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Brake Dive

630 views 9 replies 5 participants last post by  Mike K. 
#1 ·
So for the first time since buying my bike 3.5 years ago, I decided to change the fork oil. Afterwards, I took some measurements, including brake dive, which came out at about 35 mm. Does this sound like a lot? Static sag with me sitting on the bike is 50 mm. Fully extended, there is 163 mm of total travel. I used this stuff in 10W, 500 ml in each leg:

https://fortnine.ca/en/maxima-fork-oil

So far it feels almost exactly the same as it did with the stock oil.
 
#3 ·
How did you measure "brake dive"? If you want to see how much travel of the forks you are using while riding, attach a zip tie to one fork leg right above the dust seal. Make it tight enough to travel up the fork and stay in place, but not too tight. Go for a normal ride on roads you usually travel on and brake to the same degree as usual. You should use all except maybe 10mm of the fork travel. The distance from the zip tie to the bottom of the triple clamp is what you are measuring. The air gap is also important, not just the volume of fluid.

50mm of rider sag should be somewhat soft in the front end. Actually, quite soft.
 
#6 ·
...attach a zip tie...50mm of rider sag should be somewhat soft in the front end. Actually, quite soft.
Felony, yes, that's what I did. When I get home from my rides the zip tie is generally 78 mm from the lower clamp, so 113-78=35. Roads are quite smooth around here, and I watch the zip tie when I'm braking and it always moves up so I know all that travel is being used in the dive. The spec I found on the internet was 499 ml oil/106 mm of headspace, so I figured replacing the old oil with a nearly identical volume would give me very nearly the same headspace.

Boyracer, when you jack the bike right up off the ground the fork legs extend to their full length, exposing 163 mm of the upper tube. When you set it back down on the ground, some of that travel is taken up by the weight of the bike and then when you sit on it even more is taken up. In my case, just the weight of the wet bike uses up 46 mm of travel, then when I sit on it a further 4 mm is taken up so my total usable travel is 113 mm, so a bit less than the claimed 120 mm. I weigh about 190 lbs. fully clothed for riding.

Overall, I would characterize my forks as unresponsive. They soak up larger bumps (>1") fairly easily but barely move for little bumps ( ~1/4"). Really big bumps (>2") are felt in the bars but there is no wheel kick, so the forks seem to be doing their job, but I'm wondering if I can't dial it in with different oil, headspace adjustments and shorter/longer spacers. It's not bad enough for me to go get new springs. They work fine for my riding regime which is 70% flat, straight, well-paved county roads at 60-70 mph, and the rest is urban streets at 20-40 mph.
 
#5 ·
The last 10mm is a hydraulic bump stop to prevent metal to metal bottoming out.

I run a fairly soft setup and thats approx 43mm rider total sag, but my forks run different springs, emulators and a lower oil level.
 
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#8 ·
Maybe travel was the wrong word. I just meant that there is 163 mm of the inner fork tube exposed when the front wheel is off the ground and the forks fully extended, measured from the bottom of the lower clamp to the seal lip where it contacts the tube. Never measured anything with the spring removed.
 
#9 ·
OK now I think I understand, you don't have travel all of the way up to the bottom of the lower yoke clamp - all you have from fully extended is 110mm, that's when it stops. You could measure it the other way around, your travel will end around 53mm measured from the bottom of the lower yoke clamp..........
 
#10 ·
I always forget travel includes downward movement of the bottom legs, so I guess my bike has about 50 mm of downward travel (i.e. rider sag) to deal with dips in the road and a potential of 70 mm upwards travel to go over bumps, although the most upward travel I have used so far is approximately 35 mm from brake dive (78 mm measured from clamp to seal). I may have to seek out some bigger bumps to see how well the shocks soak them up, as well as find some potholes to see how well the forks extend. I am usually very fastidious about avoiding any imperfections in the road.

I measured the spacers and they were 235 mm long but I never measured the springs, washer thickness or the internal length. When I get my front tire changed later this year I might take the forks apart again try some stuff. I never realized how tinkerable forks are. There are quite a few adjustments one can make for just a few bucks. I am starting to think maybe a softer set-up might be in order. Seems I have two conflicting goals. One is to minimize brake dive which would indicate more pre-load (longer spacer) and maybe heavier oil but the other goal is more responsiveness over small bumps which I am guessing would mean less pre-load and maybe lighter oil. I have read that headspace only comes into play and the very end of the upward stroke.
 
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