Rear Wheel Vibration On Descents

ashcroft121

Hi all,

Never experienced this in my mountain biking days but now I am going downhills a lot faster! On my road bike (Giant Defy 4) both times I have descended steepish hills (14% gradient) recently I have had vibrations that increase in size at the back wheel. The only way to stop them ultimately throwing me off the bike is to slam on the rear brake which stops them but slows me down obviously!

Today I was at 67kph on the speedo free-wheeling down the hill and it started again. I have to confess to panicking somewhat but luckily regained my nerves enough to hang off the rear brake and stop the bike.

Both times I have been in the drops and I slow to a stop, check the back wheel (all seems fine) and carry on without any further consequences.

Normally I would suspect the wheel is out of alignment but there's no sign of any juddering until I get above 55kph then it comes on all of a sudden as above.

Any help appreciated, it's really annoying - I want to sprint down the hill in question without worrying about my rear wheel throwing me off the bike!

gavinp

Hi

What make is the wheel?

ashcroft121

Hi Gavin,

They are the stock ones that come with the Giant Defy 4. This is pulled from a website selling them: -

Rims Giant S-R2
Hubs Giant 24/28h
Spokes Stainless Steel 14g

I've fitted a set of Michellin AirComp Latex inner tubes to both wheels and fitted a set of Vittoria Open Corsa SC 700x23c tyres to the wheels. The bike has had the saddle moved forward slightly and a new in-line saddle post fitted plus the stem has been replaced with a shorter 60mm one. I've got a set of aero bars fitted too. The bike has been professionally fitted to me to achieve as close to an aggressive tri-position as possible.

When I was descending I was on the drops - don't do downhills on aero bars yet!

gavinp

Hi

From what I can gather, there are sometimes slight imperfections in the frame or wheels that could possibly be a problem that only shows itself at high speed, but I would suggest that the changes you've made on your bike and adopting a more aggressive riding position have all changed the centre of gravity on your bike which causes the same issue of high speed wobble (scary!).

Something that should help (hopefully) is to position yourself as far back as you can when descending, so that your center of gravity is over the back wheel more (you may need to experiment with this as you don't want to lose your front wheel either..). Additionally you could pin/squeeze your knees to the top tube. The theory is that changing the center of gravity and dampening the frame vibration keeps the wobble from increasing with speed.

That said, get the bike frame looked at and wheels trued just to be on the safe side.

Hope this helps. Good luck!

backtothecaves

Your wheel needs truing up. Simple as that.

ashcroft121

Hi and thanks for the replies. I checked the rear wheel last night by spinning it up with the bike upside down. There is a really small vibration when I spin it up by hand in the highest gear but it's not that much of an issue. Both wheels appear to be true both axially and laterally.

I'm erring towards what Gavin has said. We moved the saddle forward about 4cm and shortened the stem by 1cm to get the optimum triathlon fit for me. I guess that as the bike designers didn't plan for this (it is a road bike after all) that this is causing some sort of instability.

If the wheel was out of true wouldn't the vibration build up slowly as speed increases rather than come on suddenly at around 40mph?

Both times it's happened it's been around 40mph which is why I don't think it's a wheel truing issue.

gb901

The small vibration you experience when spinning the wheel with the bike upside down will obviously be accentuated when travelling at the higher speeds you mention. I imagine the wheel is very slightly out of true and would suggest trip to your LBS.

ashcroft121

Thanks,

Will get it sorted and report back!