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UTV's Off Road ( RZR, YXZ, Mini Buggy, Carts,etc.) => UTV Chassis and Suspension => Topic started by: mr.bob on December 30, 2012, 09:31:48 PM
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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
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Good questions. I have no answer other than mine have not had rake.
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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! ;D 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.
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Personal opinion only is that front rake is over rated in the first place.
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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!
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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.
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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.
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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/ (http://s1137.beta.photobucket.com/user/mr_bob2/library/)
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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.
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I would have to disagree with your assessment, its all about weight transfer.
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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?
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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 bs1 for weight transfer ;D) is more important.
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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.
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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.
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BTW I would agree with no rake on the rear arms.
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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?
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I believe,haven't really thought about it but,I believe you will lose a little travel with rake due to the compound angle just like with ta's.
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I believe,haven't really thought about it but,I believe you will lose a little travel with rake due to the compound angle just like with ta's.
How would it be different? Every thing would be the same with the mounts,arms and spindle.I do need to keep plunge low because Im using f-150 outer and 930 inters.any info will help
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With rake you will add a second angle into the equation. It will be small and not likely significant but it will slightly reduce travel. I think. Well, on second thought it wouldn't if the axles are perpendicular to the frame at full droop. I assume you will design it so. Soooooo, never mind. LOL!!!!!
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I'm pretty sure you will loose some travel angle. Look at it like a clock. No rake cv would swing from 12 to 6. Add rake now your swinging from 1 to 7, same stroke just not quit as far down. (6 compared to 7). And *8 would be quit that far, prob closer to 12:30 to 6:30 so lose maybe 1/2-3/4"??
What 930 cv are you planning on running on inside? And what length axles?
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Carl,look at it as being at 6 o'clock at full droop with or without rake. Thenn there would be no additional angle to consider.
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Carl,look at it as being at 6 o'clock at full droop with or without rake. Thenn there would be no additional angle to consider.
Agree. But if rake and you set droop at 6 for max travel then some plunge would be induced as it cycled up. Certainly no more than any plunging 930 could handle.
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If you had 18" of suspension travel clocked 8 degrees, you would have 17.825" of vertical travel.
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ok I see, it will take away from droop.Not that it would be less movement but higher so less droop=less travel.
The cvs are from protodie race prep 930s.
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Something to consider is just how much travel is needed. IMO,many cars have more than is needed. How wide is your rear ,flange to flange?
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My point was that the difference in travel is insignificant. With a trailing arm machine, the wheel moves in an arc so it has a significant amount of rake at full droop where quite a bit of travel can be lost. I think some rake/anti squat is good, but is only needed at ride height. That is all speaking in terms of a fixed amount of travel applied vertically or at an angle. It is rare to build a buggy that will utilize all the angle of the CV at full bump and very common at full droop. So CV angle is usually the limiting factor. If you start with level A-arms and a shaft straight out at ride height then rotate the entire assembly to get rake, you'll lose a very small amount of vertical travel. If you want to get anal about it, you can reposition the bearing carrier in such a way as at full droop, the outer CV is still straight out from the inner instead of forward of it. That will give you the exact same droop as without rake. At ride height, the outer CV will then be behind the inner, but won't affect travel. At full bump, the outer CV will be even farther behind the inner which would limit the travel if the CV angle is actually binding, but likely won't be.
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But if you design it so all 4 cv's ,at full droop, are in line (same plane) there will be no loss of travel rake or not.
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Something to consider is just how much travel is needed. IMO,many cars have more than is needed. How wide is your rear ,flange to flange?
the front has 23'' of travel at 70'' wide,but thats at 40* at droop on the bottom aarms and I dont know if 40* is to steep or not.
In back the spool is from protodie and I think its 6'' wide,I havent got axles yet or built anything in back. Im building the 2nd spindle and last front aarm now,mount the rack next then to the back.
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Everyone seems to shoot for 20"+ but IMO most would be better concentrating on a bit less. Maybe 16-18".
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Do you thing 40* on the bottom arms is to steep? 23'' sounds good but Im with you how much do you need. I can strap it to less, Im just wondering if 40 is to much.Could it cause problems?
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At 40* your arms are going in almost as much as their going down.
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Have you worked out bumpsteer at 40* yet?
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no but it wont be long before i start on that.lol
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Depending ona few things you may not get that much droop and seriously I doubt it is needed. Sounds cool though.
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Ive never set bump steer with that much travel it just may suck.12 up and 12 down does sound cool.
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Bob, really wish you'd post up some pics. ;) But in mean time, big wheel travel can be harder to tune suspension on, and the bumpsteer thing too. I would not get dead set on travel in front yet, dont build shock mounts. Cause on the rear, even with a good 930 on inside you prob wont go past 25-28* angle with them. And longest stock 930 axles are 28", so can't get 24" travel in back. (have since found that RCV will do longer ones for not much more than standard ones 1:) No sense have 24" in front with 18" in rear.
We Need Pics!
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Do you thing 40* on the bottom arms is to steep? 23'' sounds good but Im with you how much do you need. I can strap it to less, Im just wondering if 40 is to much.Could it cause problems?
Heim bind would be my biggest concern. When at max droop then you turn the wheel lock to lock.
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Bob, really wish you'd post up some pics. ;) But in mean time, big wheel travel can be harder to tune suspension on, and the bumpsteer thing too. I would not get dead set on travel in front yet, dont build shock mounts. Cause on the rear, even with a good 930 on inside you prob wont go past 25-28* angle with them. And longest stock 930 axles are 28", so can't get 24" travel in back. (have since found that RCV will do longer ones for not much more than standard ones 1:) No sense have 24" in front with 18" in rear.
We Need Pics!
Its a rebuild,the first it was built it was my first build and I ran out of money (had a baby)
so it got a 150 go kart motor so I could drive it.I guess it was better than nothing. So most of every thing was their shock mounts and things like that. I like to think I know more and it time to finish it right.
For pics. all I have is what I took with my phone and put on photobucket so they are small.
http://s1137.beta.photobucket.com/user/mr_bob2/library/
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When you cycle the front suspension ,what camber change are you seeing? What is it at full bump,mid travel,and full droop?
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In the pics. you cant tell camber because I dont have lug nuts yet. With the lower arms at 14* for ride height (witch may change) the camber is -9* bump and +9* droop. If ride height had a little more droop it will go to neg 13* at bump and pos. 7-9* at droop.
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ya you have to fix that.
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ya you have to fix that.
Fix what,and why?
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In the pics. you cant tell camber because I dont have lug nuts yet. With the lower arms at 14* for ride height (witch may change) the camber is -9* bump and +9* droop. If ride height had a little more droop it will go to neg 13* at bump and pos. 7-9* at droop.
-9 to +9 camber change through the stroke? with a possibility of 13? i would think 0 to -3 or so would be best and with that much camber change you'll have bad bump steer too. maybe I misread or missed a post but if the above means neg to pos camber arc through the stroke to those extremes then you need to correct that and get it rained in. this is what I meant.
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I would think camber should match body roll to keep the wheel strait up and down in a hard turn.I dident think about bump steer with that much camber,but if its follows the arc it should be ok?idk
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Listen to DS. He is correct. You have way too much camber change. It must be dealt with now.
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Ok Im here to learn.Why is that much a bad thing -9*+9*?
Im in east texas,no sand,but were I live I have miles of dirt roads with lots of hard curves and pipelines.I want it to handle good in turns.I had about the some change before with 15'' of travel and put a big wing on back and it turned like it was on a rail.
I know it will have body roll because it did before so why would I want to drag a tire around a turn at speed?
If im missing something let me know.
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So the tire goes fromt +9*(tilt out) at droop to -9*(tilt in) at bump? Thats a lot of change. 0* at ride hight going to a - at bump. Some desert and short coarse cars run higher - #'s but cant think of any reaso to have + camber.
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Ok Im here to learn.Why is that much a bad thing -9*+9*?
Im in east texas,no sand,but were I live I have miles of dirt roads with lots of hard curves and pipelines.I want it to handle good in turns.I had about the some change before with 15'' of travel and put a big wing on back and it turned like it was on a rail.
I know it will have body roll because it did before so why would I want to drag a tire around a turn at speed?
If im missing something let me know.
Body roll should never be considered in relation to camber change. On a side note a rear a arm car will inherently exhibit less body roll than a trailing arm. If that +/- camber car handled like it was on a rail a well designed camber curve would have made it amazing. Remember that most of the steering and suspension travel occur when going mostly straight. IMO,and nearly all the rest of the world,you do not want + camber in the equation.
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I see + camber being a little high and I played with the R.A calc. for a few hours and could not get good - without getting + camber.I see how 9*+ is not the best thing to have when it unloads so if I set r.h. at -1 or so that would help some.
What I may do is see were it only gets 3-4* + camber and strap it,I dont need 23 1/2'' of travel.
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Also the upright has to be leaned in the way it is to clear the break,as of now it is almost rubbing I may need to give it a touch with the grinder to be good.
this is the R.A calc,go x2.5
http://www.racingaspirations.com/suspensiongeometry.php?tcx=50&tcy=164&thx=294&thy=164&bcx=50&bcy=230&bhx=320&bhy=263&scl=1 (http://www.racingaspirations.com/suspensiongeometry.php?tcx=50&tcy=164&thx=294&thy=164&bcx=50&bcy=230&bhx=320&bhy=263&scl=1)
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I see + camber being a little high and I played with the R.A calc. for a few hours and could not get good - without getting + camber.I see how 9*+ is not the best thing to have when it unloads so if I set r.h. at -1 or so that would help some.
What I may do is see were it only gets 3-4* + camber and strap it,I dont need 23 1/2'' of travel.
The -1 at ride height is good but you need to be sure that you never have + throughout the entire travel range. See what you have with a reasonable travel of 18-20". If the target camber curve cannot be met then ,no choice about it, you will need to redesign mount points and maybe the arms themselves .
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think of it this way, there is a reason vw got rid of the swing axle design! it pivoted at the trans only so there was extreme camber change in the rear but they only used the middle 4" of wheel stroke so that minimized it for street use or there design designation. when people try and add wheel travel to the swing axle design, it looks like a bird flapping it's wings and at + camber the wheel is not square with the earth for a complete contact patch and the rear now slides or swaps out uncontrollably, but most who still utilize this design don't know any better and must think it is just how it is. the use of the swing axle was for simplicity for most i have to imagine but if you have ever tore down one of those transaxles I don't know how anyone could have thought that! lol lol
in your case for dirt runs I understand you are trying to take into consideration body roll for your reasons of -9 to +9 camber arc being acceptable but have you measured the amount of body roll to confirm that you truly need -9 * of camber? this would be extreme body roll imo. at no point do you want +* of camber at all! this is a bad thing and will amplify bump steer and grossly create problems.
I understand peoples thinking of the tie rod being parallel to the a-arms so it tracks accordingly but that is not correct in all cases. you have to find the sweet spot where the tie rod tracks the a-arm in it's curve/arc. if you cannot find this spot you will have to change the design. simply using the center or small amount of travel say the middle 10" out of 20" capable wheel stroke, to tame the problem down, still requires re-design to make it right, other wise you are just putting a band aid on it or using duct tape to fix something like red green. lol lol
I personally pre-set -.5* of camber at full droop for a total of -3 * at full bump. I square the toe with 10* of caster , 10* of kpi (sai ) and with 3* of anti dive set at the frame ( I use front brakes currently ) I get a very small caster change that effects bump steer ( from the anti dive ) so my tie rod does not run parallel with my a-arms exactly, but close. I have 1/8" of bump ( toe in ) through a 26" wheel stroke but limit the suspension to 24" travel. now I could adjust that bump out to 0 but it's not a big enough problem imo for myself to maintain control of my rail on the dirt or sand nor has it posed a problem on the asphalt either. If I were to introduce + camber I would never be able to control bump in any shape form or fashion that would allow my rail to steer or handle worth a darn.
it sounds to me like you need to cut some parts off and re-locate or re-design and this might not be what you want to hear but is what is needed to make it right. if you can't find this spot on your computer software try it the old fashion way of by hand. move the rack and tie rod to radical spots to see what changes it makes. if you have + camber then I would say you have a problem stemming from the mounting points of your a-arms at your frame in relation to your spindle to start. if your trying to induce - camber with those mounting points then the distance between the upper and lower a-arm at the frame should be less than at the spindle ie. 8.5" at the spindle and 8" at the frame. with kpi ( sai ) built into the spindle upright to get the load applied to the contact patch of the wheel on the ground you desired then you will need shorter upper a-arms to match this design thus unequal length a-arms.
when you push and pull the top of the spindle up right in and out 9* +/- there is noooooooo way the tie rod could ever follow that arc/swing! you MUST
address this problem first if you want to correct the problem.
I am not there to see the problem but if your a-arms are mounted on the out side of the frame itself at a rake or angle to following to the nose then you have 2 angles during the susp stroke. the a-arms are moving up and down and forward and back to an extent and this compounds the problem as well to a point. I looked at you pics but it was hard to see do to the size on photo bucket to help really. for a camber arc that extreme you upper a-arm must not be moving with the lower at the same time or it is arcing forward before the lower is.
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ok I looked again and it looks like your not mounted on the outside but rather on the inside looks to be a center load rack?
if you have soo much body roll that you require -9 to +9 in an attempt to keep the wheel square on the ground then I think it might also be time to re-valve your shocks and add sway bars to minimize or reduce some of that.
I am not trying to knock your rail or design in any way so don't take my posts or replies as negative, I am only trying pass along info to help from my experience with diff designs.
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yes it will have a center load rack.In a few Im going to play with it and see what I can come up with.
Thanks for all the help every one.
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when the rack is centered and where the tie rods bolt on, is the center of the tie rods in line with the pivot points of the a-arms? in other words if you ran 2 pieces of all thread through the tabs your a-arms bolt onto and then took a piece of tape from top to bottom of the all thread, would that tape intersect the center of the pivot of the tie rod at the rack mount point? just wondering is all because this is where it should be to start.
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I havent put it on yet but I think it may need a bump steer bar.But it may depend on the steer tabs,havent put that on ether.
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i am unfamiliar with a bump steer bar. if you don't have steering arms on yet don't bother until you get the camber arc addressed, this will save you time and stress.
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To simplify what ds is saying: correct the camber curve...........................before you do anything else. ;D ;D DS,bump steer bar= rack spreader. ;)
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To simplify what ds is saying: correct the camber curve...........................before you do anything else. ;D ;D DS,bump steer bar= rack spreader. ;)
that makes sense ;D
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the wheel hub was not in flush and the top and bottom arms were not to the rake of the frame.So now it has -6* bump and +5* droop@ 23'' with -1 at r.h..In the last 4'' of down travel is were camber changes fast and scrub.So with 19'' of travel it has + 3* and about 2'' less scrub.
So new #s are -6* +3*@19''.
I did a quick check on bump steer with card board and a pen and it looks good,@23'' it wasent bad ether maby 1/4'' that would be at droop.but @ 19'' I THINK all is good.
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You gotta get rid of that +. All there is to it my friend.
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In an attempt not to change anything what would it hurt to have +3* at full droop?It wont be jumped big if or anything like that.I see why it would be hard on things to land to the side on one wheel with+ anything but other than that what would it hurt.
It looks like bump steer will be fine.
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Think about what happens to the tire as the suspension compresses after full droop and the camber changes. Hint:it has to do with the tire contact patch with the ground.
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Ive read about every post about a-arm camber curve on rdc and most people say no +,but also say you dont want to be on the side wall.If it has no + or has - at droop then it will be on the side wall in a turn.
If you make a hard left turn the outside will be - and the inside will be + to keep the tire flat on the ground.If the same turn the inside is in - then it will be on the side wall.
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As said earlier,if you have that much body roll it needs controlled either with shock tuning, spring selection,sway bar addition or a combination of those. Only vehicles that should be that sloppy are rock crawlers. Believe what you have read elsewhere and we have said here. Some things are pretty much written in stone due to many,many,many trying to prove there was a better way when there was none.
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When I first started to look at long travel suspension design I had the same thoughts you have now. Trust me/us/others ,forget what you are thinking. Eventually I had to just realize that this is a subject that has been investigated since racing was begun and many people much smarter than I was right. LOL!! Damn It!!!