24 replies to this topic

[Ben]

Most race cars use turnbuckle type adjusters to lengthen/shorten the pushrods to adjust ride height. Most seem to do it using jam nuts to lock the turnbuckle following an adjustment. E.G.



However I noticed recently that the Porsche RS Spyder uses a pinch bolt like a bicycle seat tube and no lock nut on its pushrod adjusters:



Just wondering what the point of the pinch bolt approach is. Does it reduce stress concentrations at the thread root where the turnbuckle enters the push rod? Or is it something more mundane like the torque applied to a lock nut altering the length adjustment slightly and therefore the pinch bolt is more precise?

Ben

[Fat Boy]

I don't know exactly why they did it this way, but it looks tidy, huh?

[Ben]

Originally posted by Fat Boy
I don't know exactly why they did it this way, but it looks tidy, huh?

It does. I also like the way they use a very big diameter pushrod as well. Sort of screams "we did the Euler buckling equation are your pushrods are all too small..."

I believe the Jaguar XJR9 had this type of arrangement on some of its links, but I haven't checked for any images.

Ben

[Greg Locock]

I wonder how they made the tubes? It's an interesting tradeoff, drag vs weight vs buckling strength for the pushrods. The aero boys were doing that (tapered tubes for buckling strength) way back in the 20s.

Let's see - if you strip a thread you just replace a nut.

You can use a socket or ring spanner not an open ender.

Failure mode in axial load is non destructive.

Don't need left hand threads.

On the bad side, it looks like the clamp on a bicycle seat!.

Also you can't use the pushrod itself as a jack to adjust the height, you have to jack the car into the new position, or do it up semitight and apply brute force.

The last is the killer for me.

[McGuire]

I don't know about this. A pinch bolt is for assemblies requiring infrequent adjustment/disassembly. Each time it is loosened it will take a little more tightening to obtain the same clamping force upon reassembly. Ideally you want the entire body clamping together via the pinch bolt, as with a motorcycle fork or a bicycle crank, but here we have just a dinky little strap that badly wants to collapse. Who the hell am I to criticize the wizards at Porsche but in my humble opinion this setup is novel but a step backward, if anything.

Take for example the pinch bolts and split adjuster sleeves on production car tie rods (pre-rack and pinion). Replace them with solid sleeves and jam nuts and you have a far better deal. More positive, longer lasting.

[NRoshier]

greg could you not adjust the weight via the spring perch?

[Fat Boy]

Originally posted by NRoshier
greg could you not adjust the weight via the spring perch?

Not Greg, but no.

You adjust your spring perch to get the correct rocker orientation/loaded damper length. You adjust your pushrod for ride height / corner weight.

If you're changing corner weights, you'll have to do both.

Honestly, I like the shim adjustable pushrods. You have to make the adjustment in the air (really a very small issue on a car that is in the air (on air jacks) every time it stops anyway. Mechanics don't even have to know numbers. You say 'a red shim in both fronts' and it's done. Hard to screw it up, easy to repeat. Sure you have to carry a bunch of anodized ride shims, but it's not that big of a deal.

[LMP900]

This isn't too uncommon. Greg's listed some of the advantages, but another one is that it has less effect on corner weights when you lock it up. Also, you don't need two massive spanners to lock up a 1" UNEF (or whatever) nut. BTW you don't need a LH/RH thread combination in any turnbuckle as long as you have a difference in pitch. And I agree, shims are definitely nicer.

Regarding the tube diameter, there's no real aero issue for Porsche to worry about, and I've found the pushrod stiffness to affect the car, even though all have been above the buckling limit. Maybe the combined effect of the natural frequency and the compressive load?

[Charles E Taylor]

This arrangement is not new.

Take a look at the Camber Adjustment on any of the Lotus F1 cars, from the late 60s-80s.

The Adjuster is a RH inner/LH outer slotted with no lock nut.

Although this type of arrangement reduces the parts count, I think the real attraction is its short length and stiffness.

This method gave little or no problems in service. The main issues were with cracking of parent suspension component, (normally made from heat treated 4130 Steel); particularly when the clamp bolts were over-tightened.









Charlie

[McGuire]

The pushrod is fine but I'm still not impressed with the pinch clamp. Where you have pinch clamps repeatedly done and undone you have trouble.

BTW, I never thought much of pinch clamps on bicycle seats either. The far better way is a split post with a through-bolt. Been a long time since I messed with bicycles, childhood to be exact, but in those days it was one of the differences between a quality bike and a cheap one. I Have no idea how it is done on contemporary bikes. It's carbon and glued in for all I know, or fancy duct tape. Beats me.

But then we often see things on race cars that are less than best practice. Let's keep reinventing that wheel.

[Fat Boy]

Originally posted by McGuire

BTW, I never thought much of pinch clamps on bicycle seats either. The far better way is a split post with a through-bolt. Been a long time since I messed with bicycles, childhood to be exact, but in those days it was one of the differences between a quality bike and a cheap one. I Have no idea how it is done on contemporary bikes. It's carbon and glued in for all I know, or fancy duct tape. Beats me.

On top end bikes now they have an 'integrated seat mast'. The seat tube extends above the junction of the seat stays and top tube. A cap is then place over the seat mast to attach the seat on. You have to cut the tube to length and then use shims for fine tuning. The pinch bolt just holds it in place. It would be hard to use this exact approach on a racecar since at times the pushrod sees loads in tension where as a bike seat never does. The idea is similar to the Reynard shim adjust, though.

[McGuire]

Nice. Entire bicycle probably weighs less than a handlebar from a balloon-tire Schwinn.

[Fat Boy]

Originally posted by McGuire
Nice. Entire bicycle probably weighs less than a handlebar from a balloon-tire Schwinn.

It depends whether or not you have the hooks for carrying the newspaper bag installed or not.

[Greg Locock]

Believe it or not my final year project was on the softening of structures as they approach elastic buckling. So, yes, it is a real effect.

[McGuire]

Originally posted by Fat Boy


It depends whether or not you have the hooks for carrying the newspaper bag installed or not.

The Hot Setup, 1960. Women want him. Men want to be him.

[Lee Nicolle]

It seems a very backward step to me. Pinch clamps damage threads and sometimes jam.
A lock nut is simple and lighter too.

[imaginesix]

Originally posted by Fat Boy
You have to cut the tube to length and then use shims for fine tuning.

For real? You have to make a permanent modification just to fit the bike to the rider? So you can never sell your bike to someone taller than you???

[Ben]

Originally posted by imaginesix
For real? You have to make a permanent modification just to fit the bike to the rider? So you can never sell your bike to someone taller than you???

Yeah - it says about the same number of KGs that you could shed in body weight on a decent ride :-)

Ben

[Ross Stonefeld]

Originally posted by imaginesix
For real? You have to make a permanent modification just to fit the bike to the rider? So you can never sell your bike to someone taller than you???

These bikes have about as much resale value to the man on the street as a used F1 car.

[Greg Locock]

I hate pinch bolts on carbon fibre tubes. A great way of crushing tubes.

[RDV]

The one advantage of pinch bolts is that once you do your ride height or corner weight it won't change as it does locking a nut...a fiddly procedure.

Structurally I don't like them, have had problems with cracking as C.E.Tailor noted=

This method gave little or no problems in service. The main issues were with cracking of parent suspension component, (normally made from heat treated 4130 Steel); particularly when the clamp bolts were over-tightened.

...at Lotus other problems were caused by over-tightening, so some bots were oversized to requirements just for that reason (I.E.theorethicaly you could use a 1/4" bolt but went one size up because mechs would strip thread when doing-up)

[Ben]

Thanks for all the replies. With all due respect to the other replies, onece RDV, FB and Greg have replied I generally have the answer I need :-)

Ben

[McGuire]

Originally posted by Greg Locock
I hate pinch bolts on carbon fibre tubes. A great way of crushing tubes.

A pinch clamp does the same thing to a solid steel shaft, only to less obvious effect. It's a little portable blacksmith shop when you really look at it. The bolt/nut assembly stretches and digs in, the clamp crushes, and the shaft flattens -- especially threads if present. If the joint is only assembled once-ish that's ok, but every time it is moved it will have to be tightened a little more to obtain the previous clamping force.

Compare and contrast with a jam nut, which does minimal injury if any unless overtightened.

In my opinion a pinch clamp does not belong on a frequent and critical adjustment. What Lotus or Porsche did is not a ringing endorsement of it for me. Both are well known for being brilliant in conception and nightmarish in detail.

[Fat Boy]

Originally posted by imaginesix
For real? You have to make a permanent modification just to fit the bike to the rider? So you can never sell your bike to someone taller than you???

The shims give you about 30mm of adjustment. In most cases, that's enough to solve any fit issues. Resale isn't great. I suppose it's about the same as a race kart. If you get 1/2 your money back after a year, you're smilin'. If you're willing to buy used and be patient, you can get a nice bike for about 1/3 of retail. The carbon work on these things is really nice, too. The manufacturers seem to know what they're doing.

[Fat Boy]

Originally posted by Greg Locock
I hate pinch bolts on carbon fibre tubes. A great way of crushing tubes.

In any situation, they need to be used by someone who knows what they're doing . You don't need a rock-ape cranking down on the bolts with a 6 foot cheater bar. You need a guy that knows it just needs a snug to do it's job. That doesn't eliminate the problems, but it does minimize them.