Spoke tension question

andygo

Hi

How much do (or should) front wheel spokes flex under heavy braking?

I've recently built up a bike - including building up the wheels - but I'm wondering if I've tensioned the spokes correctly (enough)?

The reason I ask is I'm getting some issues with the front wheel spokes rubbing against the disc brake caliper under heavy braking - especially on steep downhills....and I'm wondering if spoke tension is to blame or the position of the caliper.

The brakes are Spyres and they came with 160mm rotors. They also came with 2 mounting brackets (for 160mm and 180mm discs). The bike is a Croix de Fer.

Using the 160mm bracket leaves a gap of less than a credit card (1mm?) between the caliper and the spokes and I can hear rubbing even under moderate braking.
Using the 180mm bracket pushes the caliper further away from the axle and increases the gap. I don't get any rubbing noise - but only half the pad comes into contact with the 160mm disc - so obviously not ideal.

I've found that inverting the 180mm mounting bracket also increases the gap (not as much) and allows full contact between pads and rotor. It worked for a while ...but I had to adjust the pads for wear recently and the rubbing noise has returned.

I've emailed TRP to ask what they think....and obviously the easy way to solve the problem is to buy a 180mm rotor...

But in the meantime I'm wondering about spoke tension - is it normal for spokes to flex so much under heavy braking - especially near the elbow?

I'd never built wheels before and used Roger Musson's book ...I also had some help on this forum too. (The wheels are made up of Novatec hubs (32h), A23 rims and alpina db spokes in a standard 3x pattern).

I don't have a tension meter. I tensioned the front until it became very difficult to tighten them any more - I also stopped when I snapped a spoke which had run out of thread. As far as I'm aware I used the correct length spokes - Ugo checked lengths for me). The spokes all make a high pitched ping - much higher than another wheel (same size and 3x pattern) - though obviously I don't know if you can compare two different wheels

I've checked them a few times and they've stayed true after a few weeks use. They're centred correctly too....

Any advice appreciated

Andy

ugo.santalucia

That's impossible, with that hub you have a clearance caliper/spokes of about 15-20 mm... Unless those TRP are seriously beefy and badly designed (which is possible, knowing the brand)
If you use a Park Tension gauge and you've used ACI DB spokes, you are looking at 21/22 reading on the left side front ... but even if you had 18 or so it still would not happen

andygo

My imagination is pretty good but this is all too real sadly. it's the caliper body touching the spokes - not the disc. I'll post a pic

ugo.santalucia

It's a problem with the caliper then, not the hub or the wheel or the spokes...
It is probably the reason why all the other manufacturers make one piston system rather than two. I have Hayes CX 5 and had BB7, the clearance is huge

andygo

Cheers for that, I'd read somewhere on here that spokes flexing at elbows was a sign of a poorly built wheel...which made me wonder...

Looks like I'll go hunt down a 180mm disc then....

Here's a pic of gap between caliper and spokes using the 160mm bracket



This is the gap using the 180mm bracket

ugo.santalucia

Irrelevant to the problem reported, but suggest you buy a tension gauge... I don't have much faith in acoustic methods and if you have snapped a spoke during the build, it suggested you've gone a bit too far, which will in turn kill the rims pretty fast.

Looking at the photos I can confirm it is a clearance problem and TRP design's fault... the clearance is too tight!

EDIT: will a 180 mm disc fit with your forks? Clearance there is tight too

andygo

Ok ta, maybe time to get the lbs to check the wheels over and check tension....
I'm hoping the forks will take a 180 disc....this guy is using one - he's also using spyres too

http://smutpedaller.blogspot.co.uk/2014 ... first.html

So to satisfy my curiosity - it is normal for spokes to flex under braking at elbow....?

ugo.santalucia

Clearly it is...

Problem is tolerances should be respected and this sounds like a Spyre cockup... maybe caused in the rush to fix the previous issue

hypster

I just checked the disc brakes on my mountain bikes. The 160mm BB7s I have on my single speeder have about 2-3mm clearance to the spokes. The 180mm Deores on the full suss' have 5+mm. I have never experienced any interference with the spokes on the SS so I'm guessing it's a problem with reduced clearance the Spyres are giving you even if the spokes do flex a little under braking.

With respect to the tensiometer I would agree with Ugo, definitely the way to go. For me it just makes getting even tension in the spokes much easier and getting ultimate tension a lot closer than I would just guessing or comparing to other wheels.

alan_sherman

Just a thought - it is normal practice on rear wheels to lace the 'drive' spoke infornt of the 'trailing' spoke (apologies about the terms if incorrect).

The theroy is that when the wheel is torqued by pedalling the drive spoke is tensioned and the trailing spoke is unstressed. The drive spoke being outside the trailing spoke prevents it hitting the rear derailleur in bottom gear.

I had a wheel laced the other way (with training spoke outside) and it would hit the derailleur when the lowest cog was selected. A re-built to the correct pattern solved the issue.

Maybe a similar problem on front wheels with close clearance issues? With disc braking the stressed spoke would be under deceleration (so pointing the opposite way to the rear wheel 'drive' spoke). Ideally I'd want more clearance but that depends on the hub, disc and caliper combination used so will vary a bit.

andygo

cheers for replies....I think I can live with 2-3mm clearance and hoping the 180 disc will fit to give me that...I'd rather not have to relace the wheel unless I have to...but it's a thought, thanks

ugo.santalucia

andygo wrote:

cheers for replies....I think I can live with 2-3mm clearance and hoping the 180 disc will fit to give me that...I'd rather not have to relace the wheel unless I have to...but it's a thought, thanks

Seen a Spyre today with 711 hub built 32 x 3 cross... the clearance is 2 mm or so, but no rubbing... I think you need to play with the caliper position, it's probably a matter of fraction of a mm.
The wheel was built (by me) with the pulling spokes head out and the training (braking) spokes head in