Anti-Dive

[Engineer]

It's been fun....  One less thing to worry about designing into my rail with no front brakes.   


Ok Fabr let's have your $.02.

[455bird]

What kind of sand rails are you running that you are worried about anti-dive? Its sand and it does not work like a street car. You can always make a car ride better but I think some people put too much into trying to get as technical as they can on a sand rail.

[fabr]

*clears throat. adjusts monocle with pinky extended*

As "rake" introduces more dive under brakes one can angle parallel arms so they have negative rake and get the opposite effect. BUT (yes fab) and it's a big but (bug sit back down) it prevents weight transfer to the front which is quite undesirable.

Now to use this principle in practice we only want to add a "little bit" of anti-dive so we can keep our suspension softer for better grip and not hit the bump stops when braking heavily. We only  want a little so we still get weight transfer and the grip it provides.

To my understanding to do this you start with a previously tested amount of rake and then, when looking from the side, angle the top arm back towards parallel, how much you angle it must be tested and the difficulty of having a fully adjustable front end to test is where I think bug is quite rightly coming from.

You gotta explain that to me.I've seen it many times and do not agree.If it is applying a force that resists dive then the stiffening of the front on braking will result in a more firm planting of the front tire upon application of the brakes.If what you are saying is that no dive allows the nose to drop and transfer weight that way I'll agree but do not see that as desirable.Therefore I maintain antidive ,at least to some balanced degree,IS needed.

[fabr]

could not dive be also prompted by weight transfer? would not momentum transfer the force forward with rear breaks only?

Yes but that has nothing to do with anti-dive as it relates to front geometry WITH front brakes.

[fabr]

What kind of sand rails are you running that you are worried about anti-dive? Its sand and it does not work like a street car. You can always make a car ride better but I think some people put too much into trying to get as technical as they can on a sand rail.

Wanna explain that? The principles ARE the same and those principles don't give a rats ass about whether there's sand beneath the tires. It's just as easy to design with or without antidive as it is to "skip"  because "it's the sand and it doesn't matter". Bottom line is no brakes =no antidive. Front brakes = anti-dive is good. The car I'm doing now has none designed in. First time without.I'll likely hate it.

[Yummi]

So, as i dozed off to sleep, the discussion on torque on spindles got me to thinking. 

If the mechanical forces of braking can be applied to stop dive under breaking, can the same forces - torque - be applied under acceleration to create weight transfer? 

Now, if the answer to that is yes can it be accomplished with a rear trailing arm design or does it require rear A arms or five link to allow for the multiple pivot points / radius movement?

Finally, if all this is occurring, it would seem that the suspension, especially one with 15 - 20" of travel would be working hard against the force of the anti dive and or transfer?  In effect negating some of the benefit in an off road application vs street car application as the travel / suspension design is more?  That is, with five inches of travel, you would "notice" 1 inch of dive (20% of stroke) much more than with 20" (5% of stroke.) 

[Admin]

Fabber, did you design anti dive into the BBQ car?

[fabr]

So, as i dozed off to sleep, the discussion on torque on spindles got me to thinking. 

If the mechanical forces of braking can be applied to stop dive under breaking, can the same forces - torque - be applied under acceleration to create weight transfer? 

Now, if the answer to that is yes can it be accomplished with a rear trailing arm design or does it require rear A arms or five link to allow for the multiple pivot points / radius movement?

Finally, if all this is occurring, it would seem that the suspension, especially one with 15 - 20" of travel would be working hard against the force of the anti dive and or transfer?  In effect negating some of the benefit in an off road application vs street car application as the travel / suspension design is more?  That is, with five inches of travel, you would "notice" 1 inch of dive (20% of stroke) much more than with 20" (5% of stroke.) 

It requires a solid rear axle to get the torque reaction through the suspension links.That is exactly how a 4 link rear suspension works and depending on the links length and relative mounting points you can vary the reaction from nearly none to severe. On my drag car I would set it to carry the front wheels about 2" off the ground for 60-100' on launch smooth as glass BUT if I was just wanting to screw around and have wheelie fun I could make a quick one hole adjustment to the top link and pull the front so high you could literally walk under the front wheels.

[fabr]

[Admin]

Yes I did.

Note to self: Fabber has personal experience, Dont doubt his knowledge on anti Dive...

[Engineer]

If the mechanical forces of braking can be applied to stop dive under breaking, can the same forces - torque - be applied under acceleration to create weight transfer? 

Yes.  And they are.  I like to look at it as the pinion gear climing the ring gear.  The tires are resisting movement, and that resistance causes a twisting motion on the rear end (car) / Tranny housing (IFS).  This is why when you hit the gas the front end lifts, and when you hit the brakes the front end falls.

Now, if the answer to that is yes can it be accomplished with a rear trailing arm design or does it require rear A arms or five link to allow for the multiple pivot points / radius movement?

This is where it gets trickier.  With a IRS design, the acceleration has no (read very little) affect on the suspension, because there is no torque applied to the hub (5-link) or trailing arm.  On a IRS the ring and pinion is in a case connected solidly to the frame.  When acceleration or deceleration is applied, it puts that torque directly into the frame.  (Lift or dive).

Now when it comes to braking there can be more consequences.  If the brakes are inboard, they work directly on the frame just like the ring and pinion on an IRS.  If the brakes are outboard, now it depends if your a 5-link or trailing arm.  On a trailing arm with outboard brakes, braking action will try to push the front of the trailing arm down.  It will try to compress the suspension.  It can be argued whether this is good or bad, but the point is some of the torque will compress the suspension, and that in turn will cause some dive.

If you have a 5-link with inboard brakes, then braking force is not induced into the rear suspension.  If you have outboard brakes, then it is and can cause multiple affects depending on the 5-link geometry.  If the 5-link is "parallel" then the affect will be similar to having the brakes inboard, except that when applied the forces on the 5-link will cause it to "tighten" because of the forces induced and friction.  That being said, depending if the 5-link arms are above or below horizontal, when braking force is applied , it will try to bring them to horizontal.  Think about pulling on a rope, it straightens.  When the tire is trying to hold the car back, it is going to try to straighten the link mounting points.

If the 5-link is not parrallel with outboard brakes, depending on the geometry, it could cause the rear to squat under braking or it could cause it to lift.  It will occur from the torque on the rear carrier and its motion relative to the frame.  Similar to how the anti-dive works.

Finally, if all this is occurring, it would seem that the suspension, especially one with 15 - 20" of travel would be working hard against the force of the anti dive and or transfer?  In effect negating some of the benefit in an off road application vs street car application as the travel / suspension design is more?  That is, with five inches of travel, you would "notice" 1 inch of dive (20% of stroke) much more than with 20" (5% of stroke.) 

First, the problem with all these ideas with reference to a suspension that travels 15-20" is that we want anti-dive to work, we want camber to work, etc.  But they are really needed in a smaller part of the travel.  However if you build it in for that part of the travel it will have affects throughout the whole travel.  If you build in anti-dive, it means that you will lose caster the further the suspension travels.  This in turn makes it difficult to control bumpsteer.  Camber change is good, but where is it needed?  Probably a few inches from ride heigth, but we have to limit how much is put in at a few inches from ride heigth, or it will get plum crazy at full bump and full droop.


I believe you are somewhat correct with you analogy to the street car, however their spring rate is much more sever, with only 5" of travel.  We want our buggy's to ride nice over the chop, so the spring rate is much softer in comparison to the weight, and that allows things like acceleration and brake dive to have a bigger affect.

Hope it helps...... Tell me what to clarify.

[fabr]

Good in depth explanation.

[fabr]

[Yoshi]

I think antidive is fine for vehicles heavy in the nose with very little travel, light front ends with long travel just have too many disadvantages in changes as the suspension cycles,...IMO

[Yummi]

Hope it helps...... Tell me what to clarify.

Wow, thanks for all that.  I think I get it.