Mechanics - how far should a screw reach?

[Chrispy]

OK - so I've not my nice new Titus El Guapo frame - assembled it up - and two of the bolts connecting the shock yolk to the upper stays seem a bit short...the've got six mil of penetration into the stay, which does not seem like enough to me. I realise that the shock (and mountings) takes the main strain, but it looks short to me - check the photo.



I've contacted Titus but not heard back as yet...what do you think?

 

[{ScarFace}]

There doesn't seem like there will be a particularly large amount of sideways force on the thread themselves. The actual bolt and bushings (i assume) will take most of the stress. If you're really worried about it, why not make a nut and bolt set up and have it the entire way through?

 

[Dales Cannon]

In theory two or three threads of a high tensile bolt into a high tensile nut are enough engagement but it depends hugely on materials. If the female thread is aluminium then you need more engagement. Practically you should have one to two threads exposed but flush or just short of flush is ok. Your set up is probably ok but I would run longer bolts.

 

[Dirt Devil]

I wouldn't run it like that if it was my bike. Time for a drive to the bolt shop.

 

[Dozer]

The bolts of shock eyes don't just hinge the shock, they are a big part in distributing the force to the correct area of the frame. I would not run the bike like that as the amount of torque put into the bolt is only partial. It is only a few turns short but it still doesn't look solid.

 

[hellmansam]

Where's Grip, he could explain this better than me but here I go anyway - the bolt itself is subject mostly to shear loads, not torque apart from tightening it up.

Part thread engagement is not ideal more because in aluminium you may strip threads if heavy handed and there's only a couple of turns engaged. I'd prefer all the thread to be used if it were mine. Anything more isn't doing anything useful

 

[ajay]

Where's Grip, he could explain this better than me but here I go anyway - the bolt itself is subject mostly to shear loads, not torque apart from tightening it up.

Part thread engagement is not ideal more because in aluminium you may strip threads if heavy handed and there's only a couple of turns engaged. I'd prefer all the thread to be used if it were mine. Anything more isn't doing anything useful

What's the pitch of the thread? If it has roughly 6 mm of purchase, at say (I'm
Guessing) 1.5mm pitch, you have 4 full turns. If its a smaller pitch you'll have even more. I doubts it's bigger than 1.5 though.
Also, just because the thread is there, it does not mean it must be used, what if the hole was 50mm deep? You wouldn't need a bolt that would go right through...
I'm not saying replace it or leave it, but if it does concern you, there's no harm in fitting a longer bolt.

 

[ajay]

Where's Grip, he could explain this better than me but here I go anyway - the bolt itself is subject mostly to shear loads, not torque apart from tightening it up.

Part thread engagement is not ideal more because in aluminium you may strip threads if heavy handed and there's only a couple of turns engaged. I'd prefer all the thread to be used if it were mine. Anything more isn't doing anything useful

What's the pitch of the thread? If it has roughly 6 mm of purchase, at say (I'm
Guessing) 1.5mm pitch, you have 4 full turns. If its a smaller pitch you'll have even more. I doubts it's bigger than 1.5 though.
Also, just because the thread is there, it does not mean it must be used, what if the hole was 50mm deep? You wouldn't need a bolt that would go right through...
I'm not saying replace it or leave it, but if it does concern you, there's no harm in fitting a longer bolt.

 

[T-Rex]

No

That's just wrong. For the sake of about $3 worth of M8 (or M6 can't quite tell from the photo) HT bolts, vs whatever your new frame set you back, it's not even worth having a discussion. Off to the fastener shop you go.

 

[tomacropod]

metric bolts typically come in 5mm length increments. Given a choice of having it a little bit short of the end, or protruding past the end, the manufacturer has taken the neater-looking option. If it bothers you, get a longer bolt and leave it protrude, or cut off the end. It seems that 6 of a potential 8mm of thread is engaged. I don't believe that adding another 2mm will significantly affect the integrity of the setup.

- Joel

 

[Banya]

If i remember correctly

As a general rule, the bolt should penetrate the thread to at least the same length as the diameter of the bolt for maximum strength. Just something an engineer told me once.

 

[Chrispy]

Thanks for all that - the bolts are aluminium, as is the frame - they are M8 bolts 16mm long (you don't measure the head as I've learned). Titus said they were 17mm long - but (and I've got a nice steel German ruler with half mil marks) I measured 16mm.

Now, pitch - that's the size of the space between the thread is it? I'm not sure on that. Titus did say they were standard M8 bolts - still waiting to hear back about what they think about.

I'll wait a bit more and then probably get some stainless steel ones - I was thinking about these:
http://www.boltsnutsscrewsonline.co...-Screw-Grade-316-Marine-Grade-Stainless.136/3
the only difference is that the thread run all the way down to the head - rather than a threadless bit where it goes through the bearing - will this matter?

thanks again for all the help

 

[spinner]

Aluminium bolts ? Really ?

I'd use high tensile steel bolts that use all the available thread in the frame. Not stainless , not alum (wtf?)
End of story.