24 replies to this topic

[NRoshier]

In an application that is not a single seater, but still using a pushrod actuated spring/damper via a bellcrank, is there any real need to continue to use the expensive bearings that single seaters use?
The bellcrank shaft can be mounted in double shear to a steel spaceframe.
Would two spherical bearings side by side with a 10mm spacer be sufficient or a double cone and cap needle roller bearing arrangement (ala wheel bearings) be sufficient?
Of course sealed is better, but what else is out there?
I have looked at PTFE top-hat shell bearings as well.

[imaginesix]

I've modded a formula car to accept 2 spherical bearings (in place of the stock brass bushings) on the bellcrank. The preferred option would have been to use 1 spherical bearing and one straight needle bearing as it can be very difficult to align 2 spherical bearings without binding them, and one alone should have been sufficient to take the axial loading of the bellcrank (your results may vary).

[Greg Locock]

Neil, why don't you introduce yourself (if you haven't before)?

Anyway, using a pair of ball joints to form a rotational joint is somewhat sucky, but several million front uprights a year says it can be acceptable.

Obviously increasing the span between the joints will increase the accuracy.

I suppose one simple argument about the loads is that they can be no higher than the total load in each of the connecting arms - so what joints were you proposing to use there?

I'm almost tempted to say that a rubber bush would do, but that depends on the artulation of the arm, bushes tend to be designed for +/- 20 degrees.

So, what is the conventional long life solution? plain bearings in suspension arms give me the heeby-jeebies, since I once owned a Commer van with the bloody things.

[NRoshier]

Imagine, thanks for your input. I was planning on a relatively easy to fabricate arrangement, so the bellcrank would be fabricated, the centre tube of which would be turned up on a lathe to accept the spherical bearings, perhaps minimising the allingment issues? The 'arms' of the bellcrack would be welded to the tube centre section.
Greg, good to hear from you. I've lurked here for a while but I would not know where to introduce myself.
The arms are going to use Sperical bearing/rod ends and I take your point re the loads, for some reason I had not thought of it that way. How about the conical needle roller bearings? They are certainly cheap and easy to get.
Rubber bushes...I will think on this. Do you mean ala' suspension bush. I have a suspicion that the compliance would be undesirable if the motion ratio was getting low. The racecars I have seen seem to go to great lengths to minimise compliace at this point.

[imaginesix]

I guess my first reply should have been "Is it 'sufficient' for what?"

You mentionned cost and easy of fabrication, but don't appear to be concerned with stiction or compliance. What in fact are your goals?

[NRoshier]

Imagine, I am the publisher of Race Magazine and within this I am building a spaceframe mid engined sports car for Tarmac events and circuit work. The suspension is for this. I am currently building the attribute prototype to assist with component selection.
I am concerned about compliance as I noted in my response to Greg's post. Stiction is an issue, but the suspension movement will be relatively large in comparison to a formula car, so perhaps it is less of an issue? The suspension will be using a minimum of 6 sperical bearings at each corner.

[imaginesix]

Then I must be missing something because base based on that goal I would have expected rubber bushes to be dismissed out-of-hand. Or is that what you did in a polite manner?

[NRoshier]

Imagine, if Greg suggested it then I will consider it, after all it is an area of his expertise. Having said that it seems counterintuitive to me and the compliance within a bush would be very undesirable, though I will ask Greg for more details as I might be not fully understanding his suggestion?
The double spherical bearing solution has the advantage of being sealable, but do you feel any particular solution would have any advantage as far as stiction?

[imaginesix]

Needle bearings, tapered or straight, would be better than spherical bearings as far as stiction (isn't there a more technical term for that?). I've never opened up a bellcrank bearing but from their shape I assume they are constructed with side-thrust needle bearings for that reason.

I like your idea of twin tapered needle bearings but I expect they'd require a high degree of precision machining to get their relative positioning right. If that presents an obstacle then I'd look at a combination spherical bearing and flat needle bearing. Cheap, easy, rigid. Kinda like your local crack whore.

[Greg Locock]

Tapered roller bearings, is what I think you are talking about and they'd be an excellent solution.

For my sins I am well aware of the compliance figures for ball joints and rubber bushes.

I have rubber bushes that are as stiff as 27000 N/mm, radially. I have tie rod ends that are as soft as 15000 N/mm (bigger ball joints tend to be stiffer). It is very hard to design steel sheet clevis systems, in 1.2mm steel, that are much stiffer than 20000 N/mm, so the difference between these numbers gets masked. For that matter, the local stiffness of the chassis can easily drop to 10000 N/mm.

[NRoshier]

The bellcrank bearings I have seen are a top hat type, flat roller bearing at the bottom and another roller bearing in the vertical cylinder that locates the bellcrank.
Tapered roller bearings it is then. They are easy to source and the machining should not be a problem (wheel bearings after all).
WRT the rubber bushes, the problem for a non OEM engineer such as I is where do you get the information on a nice list with all the measurements with a big red arrow pointing to one with a not saying *use this one*!
I am considering the bellcrank as the rocker arm idea places the rear dampers over to top of the exhaust from the rear bank of cylinders, which does not seem like a good idea.

[Catalina Park]

My road car uses tapered roller bearings in the upper arm. (Morris 1100!)
I think they would work well in a bellcrank. You can get them cheaply and seal them against the weather and lube them for a long time. It would be an easy solution at the expense of a little more weight.

[Greg Locock]

Tapered roller bearings are the obvious solution. The only bad thing is that if you run into stiffness or bending problems then they may tend to get overloaded, locally. As an example on the solar car we used self aligning ball races to minimise friction, since we ran a fairly light structure the deflections were enough to bend axles etc. Timken and NTN have useful catalogues.

Bushes that are very stiff radially are the shock absorber to lower control arm, and the lower control arm to subframe, on the Territory's front suspension. But I repeat, don't exceed 20 degrees articulation

[Fat Boy]

I would say that 2 sphericals is not the way you want to go about this. I've seen 2 thin ball bearings seperated by a spacer, flat needle thrust bearings top & bottom (axial load) and a wide needle bearing inside (radial load) the rocker both work well. The 'top hat' bearing that you were talking about is called a Nadella. I'd say they're pretty good for Formula Ford type loads. They're a variation of the 2 thrust/1 radial bearing idea.

Getting rid of stiction in the suspension is really important in any type of racing I know about. Tapered bearings are bound to induce some. Sphericals with the PTFE liner also are really bad for stiction. The liner reduces the _friction_, but the breakaway load is unacceptable for most racing suspensions. Greg's bushing approach is probably the best when it comes to stiction issues. There isn't any. It would still make me feel weird to run a rubber bushing on a racecar.

A lot of older Formula Ford ran Oilite thrust washers and bearings. I don't think that was the best way to go, but it was light and cheap. When they were serviced religiously, they probably didn't have much more friction than needle bearings. They took a lot of work to keep happy, though. With that in mind, I'd go with the needle bearings.

[imaginesix]

Is it just my imagination that setting up 2 tapered roller bearings is a delicate affair? Is it simply a matter of shimming them during installation to find that loose/tight sweet spot (either binding the rollers or getting freeplay)?

[Fat Boy]

Originally posted by imaginesix
Is it just my imagination that setting up 2 tapered roller bearings is a delicate affair?

It's tricky enough that I wouldn't want to use that arrangement in a suspension.

[bobqzzi]

Nadella bearings don't strike me as being terribly expensive if you don't mind using imperial spec ones. Metric ones are pricey.

I find it difficult to believe that teflon lined spherical bearings would create any significant stiction in a bell crank in a sedan. The loads are far too high for them to make any difference. They also have the advantage of being dirt cheap, easy to find and easy to fit.

[Greg Locock]

Go and buy one. Try and crank it over by hand. Several Nm of stiction. Several Nm of running friction . Is this important? kind of depends on your lever ratio and so on.

Where this, and compliance, and free-play, are vitally important is on smooth road damping, there is some (real) danger that the only damping will be the tire sidewalls.

So you really want a chart that characterises the following

stiction
running friction
compliance
freeplay
ability to cope with misalignment
long life
maintenance
cost
package space
conical compliance (if all the arms are in the same plane exactly then a single row ball bearing or needle bearing would be sufficient or even a single ball joint)
articulation angle

Typically I'd also build a compliance budget for the entire system, a friction budget for the suspension, and I suppose a free play budget, although I'd be very keen to see no free play at all.

These budgets could then feed back into the chart above to aid in selection.

My feeling is that the ball joints will be fine, because you have enormous forces due to your lever arm setup, and they are certainly robust. Just thinking about it, the best joints on the sta bar to arm link are ball joints, and they do a very similar job. But if I just wanted to do it properly, once, I'd use TRBs, and make them more robust than the surrounding structure so they pull it true. What do recirculating ball steering gears use for their idler arms?

Oh, that's a thought. When I have a clevis type system (or any pair of abutments) I try and make sure that one of the two is flexible, axially, and the other is the stiff geometrical locator. So, as you tighten the bolt up the whole assembly gets pulled towards the stiff abutment, and all the bolt preload goes into useful stuff, not bending welds etc.

[NRoshier]

Packaging that interests me includes the following:

and

The top one in particular seems a neat solution to the rear damper placement. In such a position I will be able to ensure minimal heat soak from other components and also ducting of cooling air should it be required. I anticipate a motion ratio of 70%.

[NRoshier]

System originally envisaged uses a similar top arm as this Pag:

[imaginesix]

Originally posted by NRoshier
Packaging that interests me includes the following:

That's certainly a swooshy bellcrank. I guess that's something that should be added to Greg's checklist; sex appeal. In this picture sex appeal seems to be higher priority for the bellcrank than compliance or articulation.

[bobqzzi]

Originally posted by Greg Locock
Go and buy one. Try and crank it over by hand. Several Nm of stiction. Several Nm of running friction . Is this important? kind of depends on your lever ratio and so on.

.

I agree- there are several NM of stiction-I just don't happen to think that is a significant amount in the stated use.

I like the idea of a "budget".

[Fat Boy]

Originally posted by bobqzzi


I agree- there are several NM of stiction-I just don't happen to think that is a significant amount in the stated use.

I like the idea of a "budget".

You might be suprized at how just a small amount of stiction/friction in a suspension system causes a serious loss of grip. If you spend a little time on a shaker rig you can really see it.

I guess I missed the 'stated use', what actual car are we talking about? The DP car that is shown in one of the pictures is definitely sensitive to this amount of friction in the suspension. The road car might be less of a deal, but I bet you can feel it on rough pavement.

You can screw up a perfectly good racecar by putting in a spherical in a ball joint that is too tight. All of the sudden the steering is vague and the grip goes to hell. It's all because of some thinking 'this will loosen up in a few laps' during sub-assembly. Taking care of the rod-ends won't make a bad car good, but it has the ability to make a good car bad.

[Greg Locock]

I vaguely remember the total allowable running friction in the suspension, vertically, is 300N, or 75 of your earth-units. The shock will be some huge proportion of that particularly with a strut type setup.

I haven't got any idea for stiction - twice as much maybe? Sounds like an awful lot in the context of shock absorber curves, say 1000 N/ m/s

[NRoshier]

Fat Boy, the car is intended to be used on the road and also used for 'Tarmac' rally events and some circuit work. Full suspension travel will be around 180mm and the chassis is planned along the lines as shown below. Wheelbase is currently 2570mm and rear track 1570mm and front track 1630mm, weight anticipated to be 900kg or so. As the car has to meet Australian Design Rules (ADR's) so it can be used on the road, then the majority of the components have to be from an ADR complianced production vehicle. I hope this helps fill you in on some of the details.
I imagine that the sticion will also be somewhat weight dependant, thus the lighter the vehicle the lower the stiction limit would be. Having said that the majority of 'clubman-lotus 7' type cars use similar suspensions and they weigh some 5-600kg.
Imagine 6 my interest is not in 'Sexy', but rather that an expensive component is more out of harms way and is easy to access. In my current car adjusting the dampers/sway bar requires a wheel off and much wriggling and swearing whilst under the car...best totally avoided when wet too.