seat stay deflection when braking

eoinf

Hi

My 11 month old Cube SL Cross is having a problem with deflection on the seat stays where the rear brakes are mounted.

I have had it in a few times (locally) for brake adjustment but the most recent time i was told that the problem is due to the visibly noticeable deflection on the seat stays.

Is this normal?

it has resulted in what used to be very sensitive and responsive brakes being a little less reliable.

The service guy also thought it is something which may get worse.

The problem is i bought the bike online. I have contacted the suppliers and they have suggested it is normal to see some deflection in the stays and suggested i replace pads and cables etc to get the brakes back to what they were. the service guy tends to disagree.

has anyone experienced this before? should i be concerned? is it normal to see deflection in the frame like this?

Cheers

EF

kenmc

I wouldn't have thought there should be a visibly noticeable deflection - in order for your eye to spot such a deflection it would need to move quite a lot, which would suspect a cracked frame to be honest. How are you producting the deflection? Is it on a work stand?

Try the manufacturer and see what they say - maybe with a little video of it happening if you can.

doozerie

Many years ago a friend of mine had a MTB with the rear brake cantilevers mounted under the chainstays. He had big meaty brake levers on it too. When you swung from the brake lever you could see some flex in the (steel) chainstays. Eventually one of the chainstays cracked, though it took a few years of heavy use.

Seatstays tend to be less robustly built than chainstays so I would expect that you might see some flex in them, but it probably depends on a number of factors: what material your frame is made of; are you using cantilevers or v-brakes (I suspect v-brakes result in less flex in the bike frame itself); whether the brake levers are substantial enough to allow a lot of force to be transmitted to the cantis/v-brakes; how much give there is in your brake pads; how much give there is in your brake cable; and possibly other things too.

If it is a real concern, or if you want to test the theory that it's frame flex that is affecting your braking performance, then you could fit a brake booster. These used to be easy enough to find in the past, I'm not sure how available/popular they are these days (I'm not sure if they are compatible with v-brakes, for example). It's basically a large U-shaped piece of aluminium which sits "on" your cantilevers - you remove the bolts which fix the cantis to the frame, sit the booster against the cantis, and slot in longer bolts to hold booster + cantis to the frame. It adds reinforcement to the frame to stop it from flexing. I had one on my old MTB for a few years and liked it but I seem to recall that there were some concerns over whether it was detrimental (to the frame I think) though I can't recall the details.

eoinf

kenmc wrote: »

I wouldn't have thought there should be a visibly noticeable deflection - in order for your eye to spot such a deflection it would need to move quite a lot, which would suspect a cracked frame to be honest. How are you producting the deflection? Is it on a work stand?

Try the manufacturer and see what they say - maybe with a little video of it happening if you can.

you cant definitely see it bulge/deflect , left and right, adjacent to where the pads push against the rim.

the frame doesnt seem to have any defects or cracks the bike hasn't got a lot of use maybe 300k on decent enough city roads in a 12 month period (not sure if this is a lot or not).

the deflection is visible with the bike on the ground just pulling the rear brake.

as i said it has resulted in a less responsive brake with a slightly more force required to stop where as a simple tap with relatively little force was enough to begin with.

i will try and take a photo or video tomorrow to see if i am just being paranoid.

thanks for your reply.

eoinf

doozerie wrote: »

Seatstays tend to be less robustly built than chainstays so I would expect that you might see some flex in them, but it probably depends on a number of factors: what material your frame is made of; are you using cantilevers or v-brakes (I suspect v-brakes result in less flex in the bike frame itself); whether the brake levers are substantial enough to allow a lot of force to be transmitted to the cantis/v-brakes; how much give there is in your brake pads; how much give there is in your brake cable; and possibly other things too.

Its V brakes against a rather lightweigth aliminium frame. as i siad the brake levers themselfs dont require much force to show the deflection.

doozerie wrote: »

If it is a real concern, or if you want to test the theory that it's frame flex that is affecting your braking performance, then you could fit a brake booster. These used to be easy enough to find in the past, I'm not sure how available/popular they are these days (I'm not sure if they are compatible with v-brakes, for example). It's basically a large U-shaped piece of aluminium which sits "on" your cantilevers - you remove the bolts which fix the cantis to the frame, sit the booster against the cantis, and slot in longer bolts to hold booster + cantis to the frame. It adds reinforcement to the frame to stop it from flexing. I had one on my old MTB for a few years and liked it but I seem to recall that there were some concerns over whether it was detrimental (to the frame I think) though I can't recall the details.

i am not familar with what you describe above. can you direct me towards an example?

thanks for your reply

Holyboy

You could always just fit an old school brake booster, you should be able to pick one up fairly cheap as they arent used much any more.

GTDolanator

ive a klein pulse pro with an xtr v brake on the back,the frame has a lot of flex whilst braking,do what holyboy says and get a booster,i think their a tenner on chain reaction,dosent stamp it out but noticably less flex

doozerie

Here is an example of a brake booster, this one fitted over v-brakes:



The, er, "stylish", colour combination above is optional.