Rear aarm rake

[mr.bob]

Hey its been a long time,no type.I wanted to know if any of you have rake on rear aarms. Im building a buggy 70'' wide and about 90''wb 900-1000lbs or so, gsxr 750. Ive been thinking about building in about 8* rake in rear aarms(I have 8* in front).So would having 8* act as anti squat and if so how much. I can see how it could by the wheel running under the buggy when hammering on it,but this could make suspension stiff in the ruff. Would it handle better without any rake in back(ruff and turns)?
thanks

[fabr]

Good questions. I have no answer other than mine have not had rake.

[Engineer]

Hey its been a long time,no type.I wanted to know if any of you have rake on rear aarms. Im building a buggy 70'' wide and about 90''wb 900-1000lbs or so, gsxr 750. Ive been thinking about building in about 8* rake in rear aarms(I have 8* in front).So would having 8* act as anti squat and if so how much. I can see how it could by the wheel running under the buggy when hammering on it,but this could make suspension stiff in the ruff. Would it handle better without any rake in back(ruff and turns)?
thanks

Very complex question!    I see what you are saying about anti squat because the more rake that you have, the more lift that the forward thrust is going to apply to the suspension.  If we have rake and you push forward on the bearing carrier it is going to want to go down and thus move the car up, helping resist the weight transfer in the car.

I don't think we have ever talked about this specific side affect of rear rake.

IMO the affects of 8 degrees of rear rake would be negligible with regards to stiffening the suspension in the rough.  My main reasoning is that most trailing arm cars or 5 link cars have at least 8 degrees of rake or more at ride height. A trailing arm or 5 link goes from 20-30 degrees of rake at full extension to zero rake at the point the arm crosses the horizontal plane then to negative rake as it reaches bottom out.  Longer links or arms would minimize the angles involved.  I have never seen it discuss as a design parameter so I would guess that it isn't something that would have major affect or people would be designing for longer or shorter arms to take advantage, or moving the front trailing arm mounts up and down for tuning.

[fabr]

Personal opinion only is that front rake  is over rated in the first place.

[Carlriddle]

I think the rake thing is more needed in smaller tire cars, under 25".  I dont see a reason to complicate life anymore with trying to design it into rear.  I didnt build any rake in rear, but have 7* in front.

Rake desinged into front could be handy if you land nose first on jumps, but better off learning how to not do that cause your luck won't last.  Use more gas!

[fabr]

Rake and acceleration forces,I would think,would work against each other somewhat on the rear of an a arm car. Trailing arms use "rake" to some degree but it's just the nature of the trailing arm design. "Rake" being defined as rearward tire movement on bump in regard to trailing arms. I do not know if it is a benefit or hinderance to performance though.

[mr.bob]

[

IMO the affects of 8 degrees of rear rake would be negligible with regards to stiffening the suspension in the rough.  My main reasoning is that most trailing arm cars or 5 link cars have at least 8 degrees of rake or more at ride height. A trailing arm or 5 link goes from 20-30 degrees of rake at full extension to zero rake at the point the arm crosses the horizontal plane then to negative rake as it reaches bottom out.  Longer links or arms would minimize the angles involved.  I have never seen it discuss as a design parameter so I would guess that it isn't something that would have major affect or people would be designing for longer or shorter arms to take advantage, or moving the front trailing arm mounts up and down for tuning.
[/quote]

To me it seems like aarms would make it stiffer than trailing arms because they are out to the side instead of inline. So it would be able to get out in front of the mounts more. My arms will be around 26'' long on bottom. Not saying you are wrong, just thinking.

If this is so it could be good in some ways and bad,going around a turn it could be good to keep role down a bit, but in woops hadr on the gas could be bad.
Ok what about woops?On the down side of woops would it sweep forward on the down side and be stiffer on the up side?But if it did this it would have more bump travel to soak it up on the up side?

idk like I said just thinking about it,I still have a few weeks before I get to the rear of the buggy.
this is a rebuild of my first buggy if any wants to see.the pics with body panels are the first time around the others are new.

http://s1137.beta.photobucket.com/user/mr_bob2/library/

[Engineer]

Pics are good!  looks nice.

Rake and acceleration forces,I would think,would work against each other somewhat on the rear of an a arm car. Trailing arms use "rake" to some degree but it's just the nature of the trailing arm design. "Rake" being defined as rearward tire movement on bump in regard to trailing arms. I do not know if it is a benefit or hinderance to performance though.

Acceleration forces...  could get complex.  When you hit a bump that bump generally has some angle say 20-30 degrees, That angle combined with tire size, rake and the attitude of the car determines how much force goes into the suspension mounts and against the resistance of the shock and spring package.

Notice that when you go into a whoop section, if you stay on the gas everything works well, but if you drop off, everything goes to shit in a hurry?  What changed?  On the gas you raise the front of the car and increase the effective rake of your suspension.  As soon as you drop off the gas the front drops and you lose effective rake angle.  The rake and weight transfer are really the only things that changed.

[MC]

I would have to disagree with your assessment, its all about weight transfer.

[mr.bob]

Say if I go 8* and build mounts to also go 0*, what would that do for cv angle and plunge at both mounts?One good and the other not so good?

[fabr]

I would have to disagree with your assessment, its all about weight transfer.

+1. While engineers points are valid,IMO,the weight bias shift(fancy for weight transfer  ) is more important.

[fabr]

Consider this,the front of car approaches a bump. When cresting the bump ,on the throttle,the front end is riding high. Maybe 8 degrees or so high. Get my drift......................? You already have 8 degrees rake rear if that's the case. With a well tuned suspension that may not be the case some small percentage of the time but even then will usually still be running front end high/light.  On the other hand ,if running whoops at a steady throttle I'd think a "nuetral" rear suspension would be best anyway. I truly feel that no rear rake is best but am quite curious as to others opinions and why.

[MC]

By far and away the single biggest issue is weight placement when designing the car if you want it to go thru whoops. Note mid-engine cars dont go thru whoops like rear engine cars and the few mid-engine cars that race off road and do well are real long, heavy, and run a bunch of weight behind the rear tires just like the trucks.

[MC]

BTW I would agree with no rake on the rear arms.

[mr.bob]

Say if I go 8* and build mounts to also go 0*, what would that do for cv angle and plunge at both mounts?One good and the other not so good?

all good points so what about this?