What are the advantages, if any, of using push/pull rods when compared to regular coilovers in a FR sports car like an RX-7, Corvette, etc...? Is it just packaging efficiency (in an open wheeled race car) or does it offer any kinematic advantages? Details would be nice... thanks
What are the advantages of a push/pullrod suspension layout?
Started by
Earthling
, Jan 31 2002 23:26
18 replies to this topic
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#2
jpf
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Posted 01 February 2002 - 00:14
Well, for one thing, don't poo-poo packaging efficiency, because for open wheelers, that's really just another term for "aerodynamic efficiency" which we all know is a big deal.
Another advantage is that when using a push/pull system, there is a rocker arm between the actuating rod and the spring/damper assembly. So by varying the leverage that the actuating rod has on the spring/damper, you can create a situation where a small rod motion is translated into a large linear motion at the spring/damper, which can allow for significantly more precise and accurate control of the damping. You can also change the leverage in an effort to change the spring stiffness needed for a given wheel rate.
Another thing is, with pushrods, lots of times the dampers are sort of high up (not great for c of g), which makes them very accessible for adjustments or changes.
It's also conceivable that the smaller size of a push/pull rod in the midst of all the wishbones and such could allow for different suspension geometry. I can especially see this as being relevant in locating the steering arm where you want it without having to worry about a bluky coil-over in the way.
Hope this helps! Now let's wait for the experts to chime in...
Another advantage is that when using a push/pull system, there is a rocker arm between the actuating rod and the spring/damper assembly. So by varying the leverage that the actuating rod has on the spring/damper, you can create a situation where a small rod motion is translated into a large linear motion at the spring/damper, which can allow for significantly more precise and accurate control of the damping. You can also change the leverage in an effort to change the spring stiffness needed for a given wheel rate.
Another thing is, with pushrods, lots of times the dampers are sort of high up (not great for c of g), which makes them very accessible for adjustments or changes.
It's also conceivable that the smaller size of a push/pull rod in the midst of all the wishbones and such could allow for different suspension geometry. I can especially see this as being relevant in locating the steering arm where you want it without having to worry about a bluky coil-over in the way.
Hope this helps! Now let's wait for the experts to chime in...
#3
Bex37
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Posted 01 February 2002 - 02:31
Reduction in unsprung weight.
Moves the springs and shocks inside the bodywork for better aerodynamic efficiency.
Moves the springs and shocks inside the bodywork for better aerodynamic efficiency.
#4
Ray Bell
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Posted 01 February 2002 - 02:52
All true, but to open up another area, the linkages may be set to permit an effective rate-rise in the springing. As the spring compresses, which is naturally always a regular progression (ie. 1" = 200lb, 2" = 400lb etc) unless the spring is progressively wound, the angles of the links and the leverage effects involved will cause the effective spring rate to rise.
#5
BRIAN GLOVER
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Posted 01 February 2002 - 05:19
The Corvette has transverse leaf springs and not coil overs, and like the pull/push rod suspensions, loads are taken near the centre line of the car reducing torsional loads on the chassis.
Appart from the afore mentioned reasons for their use, a few more things to consider. The push rod is more commonly used in F1 and Gt1 and open wheel race cars, rather than pull rods.In the case of sports cars, the loads can be fed vertically downward via bell cranks to reinforced members with coil/overs to lower CG. F1 cars use horizontal torsion bars in the front, so the CG is higher and roll is controlled by a longitudinal torsion bar and a vertical shock.
The advantage of a push rod over a pull rod, is that the a arms stay in tension, whereas the pull rod a arms are in compression, which requires a heavier section. Also pull rods have more opporational friction and binding takes place in roll control and retains a certain degree of coupling in roll and bump. Minardi used pull rods for a while.
Unfortunately in F1 cars, the trade of for aerodynamic advantages of the high nose, forces designers to raise CG for suspention loads and also pedals and drivers legs are higher than they would like.
Appart from the afore mentioned reasons for their use, a few more things to consider. The push rod is more commonly used in F1 and Gt1 and open wheel race cars, rather than pull rods.In the case of sports cars, the loads can be fed vertically downward via bell cranks to reinforced members with coil/overs to lower CG. F1 cars use horizontal torsion bars in the front, so the CG is higher and roll is controlled by a longitudinal torsion bar and a vertical shock.
The advantage of a push rod over a pull rod, is that the a arms stay in tension, whereas the pull rod a arms are in compression, which requires a heavier section. Also pull rods have more opporational friction and binding takes place in roll control and retains a certain degree of coupling in roll and bump. Minardi used pull rods for a while.
Unfortunately in F1 cars, the trade of for aerodynamic advantages of the high nose, forces designers to raise CG for suspention loads and also pedals and drivers legs are higher than they would like.
Originally posted by Ray Bell
All true, but to open up another area, the linkages may be set to permit an effective rate-rise in the springing. As the spring compresses, which is naturally always a regular progression (ie. 1" = 200lb, 2" = 400lb etc) unless the spring is progressively wound, the angles of the links and the leverage effects involved will cause the effective spring rate to rise.
#6
Earthling
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Posted 01 February 2002 - 06:38
I guess I should have explained myself better when I posed the question... If one were to commence design of an FR sports car from scratch, what would be the pros and cons of using pull/pushrods. Again, this is to be a closed wheel FR sports car a la Corvette or RX-7, with ~400bhp, 2450lb weight, and a strong emphasis on handling.
#7
Ray Bell
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Posted 01 February 2002 - 07:33
Cons include additional wear points, ball joints etc that carry high loads don' t last forever.
#8
jpf
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Posted 01 February 2002 - 16:20
Pros would include the stuff mentioned -- multiplication of wheel movement at the spring/damper (for more or less movement), adjustability, reducing unsprung weight, and more flexibility in building a rising-rate geometry.
#9
cpurvis
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Posted 01 February 2002 - 17:17
don't forget lower polar moment of inertia.
#10
BRIAN GLOVER
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Posted 02 February 2002 - 15:44
What materials will you use? What budget? What are the desgn objectives?????? You mention 400hp and 2350lbls. How did you come by that? A Corvette has a perimeter frame that will twist like a pretzel if loaded at the corners, so corner mounted coil overs is out. A stock Z06 wighs 3200 lbs but the C5r at Lemans was 2500lbs. RX7 is a unibody. What is your preference in chassis design? Will you go with triangulated steel space frame?. Square or round tube? Will you work with composites? Or a 2 tube reinforced composite tub.
Rectangular box sections connected to fabricated bulkheads with employment of adhesive bonding two part epoxy offers cheap and effective construction and tremendous torsion stiffness and stops longitudinal bending. This will lighten your car considerably.
This chassis is found Opel speedster and Lotus Elise. If you go with a backbone or ladder frame like in a Lotus Elan you would definately would have to go with push rods. Will you design with a CAD? Good Heim joints cost $60 a piece. You may need 24.
Custom bushes can go into the thousands. You may have to try many shock and springs rates and suspension geometries. A cheap set of coil/ overs with cost $iooo. You may encounter packaging problems.
What ever you decide, start with the tires. Learn all you can about them even if you have to measure certain properties yourself. Tire manufacturers are not good at passing this information on to customers. The whole car is designed around the tires. take great care choosing them.
Is it a steet/`track car? Talk to a lot of chassis designers. Many colleges world wide have race car design courses. Do your homework. Will the uprights be your own design or will you use donar parts? What rear end will you use.Have you got milling machines?
Read some books by Alan Staniforth and talk to Ben.
Push rods are the way to go on much lighter cars than 2500lbs, but a Vette doesnt use them and niether does a Viper. No production cars can beat them on the street.
Few production cars use them but more and more Hot Rods are using them such as the the Prowler by Plymouth. But that is usually only for sex. 
QUOTE]Originally posted by Earthling
I guess I should have explained myself better when I posed the question... If one were to commence design of an FR sports car from scratch, what would be the pros and cons of using pull/pushrods. Again, this is to be a closed wheel FR sports car a la Corvette or RX-7, with ~400bhp, 2450lb weight, and a strong emphasis on handling. [/QUOTE]
Rectangular box sections connected to fabricated bulkheads with employment of adhesive bonding two part epoxy offers cheap and effective construction and tremendous torsion stiffness and stops longitudinal bending. This will lighten your car considerably.
This chassis is found Opel speedster and Lotus Elise. If you go with a backbone or ladder frame like in a Lotus Elan you would definately would have to go with push rods. Will you design with a CAD? Good Heim joints cost $60 a piece. You may need 24.
Custom bushes can go into the thousands. You may have to try many shock and springs rates and suspension geometries. A cheap set of coil/ overs with cost $iooo. You may encounter packaging problems.
What ever you decide, start with the tires. Learn all you can about them even if you have to measure certain properties yourself. Tire manufacturers are not good at passing this information on to customers. The whole car is designed around the tires. take great care choosing them.
Is it a steet/`track car? Talk to a lot of chassis designers. Many colleges world wide have race car design courses. Do your homework. Will the uprights be your own design or will you use donar parts? What rear end will you use.Have you got milling machines?
Read some books by Alan Staniforth and talk to Ben.
Push rods are the way to go on much lighter cars than 2500lbs, but a Vette doesnt use them and niether does a Viper. No production cars can beat them on the street.
Few production cars use them but more and more Hot Rods are using them such as the the Prowler by Plymouth. But that is usually only for sex. QUOTE]Originally posted by Earthling
I guess I should have explained myself better when I posed the question... If one were to commence design of an FR sports car from scratch, what would be the pros and cons of using pull/pushrods. Again, this is to be a closed wheel FR sports car a la Corvette or RX-7, with ~400bhp, 2450lb weight, and a strong emphasis on handling. [/QUOTE]
#11
Earthling
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Posted 02 February 2002 - 20:07
The general brief for this car is that it is to have a very low weight, blisteringly fast performance, excellent handling, and above all, an unbelievably low price (for the performance), somewhere in the region of $50k... Of course, equal weight distribution, low C of G and PMI are priorities of the highest order. I'm taking the 3rd gen RX-7 as a dynamic benchmark, as having driven one, I doubt there are any better handling FR cars out there. Except this one is going to be smaller, lighter,(much) faster, and even better handling!
Approx dimensions are a >420cm length, ~175cm width, and a ~115cm height
Proposed drivetrain is probably going to be, and I know you'll like this Brian, an LS1 with either a T56 or an MM6 tranny, I still don't know if I want the tranny right behind the engine or on the rear axle a la Corvette... Its probably going to come down to cost and packaging. ~400bhp is going to be no sweat for an LS1 with a mildish cam, headers, and a low restriction intake and exhaust minus cats and emissions equipment (where I live, leaded fuel is still the status quo)
As for the frame, I really like the bonded aluminum extrusion frame of the Elise, it is simple, light, strong in torsion and bending, and not too expensive either (or is it?) There is a .pdf document out there about a chassis featuring honeycomb sandwitch construction, and that looks pretty interesting too... I'd like to hear your thoughts on it, Here is the link to it:
http://www.tech.plym...ation/chass.pdf
I'm not terribly keen on spaceframe construction, which I think is pretty antiquated. It is heaver, more complex (hence harder to build and repair) and is inferior from a packaging and possibly safety perspective to the previous alternatives. Plus I've read that with the Lotus Elise, they got chassis tolerances for the suspension pickup points down to an unheard of 0.3mm as opposed to a more typical 3-5mm for a conventionally built spaceframe. I'd like to do some more reading on advanced chassis construction techniques, so if any of you could direct me to some .pdf docs, websites or even SAE tech papers I'd really appreciate that. I've perused a couple books on the subject, but they were more oriented towards single seater and/or spaceframe construction.
As for suspension, we're looking at standard sports car fare... double wishbones at each corner, except I'd probably make em out of aluminum to keep unsprung weight down... I'm in the process of reading Advanced Vehicle Technology by Heisler and another textbook called Suspension Design and Dynamics or something, by a former Opel engineer, published in '96... The name escapes me, but I'll post it tomorrow. Anyway, I am MUCH more knowledgeable on the finer points of elastokinematics and suspension design than I was a few weeks ago, but theory is one thing, putting it into practice is a whole other ball game! I still don't know if I want to go for a conventional coilover setup or go for pushrods... If it starts getting too expensive then that's no good! I also don't know whether or not the suspension will be subframe mounted or chassis mounted.. Is it just a matter of stiffness vs. NVH transmission supression or is there something else I haven't taken into account? I'd probably want to keep heim joint usage to a minimum on account of their harshness and expen$e .
Body is gonna be fiberglass, so no big surprises there, but the emphasis is on a low CdA, which I would like to keep under 0.5. Also going for a flat bottom and underbody venturi for useful downforce at speed (yes I know it won't be completely flat due to FR layout) I've got Hucho's Aerodynamics of Road Vehicles as a reference. Excellent text, I'd say its an essential book for the vehicle engineer, designer and stylist and for any engineering student with a serious interest in cars. Its the first edition though... Has anyone got the newer edition? Looks like its even better and I would love to know see if any of you guys have it....
This has been a looooongass post, I'd really appreciate your comments and feedback on the matter, we've got some real brains on this forum... Also is there another technical forum out there for road/sports cars? Atlas F1 is great and all, but its mostly oriented towards (single seater) race car technology. Thanks again
Approx dimensions are a >420cm length, ~175cm width, and a ~115cm height
Proposed drivetrain is probably going to be, and I know you'll like this Brian, an LS1 with either a T56 or an MM6 tranny, I still don't know if I want the tranny right behind the engine or on the rear axle a la Corvette... Its probably going to come down to cost and packaging. ~400bhp is going to be no sweat for an LS1 with a mildish cam, headers, and a low restriction intake and exhaust minus cats and emissions equipment (where I live, leaded fuel is still the status quo)
As for the frame, I really like the bonded aluminum extrusion frame of the Elise, it is simple, light, strong in torsion and bending, and not too expensive either (or is it?) There is a .pdf document out there about a chassis featuring honeycomb sandwitch construction, and that looks pretty interesting too... I'd like to hear your thoughts on it, Here is the link to it:
http://www.tech.plym...ation/chass.pdf
I'm not terribly keen on spaceframe construction, which I think is pretty antiquated. It is heaver, more complex (hence harder to build and repair) and is inferior from a packaging and possibly safety perspective to the previous alternatives. Plus I've read that with the Lotus Elise, they got chassis tolerances for the suspension pickup points down to an unheard of 0.3mm as opposed to a more typical 3-5mm for a conventionally built spaceframe. I'd like to do some more reading on advanced chassis construction techniques, so if any of you could direct me to some .pdf docs, websites or even SAE tech papers I'd really appreciate that. I've perused a couple books on the subject, but they were more oriented towards single seater and/or spaceframe construction.
As for suspension, we're looking at standard sports car fare... double wishbones at each corner, except I'd probably make em out of aluminum to keep unsprung weight down... I'm in the process of reading Advanced Vehicle Technology by Heisler and another textbook called Suspension Design and Dynamics or something, by a former Opel engineer, published in '96... The name escapes me, but I'll post it tomorrow. Anyway, I am MUCH more knowledgeable on the finer points of elastokinematics and suspension design than I was a few weeks ago, but theory is one thing, putting it into practice is a whole other ball game! I still don't know if I want to go for a conventional coilover setup or go for pushrods... If it starts getting too expensive then that's no good! I also don't know whether or not the suspension will be subframe mounted or chassis mounted.. Is it just a matter of stiffness vs. NVH transmission supression or is there something else I haven't taken into account? I'd probably want to keep heim joint usage to a minimum on account of their harshness and expen$e .
Body is gonna be fiberglass, so no big surprises there, but the emphasis is on a low CdA, which I would like to keep under 0.5. Also going for a flat bottom and underbody venturi for useful downforce at speed (yes I know it won't be completely flat due to FR layout) I've got Hucho's Aerodynamics of Road Vehicles as a reference. Excellent text, I'd say its an essential book for the vehicle engineer, designer and stylist and for any engineering student with a serious interest in cars. Its the first edition though... Has anyone got the newer edition? Looks like its even better and I would love to know see if any of you guys have it....
This has been a looooongass post, I'd really appreciate your comments and feedback on the matter, we've got some real brains on this forum... Also is there another technical forum out there for road/sports cars? Atlas F1 is great and all, but its mostly oriented towards (single seater) race car technology. Thanks again
#13
Crazy Canuck
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Posted 04 February 2002 - 17:58
kind of a stoopid question...but what is the FR that you keep referring to?
CC
CC
#14
Earthling
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Posted 05 February 2002 - 17:27
FR = Front Engined- Rear Wheel Drive.
Now can anybody answer my question(s) ?
Now can anybody answer my question(s) ?
#15
BRIAN GLOVER
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Posted 06 February 2002 - 01:09
Now dont cry E. What question?. Did you get my private mail?. Send me your e mail and Ill send you something. I cant seem to post pictures here.
A tranaxle for a front engine car is generally not a good idea. In the case of Corvettes and F550s, it is more for passenger comfort. You can still get your ideal 52% rear weight bias with a front mounted gearbox. The T56 is good and is used in vettes and Vipers. The Vette is technically not a transaxle. It is a stock T56 bolted onto a differential housing. The Viper Xmission T56 bolts directly onto the engine and can be mounted further behind the front wheels.
Even though the weight distribution is the same, the Viper accumplishes it with one large centroid and one small one. The Vettes drive trane is achieved with two nearly equal masses making transitions slower. Rotation about the vertical axis(Yaw)is more difficult to initiate and stop with the Vette. Fortunately the LS1 is very light. 5230lbs for engine and xmission together.The chassis you prefere will bring the car to 1800lbs or less. front engine cars have poorer aerodynamics and intake and exhaust are bigger problems. The driver is hotter and transfers in braking and acceleration are not ideal. My street cars must be front engine though. If you have to meet any side impact or front crash test criteria, you will have much easier time making the chassis in steel or aluminum and a front engine. The absorbtion is linear. What tires will you use.
A tranaxle for a front engine car is generally not a good idea. In the case of Corvettes and F550s, it is more for passenger comfort. You can still get your ideal 52% rear weight bias with a front mounted gearbox. The T56 is good and is used in vettes and Vipers. The Vette is technically not a transaxle. It is a stock T56 bolted onto a differential housing. The Viper Xmission T56 bolts directly onto the engine and can be mounted further behind the front wheels.
Even though the weight distribution is the same, the Viper accumplishes it with one large centroid and one small one. The Vettes drive trane is achieved with two nearly equal masses making transitions slower. Rotation about the vertical axis(Yaw)is more difficult to initiate and stop with the Vette. Fortunately the LS1 is very light. 5230lbs for engine and xmission together.The chassis you prefere will bring the car to 1800lbs or less. front engine cars have poorer aerodynamics and intake and exhaust are bigger problems. The driver is hotter and transfers in braking and acceleration are not ideal. My street cars must be front engine though. If you have to meet any side impact or front crash test criteria, you will have much easier time making the chassis in steel or aluminum and a front engine. The absorbtion is linear. What tires will you use.
Originally posted by Earthling
FR = Front Engined- Rear Wheel Drive.
Now can anybody answer my question(s) ?
#16
Crazy Canuck
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Posted 08 February 2002 - 17:53
I can tell you some disadvantages of a rocker arm type suspension vs push/pull rod suspension:
1. The rocker arm is quite large, and therefore heavy, relative to an A-arm. The weight is also up high which is bad for the CG and adds to the unsprung weight.
2. The rocker arm acts like a beam hence has some springiness and is undamped springiness so your overall spring rates are difficult to quantify.
Hope this helps, if you need more clarification let me know.....
CC
1. The rocker arm is quite large, and therefore heavy, relative to an A-arm. The weight is also up high which is bad for the CG and adds to the unsprung weight.
2. The rocker arm acts like a beam hence has some springiness and is undamped springiness so your overall spring rates are difficult to quantify.
Hope this helps, if you need more clarification let me know.....
CC
#17
imaginesix
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Posted 09 February 2002 - 22:15
One of the advantages of pull/push rod that has been stated; it's ability to allow a highly progressive wheel rate, is also a disadvantage in that it can add complexity where it realy isn't needed.
Increasing the rocker ratio, or varying the wheel rate with travel, are not necessary for relatively softly sprung, large-suspension travel designs (like non-aero cars). So a very important consideration is how much aero force will this this car have, and how much suspension travel it will require.
Another disadvantage is that push/pull rods actually cause INCREASED unsprung weight, since the weight of the pushrod and bellcranck must be also be accelerated, along with the other suspension components over bumps.
An advantage that has not yet been mentioned is the possibility of designing the pull/push rod itself as the 'failure point' in the suspension, to reduce the risk of damage to more costly suspension components in an accident. It would depend, I suppose, on whether or not these cars are intended to be raced regularly. If they are, then that should also affect your choice of frame construction methods. A spaceframe is cheaper and easier to repair in an accident.
With regards to the frame, I must point also out that Lotus were unable to adapt the bonded unibody concept to the engine/gearbox/rear suspension cradle of the Elise, because there were too many mounting points that needed to be located in close proximity, that caused insurmountable structural problems. The engine bay for the Elise is a steel spaceframe bolted to the aluminium 'tub'.
From what I understand so far, I see no advantage to using a pull/push rod suspension for your application. For the frame thought, if you can get the bonded unibody to work for you, then that would definitely be the way to go.
Increasing the rocker ratio, or varying the wheel rate with travel, are not necessary for relatively softly sprung, large-suspension travel designs (like non-aero cars). So a very important consideration is how much aero force will this this car have, and how much suspension travel it will require.
Another disadvantage is that push/pull rods actually cause INCREASED unsprung weight, since the weight of the pushrod and bellcranck must be also be accelerated, along with the other suspension components over bumps.
An advantage that has not yet been mentioned is the possibility of designing the pull/push rod itself as the 'failure point' in the suspension, to reduce the risk of damage to more costly suspension components in an accident. It would depend, I suppose, on whether or not these cars are intended to be raced regularly. If they are, then that should also affect your choice of frame construction methods. A spaceframe is cheaper and easier to repair in an accident.
With regards to the frame, I must point also out that Lotus were unable to adapt the bonded unibody concept to the engine/gearbox/rear suspension cradle of the Elise, because there were too many mounting points that needed to be located in close proximity, that caused insurmountable structural problems. The engine bay for the Elise is a steel spaceframe bolted to the aluminium 'tub'.
From what I understand so far, I see no advantage to using a pull/push rod suspension for your application. For the frame thought, if you can get the bonded unibody to work for you, then that would definitely be the way to go.
#18
Earthling
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Posted 10 February 2002 - 09:01
Excellent contributions, all of you... I figured push/pull rods would be overkill for my application. I want to discuss different methods of chassis construction so I'm going to continue this discussion under a new thread...
#19
Crazy Canuck
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Posted 11 February 2002 - 04:15
Originally posted by imaginesix
stuff...
more stuff...
Another disadvantage is that push/pull rods actually cause INCREASED unsprung weight, since the weight of the pushrod and bellcranck must be also be accelerated, along with the other suspension components over bumps.
...
I didn't realize that! Good point...but I would think that a pull rod system would have less unsprung weight than a push rod system because you could design a pull rod to be smaller than a push rod 'cause a pull rod is a tensile member and a push rod is a compressive member.
CC
