat, since the FVIC remains at the same lateral position but also drops by 38mm just like the chassis pivots and balljoints. How much the RC drop amounts to will depend on the FVIC lateral position and the track, but I'd guess somewhere between zero (for arms that are truly parallel in front view) and a few mm (if they converge slightly going toward the chassis).

As to anti-dive, I think it's increased slightly, perhaps 8% or so due to the change in side view inclinations and maybe another 4% due to the reduced CG height.

I also think that there will be slight losses in both anti-squat and anti-lift at the rear.


Norm

 

[GregLocock]

OOPS sorry, I meant -100%, Norm is right

Cheers

Greg Locock

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

 

[colinmseries]

Thanks for your replies. I've read the FAQs and understand the issues regading the forums not being a substitute for professional advice. I have however, tried hard to find a coach and the material to teach myself, both without much success.
If anyone could direct me to someone who professionally could advise I would appreciate it, as I am realising it's going to be a long road getting to fully understand the issues for myself. Thanks again, Colin.

 

[Fabrico]

There are lots of books on that and related subjects, try a search for automotive suspension books or for racing.

The simplist way to picture dive is to think of it as self actuating. If the wheels move up and back, then hitting the brakes will simply cause them to move easier and farther in that same direction. If the wheels move straight up, or, up and forward, they will be moving at an angle to the dive force. You will be very slightly raising resistance to dive.

 

[GregLocock]

or you could email me via the link on my website

http://www.geocities.com/greglocock/

The best book for this is probably Gillespie, but in general most people seem happy enough with Milliken and Milliken.



Cheers

Greg Locock

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

 

[BillyShope]

Does the front RCH change at all? Initially, it was at road surface level. Seems to me you could rotate 45 degrees...as long as the arms remain parallel...and parallel lines still meet at infinity.

 

[boatmotorgod]

If the control arm chassis mounts are parallel from a side view then there is no anti-dive, stock or lowered. I believe anti-dive comes from a change in caster. If there would be a positive change in caster during braking, then a load would work against the direction of the brake rotor to hold the chassis up(anti-dive). A negative change in caster would work with the brake rotor rotation to pull the chassis down (pro-dive). Parallel mounts have no caster change hence no anti-dive.

 

[NormPeterson]

If the chassis mounts are parallel, all that means is that the anti-dive is not progressive by virtue of any length change in the SVSA. The "ramp analogy" described by Fabrico still applies, as the ball joints will be moving forward as they rise relative to the chassis.

Kind of a separate issue, but I guess that we have all been assuming that the static caster spec can be restored following the -1° or so change in rake.

With respect to RC height, it's somewhat less than clear (to me at least) whether the planes containing the arms are parallel or if the parallelism is limited to just the side view projection of the chassis pivot axes. Hence the 0 to some few mm range (I will assume that the arms would converge toward the chassis in front view rather than diverge).


Norm

 

[colinmseries]

Thanks for this so far, I think I'm getting there. My chassis mounts are parallel in side view and the arms do converge in front view as Norm assumed. The original static front caster of 3deg. is to be replaced with the appropiate new top arms.
Could you please confirm/correct my understanding:
- the parallel, in side view, arms slope down to the front and will cause an amount of anti-dive at the front and pro-squat at the rear?
- that though able to work as inclined parallels, the anti features desired are better found through non parallel arms?
I ask this specifically as in my research I've found a number of articles stating that for anti-dive the front top arm has to incline with its front pick-up higher than its rear ie. the opposite to mine.
As a general point, would you say the original horizontal, parallel arms I started with are a better compromise or are the advatages of anti-dive and anti-squat well worth the trouble?
Thanks, Colin.

 

[GregLocock]

If the car dives under braking at the moment it might be worth worrying about, but I suspect that (a) it doesn't and (b) bear in mind that antidive is robbing Peter to pay Paul, you may like the extra initial bite it gives you, but a second or so later you'll have to pay back the load transfer you've just enjoyed.

Cheers

Greg Locock

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