Are High Tensile Bolts Needed?

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Originally Posted by Jeff

Does using high tensile bolts create the potential for damage to the chassis?

Everything needs a weak point and if the bolts will not stretch, the chassis could be damaged as the tow bar is of thicker steel than the chassis.


Jeff

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The mounting bolts are in tubes through the chassis, so thickness is not directly relevant. If you load the tow bar sufficiently to deform the chassis (usually by dragging it on a rock) you have exceeded the design load of the towbar.

With a towbar the last thing you want is a weak link that will fail, and a bolt that stretches is on the point of failure - deformation of the cross member is always preferable to a failure that could see the loss of a trailer.

Note that tow bars are not designed for snatch recoveries, although widely used for these!

John

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Originally Posted by isuzurover

do you want your tow-bar to break off and break someones legs (or worse) when doing a recovery
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Well not really. If doing a recovery people should be made to stand far enough away. This may not always happen.

I have seen the after affects of a similar thing with HT bolts. One tore off the rear cross member, but it was possibly rusty. The other pulled the bolts through the cross member.

Jeff

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Originally Posted by amshaw

For some reason I thought that HT bolts had to be black....but Ive had a look at trhe bolts Ive just used, and they have 8.8 stamped on the head but are silver.....:)

In my case the bar is rated to 3000kg

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Most HT bolts are black, some are zinc plated, which yours apparently are, but although Isuzurover says galvanised ones are readily available, I cannot recall ever seeing any, but I would be surprised if they are not available unless the heat of the process interferes with the heat treatment of the steel. It is also possible that the extra clearance required on the thread (for the zinc layer - normally the thread on nuts is not galvanised) reduces the strength of the bolt.

John

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Originally Posted by Jeff

Well not really. If doing a recovery people should be made to stand far enough away. This may not always happen.

I have seen the after affects of a similar thing with HT bolts. One tore off the rear cross member, but it was possibly rusty. The other pulled the bolts through the cross member.

Jeff

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The driver of each vehicle cannot stand away - there is a pic floating around of a jeep where the recovery point on the vehicle in front failed and went through the windscreen - missing the driver's head by inches.

As John said - there should be NO weak links.

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Originally Posted by JDNSW

but although Isuzurover says galvanised ones are readily available,

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(I should have said zinc plated bolts in the first post) I have never had a problem buying ZINC PLATED grade 5 (3 lines on head) bolts from bolt suppliers. However zinc plated Grade 8 or Metric class 8.8 bolts are much harder to find (which is strange because Metric 8.8 = Grade 5). I have a few zinc plated grade 8 bolts - but I found them at a swap meet so would not know where to get more.

You won't easily find graded gal bolts - as the heating affects the strength, but I don't know why zinc electroplated bolts are not more readily available.

I used HT Bolts on my ARB Rear Step for the Rangie, they are about M12 of M14 ISO Metric Bolts they are readily available from Coventry Fastners.

Blackwoods & Atkins (previously Atkins Carlyle) Has grade 8.8 or 10.9 HT Bolts in Metric or UNF in Coarse and Fine threads.

I thought that 8.8 was regarded as the norm, below this was low tensile and above this was high tensile.
Most brake components by law have to be 10.9 or above.
They arn't hard to come by, it's just that they arn't a common moving stock, so bolt shops don't want to stock them, and they would rather sell you something there and then.
Whilst rebuilding my D90 I was often into the local Coventry Fastners, they had no dramas with ordering 12 rated cadnium plated bolts for me. And they had almost anything in zinc 8.8.
Just a personal choice but I would never install black bolts into a car, to many dramas down the line.

But to answer the original Q, I believe that's its a ADR (I'll have a look) that all towing assemblies must be installed with HT bolts. It was one of the things my 90 was knocked back on when I was having it engineered for rego.

Cheers

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Originally Posted by JDNSW

Note that tow bars are not designed for snatch recoveries, although widely used for these!

John

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here here

i was discussing this with an ex police automotive engineer / ex coronial inspector just last week and just confirmed it with him again on the phone now

it is an offence to modify in anyway a towbar or bullbar or its fixings..

it is not recommended that you use any towbar or bullbar as a recovery point

case history suggests that if you do so you will be apportioned the majority of any blame in an enquiry or inquest involving such equipment.

towbars and bullbars must be fitted to a vehicle using the kit supplied by the manufacturer using the method documented by the manufacturer.

most towbars and bullbars use grade 8 bolts but many still dont as they employ shear bolts.

some towbars and bullbars are manufactured with shear points as well as shear bolts in the fitting kits..

so there you go...

straight from the informed horses mouth.

Ok I have never had an issue getting 8.8 HT bolts in bright (Zinc coated) which in some ways is just as good if not better than gal because of the heat treatment aspect.

When I first walked into the workshop at the golf club, there was not 1 HT bolt in the place as the previous mechanic just used the cheapest he could get or went to bunnings.

From my experiance in most cases it is better to use HT bolts in steel if it doesnt need a sheer point to make it safe.

Non rated or LT bolts are for timber not steel and thats why they are sold at bunnings.

I have 1 machine at work which uses LT bolts which have to be torqued to specs so that if it hits something which can break something expensive it will just break the bolts.

If you put HT bolts in it as the previous mechanic did because they kept breaking bolts, it sends a shock through the gearbox which has 50hp at 540rpm running through it and makes mince out of it.

This is why you should always use HT bolts in a towbar and make sure their tension is FT all the time.

This might sound a bit odd re this chat, but, our all steel billycart (which ran so well at the Bangalow Billycart Derby last Sunday), sheared several bolts (not HT) after an impact from another competitor and rollover...

GQ

crikey!

:P