Mercury Mk3 - securing guards

[Magnum3127]

Hello.

Having removed both front and rear guards for the summer I now want to refit them. Slightly concerned about securing the studs that go, vertically, into fixing holes. Don't want to over tighten and crack guards. Equally, don't want the bolts coming loose and rubbing on tyres. Is there a recommended torque? Would anyone use a thread-lock fluid to ensure secure fitting?

Thanks in anticipation.

Magnum3127

[Danneaux]

On my old 8mm nutted through-bolts, I used thread-locker to keep them secure, but it has not been necessary since I upgraded my fleet with PlanetBike 10mm through-bolts with Nylock nuts (all stainless hardware). PB has a nice stock of reasonably priced spares they will mail for free domestically (US) because they feel it will allow repairs to be made to their existing products and so keep them out of the landfills. They also make dandy upgrades, along with their stainless fender (mudguard) stays in single- and v-configurations. See...
https://www.planetbike.com/service-parts-free-shipping/

I have to confess to replacing the Thorn supplied nuts, through-bolts and stays on my original Sherpa and later Nomad. Thorn did a superb job mounting them and nipping the ends flush with the cross-braces, but I prefer to have my 'guards adjustable depending on season and whether I anticipate a lot of rain and mud where I would like to allow extra clearance. Cutting and capping my stays so they allow at least 10-15mm of additional clearance has been really useful for me. In normal conditions, I draw them closer to the tires, leaving about 10-12mm clearance. When it is going to be messy, it is really nice to extend that, particularly at the bottom of the front mudguard where a lot of mud, muck, and debris tends to collect. Having a bit more clearance there allows the accumulation to drop off under its own weight when I hit a pothole.

Don't want to over tighten and crack guards. Equally, don't want the bolts coming loose and rubbing on tyres.

I find holding the stay along its length with one hand below the nut/bolt assembly while I tighten the nut with the other hand keeps the cross-brace from torquing in the plastic mudguard, minimizing any possibility of cracking the blade where the bracket is riveted (the most common point of cracking). Conversely, I've see them crack when people got careless and just started cranking on the nuts which bows the stays slightly and pulls the brackets at an angle to the fender blade, causing stress at the rivets.

Is there a recommended torque?

Mudguard strut nuts should be about 20-25 lb/in (2.3-2.9 Nm) according to this source, which correlates pretty well with my experience...
https://www.dedhambike.com/articles/torque-table-pg186.htm

Best wishes and good luck, Dan.

[PH]

Dan's given a very thorough answer about the stays. but I thought the question was about the direct fitting of the guards to the frame and forks? I could be wrong, it does happen
For those direct fittings to fork and frame, fairly tight and using a bit of general purpose medium thread lock will help, though if you're going to make a habit of fitting and removing it can be a pain re-applying threadlock.
That's it really, unless you're a bit obsessive... Here's what I do, it's not a recommendation, but it keeps me happy and I don't have problems with them.
I use flange button head Torx screws, I use these for a few things so I have stock, the advantages are - The button head collects less dirt under the guard than a cap or hex head, the flange distributes the stress and the Torx head is less likely to round off than an alan head.  Also, with any M5 button head, you'll round it out before damaging the thread, and though you shouldn't do either, that one's an easier problem to deal with.
Something like these:
https://www.accu.co.uk/torx-flanged-button-screws/485122-SHBF-M5-14-A4?uk_google_shopping=1&c=3&gclid=EAIaIQobChMI9qDgoPPT-QIVhBCLCh06sgpQEAEYASABEgL9efD_BwE

Then I like to use a rubber washer between guard and frame, stops any rattling and protects paintwork, traditionally this would be a leather washer, but you don't get those in Wilco's.
I use both holes in the fork, I don't know why there's two, maybe its for choice or maybe it's just more secure, maybe someone who has has them fitted by Thorn will say what they do.  I find doing them both up tight distorts the shape of the guard a bit, so I use a couple of washers under one. I also like to use longish screws, longer than the gap between guard and tyre, in the unlikely event they both loosened the guard would be sat on the tyre but still attached to the fork. Frame fittings are a bit less safety critical, you'd have to loose both the chaistay and seatstay screws before it went anywhere.

[mickeg]

...
Having removed both front and rear guards for the summer I now want to refit them. Slightly concerned about securing the studs that go, vertically, into fixing holes. Don't want to over tighten and crack guards. ...

I am not sure what you could crack.  I have some fenders that have plastic parts that could be cracked (have cracked a plastic part on a fender or mudguard on a Bontrager).

Could you elaborate on what you are afraid of damaging?  Photos help.

[PH]

I am not sure what you could crack.

I've seen quite a few examples of cracked chromoplastic guards, always at the fittings, never whilst being fitted, but sometimes after not a lot of use and nearly always guards that could IMO have been fitted better.
It is quite a rigid plastic, the key is for it not to be under strain, there's a very simple test to do for this - after fitting, remove and re-fit one fixed point at a time, the guard shouldn't move, each fixed point should be holding it there, not pulling it into place.
Or - treat them as a consumable and expect to replace every few years. 

[mickeg]

I am not sure what you could crack.

I've seen quite a few examples of cracked chromoplastic guards, always at the fittings, never whilst being fitted, but sometimes after not a lot of use and nearly always guards that could IMO have been fitted better.
It is quite a rigid plastic, the key is for it not to be under strain, there's a very simple test to do for this - after fitting, remove and re-fit one fixed point at a time, the guard shouldn't move, each fixed point should be holding it there, not pulling it into place.
Or - treat them as a consumable and expect to replace every few years.

I am not seeing frequent cracking of plastic fenders.  Decades ago when most fenders were metal, those were the ones that I often saw cracking.

The only cracked plastic fenders that I have seen were on mine, the bike was on a rack on the back of a vehicle and when driving down the highway the strong side wind for a few thousand miles of distance caused a resonance that I could see in the rear view mirror.  That resonance eventually caused cracking.  A bit frustrating, but not the fault of the manufacturer which was Planet Bike.  They sent me a new bracket,  I had to drill out the rivets to replace.  First photo after I bent the stays outwards a bit.  The previous bike owner installed them before I bought the frame (Thorn Sherpa) in 2010.

The Zefal fenders on my rando bike have been in nearly continuous use since 2004, no cracks,  second photo.  I have not seen that model sold in over a decade, unfortunately.  I like them.  The mud flaps and taillight were my addition.

I had a bracket (plastic) crack on a Bontrager fender, but my fenders were the only ones I have ever seen with that specific bracket, I suspect the manufacturer quickly redesigned it after they sold a few.

You mention Chromoplastic, SKS fenders are a rarity in my region.  If those have an affinity for cracking, that would explain why it is common in your region and not mine.  Most fenders in my region are Planet Bike with Bontrager being in a distant second place.  I almost never see those fenders crack.

I wonder why the formatting on this website is putting my comment in quotes?

[Magnum3127]

Hello all,

Thank you for your helpful responses. Much appreciated.

I was concerned about over tensioning the studs and cracking the plastic around the securing point.

PH - Spot on! The guards were removed for one specific ride but remained off for the summer. They will be fitted permanently from now on as the mercury will be my autumn/winter bike. I'll 'snug' things up finger tight using a thread lock and then use your strain test.

Dan - Thank you for the link to the torques table. That will be very useful.

[PH]

You mention Chromoplastic, SKS fenders are a rarity in my region.  If those have an affinity for cracking, that would explain why it is common in your region and not mine.

Maybe that's it.  If I put the appropriate words into google I get plenty of examples, several makes of mudguards available in Europe are a similar construction, it does have advantages, quite impressive strength to weight, just prone to cracking, probably fatigue,  if not fitted well.  Some of the more recent SKS guards, like the matt black ones, are a different plastic which is likely to be less brittle.  Though as I haven't cracked any, I'm hoping not to test that theory.

[PH]

I'll 'snug' things up finger tight using a thread lock and then use your strain test.

It's when I test them like this that I add the threadlock, saves it getting messy!
I do this stuff without really thinking about it, it's probably easier to do than explain...

[B cereus]

I'd second PH's recommendation for button head fixings and rubber washers. You need a fairly hard rubber and I find tap washers ideal, available from plumbers merchants or decent hardware shops. They can also beplaced between the frame and mudguard if the latter is not quite concentric with the tyre.