136 replies to this topic

[NeilR]

for the past day I've been running through suspension iterations on paper trying to make sense of all of the angular variations that occur in a 'traditional' pushrod suspension. I know I will be overlooking some things, but looking at the layout of the current project (a motorbike powered formula-like car) I've encountered what seems to be a significant problem:

The front suspension looks like this with the working angle of wishbone/damper and pushrod quickly mocked up. Essentially they are all in the same plane with the bellcrank axis perpendicular to the working angle:

 

The pushrod is angled back to the cockpit around 10 degrees.

The problem is more at the rear. The structure looks like this in side view because the plan was to run the dampers across the very rear of the car.

 

We would now prefer to run the damper longitudinally to the point in the middle of the pic where two tubes meet. Either way the problem with the desin is one of packaging, which can be partly seen in this picture:

 

In plan view the A-arm gets in the way of the pushrod - running . We can get around that by increasing the angle of the pushrod or by using a 'K' arm - basically a bent link in the aft section of the A arm with another tube going forwards. I don't like the idea too much, but am open to other solutions. The bigger issue is the deviation in plan view - should I run the pushrod further back to clear a straight link it would be really quite an accute angle and I assume this will add another variable that I have to account for?

[GrumpyYoungMan]

Can you not use rocker arm links to alter the angle?

[rachael]

The loads will shoot up if you go to an accute angle which could give you a problem with buckling of the push-rod and can also put side loads into the A arms which might prove problematic.  Have you considered a pull-rod solution?

 

Rachael.

[NeilR]

I have but packaging is an issue, but good point on the A arms. The pushrods are going to be stout 4130 tube, 32mm X 1.2mm wall if I remember correctly. It is a hill climb car so generally short runs

[gruntguru]

This really requires a proper analysis of the forces involved. If you posted some drawings i am sure someone could at least estimate the increase in pushrod load.

 

You can estimate the increase yourself. If you are not changing the pushrod angle or length in front view, the force will increase in proportion to the length of the pushrod.

 

Of course lengthening the pushrod without changing any angles has no effect on the force so you can estimate the force at many other locations as well.

 

The longitudinal component of the pushrod force is equal to F x X/L where F=pushrod force. L=pushrod length. X=how far you have displaced the top of the pushrod along the longitudinal axis from the point where the pushrod is shortest. (pushrod lateral in plan view)

[NeilR]

Thanks for that information: Front pushrod looks like this and causes less concern:

 

I have to get the next issue of Race Magazine done, so will not be doing anything to it for the next two weeks, but I'll nip down and get some rough measurements of where things are at the moment. I must say that making something from scratch has been a great learning experience even if it highlights the shortfalls in what I though was my 'knowledge'.

Edited by NeilR, 12 August 2014 - 00:29.

[Fat Boy]

The front packaging is why many end up with 3 bulkheads forward of the driver opening. Dash, middle and forward bulkheads give you places to locate the wishbones (dash and forward) and a place to locate the rockers (middle). It also give a little more of a crash structure. The rear packaging is tougher. You see a lot of college teams go with laterally located dampers for this reason. In general, I don't like those funky angles with the pushrods. It's going to cause installation stiffness issues (which may or may not actually matter) and probably make for some odd rocker geometry. It's not that you can't make it work, just that you have to be more careful. Then again, my brain works better when limited to 2 dimensions at a time (my wife would probably argue that this is being generous).

[Greg Locock]

You don't need to get too fussy when working out the rising rate, just measure the damper length at full droop, design, and full bump. The shape of the curve is pretty much a parabola through them, any error is more than outweighed by the shock characteristics themselves.

[NeilR]

Fat Boy three bulkheads would be smart! ... chassis two will have many changes. What can I say, it seemed like a good idea when we were all staring at solidworks on the screen!

Greg,  thanks for that input. I have a tendency to over complicate things I don't understand well. Most of the bike engined cars are running zero droop front suspension. I'm not sure it is a good idea given we'll start without wings. Fortunately with access to a large laser cutter making bellcranks will be straight forward.

[MatsNorway]

Check out https://www.facebook.com/LoveFab for similar stuff related to his pikes peak car and other things.

[NeilR]

Well I have moved on a little.

The original plan was to have higher mounted dampers and a conventional bellcrank, but packaging became an issue which you can see here:

 

The dampers/design just would not match to my satisfaction.

The current thing we are mocking up is a lower mount option as shown, any input welcome (note suspension shown at 30mm droop):

Edited by NeilR, 08 September 2014 - 04:40.

[carlt]

that seems eminently sensible

close to good load paths as well

[Fat Boy]

Two dimensions (for the most part) does make things a lot easier, huh? Make sure you account for the loads that will now be put to the middle of the chassis tubes. I've seen some FSAE cars that have as much wheel displacement coming from chassis deflection as comes from damper travel!

[NeilR]

I can locally reinforce the tube wall by sleeving it and can also gusset to adjacent vertical tubes.

[Fat Boy]

I can locally reinforce the tube wall by sleeving it and can also gusset to adjacent vertical tubes.

 

It's probably going to end up a little clunky, no matter how you do it. I think you could do some good things by just using 1/2" x 0.035 tubing to define some load paths. Make sure you've connected the mounts to nodes. Triangles are you friend.

[carlt]

one short tube from under the bell crank mtg to pick up adjacent to the bottom shocker mtg at the lower wishbone forward mtg point would suffice to carry a fully laden Sherman tank

[Joe Bosworth]

I absolutely, positively don't want to be be negatve but there is a very positve lesson to be learnt from this topic in the month that it has been in existence.

 

Here is case of many hours have been utilised no doubt making design calculations, formulating designs and welding up a complex tube frame before settling on a suspension design details.  The result is turniing into a problem of deciding which location of suspension forces compromises the frame the least.

 

Given that the major purpose of a frame is to resolve static and dynamic suspension forces to result in the smallest feasible corner location changes to one another it is essential that suspension details are fixed before frame design.  This insures suspension, frame and weight harmonisation and lowest design weight for the selected satisfactory deflections.

 

It is far easier to work on paper or using any number of CAD variations or even making full size wooden mockups than it is to offer pieces up to a full fledged nearly completed frame.

 

Keep the horse in front of the cart rather than trying to to design a good way to get the horse to push the cart.   

 

Regards

[NeilR]

Actually Joe you are only partly right. The problem was three fold: The car was designed in solidworks and the model we used for the dampers was probably not accurate enough, two - we did not have all of the component list finalised at the time of the chassis construction, three I decided to make changes because the reality did not look as good as the computer design. None of us involved in this project have ever designed and built such a car before and I do agree that accurately modelling things in full size would be the best way forward. However the goal was never the purest design or the lowest weight. We never thought of this car as anything but a training excercise and will run it on FF tyres just to run against ourselves for fun. So the goals were low running costs, safety, durability and ease of use.

[NeilR]

I sourced a pair of needle roller bearings for the rear bellcrank and we plan to use the same construction method: an alloy bearing block with three bolts with a laser cut steel plate either side, naturally held on by the three bolts. Side loads will be taken care of by a pair of PTFE washer either side of the alloy block.

[kikiturbo2]

I sourced a pair of needle roller bearings for the rear bellcrank and we plan to use the same construction method: an alloy bearing block with three bolts with a laser cut steel plate either side, naturally held on by the three bolts. Side loads will be taken care of by a pair of PTFE washer either side of the alloy block.

Try calculating your pivot loads and see if a set of twin plain ball bearings in an acceptable size would do the trick. I was in the same boat and had to weigh between complicated solution of using a needle bearing + two axial thrust bearings, which was a sealing nightmare or a ready made set of hybrid needle / ball axial thrust bearing bearigns which was mega expensive.

In the end I went for two "popular size" ball bearings which serve both radial and axial loads and are dirt cheap to replace... which will be more often than needle but still acceptable for low mileage track car..

[Greg Locock]

Usual solution there is to use deep groove ball bearings, or tapered roller bearings. Either way you end up using two parts instead of 3, and you can get fully sealed ball bearings. TRBs probably have less friction.

[kikiturbo2]

Usual solution there is to use deep groove ball bearings, or tapered roller bearings. Either way you end up using two parts instead of 3, and you can get fully sealed ball bearings. TRBs probably have less friction.

 

that is exactly what I was talking about... plus, I tried to use a popular size which brings the price down quite a bit..

[NeilR]

I can certainly do deep groove ball bearings. I had a look for smaller TRB's and drew a blank for a cheap common size. We picked the needle rollers because we had them in the drawer, will see if TRB's in 10mm ID are available

[Fat Boy]

I can certainly do deep groove ball bearings. I had a look for smaller TRB's and drew a blank for a cheap common size. We picked the needle rollers because we had them in the drawer, will see if TRB's in 10mm ID are available

 

I'd argue that with how you have it arranged at this point, deep groove bearings are overkill, not to mention the tapered rollers. The tapered rollers are going to add a significant amount of friction that you don't want. I'd just use a very vanilla sealed ball bearing. You're taking the lion's share of the load radially, and the loads are not a going to be a big issue axially.

[NeilR]

I took your suggestion on and bought the std BB's, 10mm ID, which were much cheaper than TRB's. I can run two with a spacer or three in each bellrank - will start with two and see how they go.

[Fat Boy]

I'm not sure where the spacers are supposed to go that you're talking about. Make sure you have a spacer between the bearings that is used as a crush sleeve. You don't want to side load the bearings unneccesarily.

[kikiturbo2]

I took your suggestion on and bought the std BB's, 10mm ID, which were much cheaper than TRB's. I can run two with a spacer or three in each bellrank - will start with two and see how they go.

did you calculate the forces on the bearings?

[NeilR]

I have tried to answer this but the message drops off.

so in short: no did not calculate the forces - was not sure of right way to do it. So phoned previous hillclimb champ who makes his own cars and am using what he uses. Not scientific, but is 'proven'. Would like to know best way to calculate forces though

[YowserUK]

I have tried to answer this but the message drops off.

so in short: no did not calculate the forces - was not sure of right way to do it. So phoned previous hillclimb champ who makes his own cars and am using what he uses. Not scientific, but is 'proven'. Would like to know best way to calculate forces though

 

If you have calculated expected peak loads for the vehicle, then SKF offer up a handy tool on their website for calculating bearing requirements. http://www.skf.com/g...oads/index.html

 

If you have not done any dynamic force calculations, or peak static calculations then you will really need to look into this.  Its relatively straight forward to work out expected peak forces as static loads under Braking, acceleration and cornering, using maximum gravitation forces for each, alongside wheelbase, track, centre of gravity ..etc , but for more complex/accurate dynamic calculations there are a number of simulation software options.

Edited by YowserUK, 01 October 2014 - 08:47.

[NeilR]

The original design looked like this prior to construction:

[Kelpiecross]

Is it possible to run the pushrod from the top of the upright? This would still involve a few funny angles but at least it would be out of the way of the A arm.

[Kelpiecross]

Further thoughts - maybe the rear member of the top A arm could be combined with the pushrod to make a solid lever-action A arm acting on the end of the spring/damper - the movement of the end of the lever would then be parallel and in line with the rear cross member.

[Fat Boy]

Is it possible to run the pushrod from the top of the upright? This would still involve a few funny angles but at least it would be out of the way of the A arm.

 

No, it's not.

[Kelpiecross]

Yes, it is. It certainly is possible (and practical) to run a pushrod from the top of the upright. I used this layout on my own little homemade car. The car uses Morris Mini suspension uprights and suspension arms. The pushrod runs from the stock Mini shock absorber mounting point on the top arm (about 3/4 of the way out along the arm) to a bellcrank mounted on top of the chassis - and the spring/damper unit mounts to the other side of the bellcrank. It all seems to work very well. The system also allows much adjustment in that the pushrod length is adjustable and the bell crank dimensions can be changed fairly easily if necessary.

If I knew how to I would post a photo of the layout.

[desmo]

Someone should perhaps explain why attaching a pushrod from the top as opposed to the bottom of a wheel upright is necessarily a bad idea. I can see obvious potential problems if the resulting pushrod angle is too horizontal relative to vertical wheel travel but if the rocker attachment end is sufficiently elevated to create the necessary angularity I'm not seeing why it shouldn't work.

[imaginesix]

Yes, it is. It certainly is possible (and practical) to run a pushrod from the top of the upright. I used this layout on my own little homemade car. The car uses Morris Mini suspension uprights and suspension arms. The pushrod runs from the stock Mini shock absorber mounting point on the top arm (about 3/4 of the way out along the arm) to a bellcrank mounted on top of the chassis - and the spring/damper unit mounts to the other side of the bellcrank. It all seems to work very well. The system also allows much adjustment in that the pushrod length is adjustable and the bell crank dimensions can be changed fairly easily if necessary.

If I knew how to I would post a photo of the layout.

That's not the same as mounting to the upright. My guess is the answer is "no" because it would introduce tremendous twisting loads into the upright with every bump and make the compliance of the whole system a huge burden in trying to tune the suspension.

[Fat Boy]

Someone should perhaps explain why attaching a pushrod from the top as opposed to the bottom of a wheel upright is necessarily a bad idea. I can see obvious potential problems if the resulting pushrod angle is too horizontal relative to vertical wheel travel but if the rocker attachment end is sufficiently elevated to create the necessary angularity I'm not seeing why it shouldn't work.

 

It just makes the suspension clunky, similar to the original arrangement where the pushrod goes aft a large amount. If you want to transfer vertical motion, then the pushrod would have to go vertical at a reasonably steep angle. Let's say 30-40 deg. w.r.t. the horizon. Anyway, if you are going up from the top of the upright, then you've got the dampers (or at least the rockers) way above the top of the upright. If you've got the rockers and dampers up that high, you'll also have to stick a bunch of frame structure up there too. It's just not something you'd want to do.

 

If you really, really wanted to pick up from the top of the upright, you'd do it as a pull-rod. You make your 30-40 degree angle going down to a rocker which works the damper.

 

There's a big difference between what you *can* make happen and what you *would* make happen. How many professional open-wheel race cars do you see with push-rods coming from the top of the upright? Why do you think that is?

[gruntguru]

Loads are all wrong as imaginesix says. Largest tyre load is lateral inwards below the bottom BJ so puts lower A arm in compression, upper in tension. Pushrod at top increases tension in upper arm. Pushrod at bottom reduces compression of lower arm. 

 

Another consideration is rim clearance. Getting the pushrod from the top of the upright past the rim/tyre can require a very canted pushrod.

[Kelpiecross]

It just makes the suspension clunky, similar to the original arrangement where the pushrod goes aft a large amount. If you want to transfer vertical motion, then the pushrod would have to go vertical at a reasonably steep angle. Let's say 30-40 deg. w.r.t. the horizon. Anyway, if you are going up from the top of the upright, then you've got the dampers (or at least the rockers) way above the top of the upright. If you've got the rockers and dampers up that high, you'll also have to stick a bunch of frame structure up there too. It's just not something you'd want to do.
 
If you really, really wanted to pick up from the top of the upright, you'd do it as a pull-rod. You make your 30-40 degree angle going down to a rocker which works the damper.
 
There's a big difference between what you *can* make happen and what you *would* make happen. How many professional open-wheel race cars do you see with push-rods coming from the top of the upright? Why do you think that is?

I see your point about having the pushrod at too flat an angle. However in my case the pushrod does indeed go up at about 45 degrees. Using a Mini upright etc. is probably a special case as due to the small dimensions of the upright and the short upper arm (and the rod attaches about 1/4 of the way in from the end). The tee-shaped bellcrank sits directly on top of the suspension cross member box without any extra structure - the height of the bellcrank gives about 4 inches extra vertically - so overall the rod is at quite a steep (and acceptable angle).

I suggested this layout for NR's car because as you can see from his photos (post No.11) with the added height of the bellcrank directly on top of his top chassis tube the pushrod angle would quite steep. Unfortunately there is no other structure in this area to mount the bellcrank/springs etc. to.

Note that on NR's chassis the suspension upright is unusually short and the inboard suspension mounting points for the arms are very close together vertically - both at the front and rear the top of the upright is well below the top of the chassis.

[Johan Lekas]

Maybe this is an option?

(pushrod on the lower wishbone)

 

http://<img src="htt...t="4h74wp.jpg">

Edited by Johan Lekas, 09 October 2014 - 07:04.

[gruntguru]

Nice - I hope it fits.

[silente]

Very interesting project, congratulations!

It looks also like you are keeping cost and complexity relatively low, which is not always easy.

 

Is this car meant for any particular class? In which country would you use it?

 

Maybe it´s trivial but i noticed that some of the suspension attachments (including the latest proposed rear rocker location) are in "weak" zones of the spaceframe, where there is no triangulation. That could affect your installation stiffness or even your local strength, although the latter doesn't seem so much of a problem with the tubes you are using.

 

Did you also do any check about suspension geometry? Are you somehow checking loads (a part from the already discussed bearing load)?

 

Are you building all yourself? Where are you taking standard components (like dampers, hubs, steering rack, etc) from?

 

Sorry for the many questions but i am really intrigued by your project! i tried to do something similar some time ago, but because of time constraints and problems on working directly on car assembly it didn't go on.

 

Anyway Congratulations!

Edited by silente, 10 October 2014 - 07:20.

[NeilR]

Johan, we tried that high-mount and could make it fit as well as we would like. Plus it puts the weight up much higher.

Silente the car is meant for Formula Libre hill climbing in the 0-750cc class. Yes there are a lot of 'poor' engineering solutions in it, which does irritate a little, but then the purpose is a rugged car, easy to use, cheap to run and something we can learn a lot on - hence the quick adjustable front suspension.

Suspension geometry has been designed in Susprog, a suspension program. The geometry is intended to mimic a late 1990's Formula Ford within the suspension mvt expected. We're using the same rubber, so it seemed a reasonable place to start. Loads? The uprights have been FEA'd by a professional engineer, with a very large safety margin - is this the sort of thing you meant?

I have generous help to make it from a commercial engineering shop. It could be made at home, but then I have two other projects as well. Hubs are Honda to match LSD and driveshafts. Discs and calipers are GMH rears and match FF sizes but a little thicker.  Dampers are Yamaha R1, Engine Suzuki GSXR750 K8, steering rack is ex-indycar off ebay.

Were I to do a mk2 it would be simpler and fully mocked up before construction in earnest. It would also be much lighter.

[Fat Boy]

You know, this deal shows how it's actually pretty impressive that a couple hundred schools can produce enough kids to turn out cars every year.

[gruntguru]

Yep. Producing an FSAE car from scratch is a monumental task.

[NeilR]

Yes the FSAE cars can be a work or art too. The thing I would point out it that few FSAE cars seem completely new designs. Most seem to lean heavily on previous years efforts - which is making the most of the resources available and keep the students out of the student bar for the least amount of time possible too!

[Fat Boy]

Question: Put a group of young engineering students in a room and tell them to build a racecar. Have a racecar in the room for them to look at and maybe even a person that they could talk to who knows a lot about racecars.

 

How many will try to copy the racecar sitting in front of them and get advice from the person they have contact with instead of forging new ground and 'doing it their way'.

 

Answer: Damned few.

[imaginesix]

Not reinventing the wheel with every new generation is as essential to progress as trying new things and screwing up sometimes.

[NeilR]

My comment is by no means a criticism as I have borrowed heavily from things that I have seen...sometimes borrowed badly.

[kikiturbo2]

Question: Put a group of young engineering students in a room and tell them to build a racecar. Have a racecar in the room for them to look at and maybe even a person that they could talk to who knows a lot about racecars.

 

How many will try to copy the racecar sitting in front of them and get advice from the person they have contact with instead of forging new ground and 'doing it their way'.

 

Answer: Damned few.

 

Quite a few years ago I was asked by a friend to take a look at a FSAE car our uni was building and try to solve some ECU issues.. First thing I saw were solid 12 mm rods used in A arms... I told them that it was a stupid thing to do because of non existent stiffness and to go buy some seamless tube at which point I was told by cheif suspension engineer that FEA says it is ok and that 12mm rod is ideal because they can cut a thread on it and thread it directly into a female rod end...

 

Well, on the first run, first corner and the car gains about 15 deg camber

 

I often find that FSAE cars have too little thought, too much 3d modeling and far too little testing in them..

Edited by kikiturbo2, 21 October 2014 - 09:53.