The New And Improved Unofficial R65 Forum V2

Technical Discussion => BMW Technical Q&A, Primarily R65 => Topic started by: rob650 on June 12, 2010, 07:26:03 PM

Title: How much front end brake dive is too much?
Post by: rob650 on June 12, 2010, 07:26:03 PM

I pushed my bike into the garage today, applied the front brakes and noticed the front end seemed to dive quite a bit at my walking speed.  How would I know how dive much is "too much"?

Never noticed any handling problems in the straights or twisties, although the ride has always been a tiny bit "cushy" compared to other bikes I've tried.

Progressive springs installed at least 6 years ago.  Fork oil changed a year ago, i step up heavier than stock (done to remove some of the "cush", not much effect)  No leaks on the forks.

Thanks!

Title: Re: How much front end brake dive is too much?
Post by: Bob_Roller on June 12, 2010, 07:29:28 PM

The front forks do 'dive' quite a bit, I've got Progressive springs on my '81, and it still moves quite a bit when moving the bike around in the garage, when you apply the brake .

Title: Re: How much front end brake dive is too much?
Post by: Barry on June 13, 2010, 03:13:19 AM

Dive under heavy braking is normal.  You could try increasing the oil level which will reduce the volume of air in the forks and increase the "air spring" effect under compression.

The forks will also dive in the way you describe at walking speed even with heavier fork oil but this is not necessarily indicative of their performance on the road. The damping really only comes into effect when the forks are moving very much faster in response to road bumps.

Heavier fork oil eliminated the floating feeling I was getting at the expense of some ride comfort. That's just the limitations of damper rod forks which provide little damping at low velocities and too much at high velocities because the damping effect through a simple orifice rises exponentially with velocity. This is the key to understanding damper rod forks and why they can't be compared to modern cartridge designs where the velocity vs damping curve is made to change by the use of clever shimmed valves. The fork oil can then be chosen to provide enough damping at low velocities and the valve is set to open up at high velocities to limit the rate of damping increase. Our forks can never be that good unless fitted with cartridge emulators.  

To go back to your question "How would I know how much dive is too much"?  If it dives a lot at walking speed than that's potentially a good thing as it indicates that you have good fork alignment and low levels of stiction. If I push my bike 1 yard and brake I get 3 1/2" of travel with ease and 4" if I try a bit harder.

Title: Re: How much front end brake dive is too much?
Post by: rob650 on June 15, 2010, 05:08:17 PM

Thanks for the explanation, Barry and Bob.