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Forks Bottoming
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Forks Bottoming - August 20, 2012, 11:58 AM

Iíve noticed the past few track days that Iíve been routinely bottoming out my forks in hard braking areas. When this happens I notice the rear feels like itís moving around a lot. Not just up, but side to side. I was planning on refreshing my forks this week and I wondered what I could do (aside from re-valving or aftermarket cartridges) to provide more resistance to bottoming.
The most common solutions I found while researching was either reducing air-space or upping compression. Some of the less common solutions were increasing pre-load or upping spring rates.

Info:
1st Gen SV650
06 GSXR Forks w/.90 Springs (unmodified)
190-200 lbs rider

Any advice would be greatly appreciated!


-Tommy

Last edited by Desmo; August 20, 2012 at 12:04 PM..
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August 20, 2012, 12:23 PM

IIRC, for that weight .90 is too low... you should be running .95.... I think.


If compression is set right, check and maybe up your oil weight/ level?
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August 20, 2012, 12:28 PM

Yeah based on the minute amount of info I know on suspension, your springs are way too soft. Even I'm supposed to be running .95 and I'm quite a bit lighter than you.


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August 20, 2012, 12:37 PM

Springs to soft means in order to set your sag setting and pre-load you are going to be shortening your suspension travel and any hard braking is going to leave you bottoming out.
Road racing rate for 200lbs is more like 1.0kg
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August 20, 2012, 12:37 PM

Those springs sound light, but getting the sag in the right range is more what the springs are about. How much preload do you have to use to get your sag numbers?

The air gap/oil height will definitely effect the bottoming situation the most.

It can also be affected by your riding style/technique. If you are not keeping yourself back in the seat and using your legs and torso to hold your weight versus putting a ton of pressure on the bars while hard braking, it will transfer more weight to the front.

It can be very normal for the rear end to move around hard on the brakes, but too much might be slow rebound on the rear or the before mentioned to forward weight position of the rider.


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August 20, 2012, 12:38 PM

The springs were my first thought, but according to the race tech calculator they recommend 0.907kg/mm springs for my weight on the SV. Of course that would be for the stock damper rod forks. I wonder if that makes a difference?


-Tommy
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August 20, 2012, 12:47 PM

Quote:
Originally Posted by Desmo View Post
The springs were my first thought, but according to the race tech calculator they recommend 0.907kg/mm springs for my weight on the SV. Of course that would be for the stock damper rod forks. I wonder if that makes a difference?
Yep, there is a difference. Even for your weight assuming track use with stock SV forks you would still be .95 to 1.0


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August 20, 2012, 01:11 PM

I just noticed the the race tech calc lists the weight of the bike "semi-wet" as 364lbs. That sounds a bit light to me.

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It can also be affected by your riding style/technique. If you are not keeping yourself back in the seat and using your legs and torso to hold your weight versus putting a ton of pressure on the bars while hard braking, it will transfer more weight to the front.
I definitely had this problem before i put tank grips on. I feel like I do a good job of keeping my weight back now, but I'll focus on it a bit more now and see if it helps.


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August 20, 2012, 01:31 PM

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I just noticed the the race tech calc lists the weight of the bike "semi-wet" as 364lbs. That sounds a bit light to me.
It is.... they are around 415-425 with a full tank of gas in street trim. Obviously depending on what else has been done to the bike to make it lighter needs to be accounted for.


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August 20, 2012, 04:05 PM

First step: Find out what the difference in sag is between the bike sitting at rest (Statics sag) and rider sag (you on the bike in riding position). If memory serves you want to see around a 15mm difference. Much more than that your springs are too weak, much less than that they are too stiff. Buy springs accordingly if needed.

Second step: Check what your fluild levels are currently and compare that to what is recomended for the forks you're running. Small changes make big differences. Less air gap can make the initial motion a bit stiffer and bottoming resistance much stiffer.

Third step: Set preload so that your rider sag is 25-30mm (I defer to the SV experts here as to what to use for a baseline)

Testing: Front end feel is a lot about personal preference. In general you want to be using almost all of the available travel without bottoming. You can fine tune this a bit with preload and/or airgap. Attach a ziptie snuggly around one fork leg and you'll be able to tell how much if any fork travel you have in reserve and adjust accordingly.

Sometimes what you think is happeneing ain't what's really happening. The ziptie doesn't lie though.

That's my free advice, likely not worth the cost.


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August 20, 2012, 04:44 PM

Lose weight? That's what I always get told.

YMMV.



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August 20, 2012, 04:46 PM

Heavier fork oil and decreasing the air level maybe? I figure those would be the cheapest and fastest ways but springs are pretty cheap too.


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August 20, 2012, 04:50 PM

Quote:
Originally Posted by n2sport View Post
Springs to soft means in order to set your sag setting and pre-load you are going to be shortening your suspension travel and any hard braking is going to leave you bottoming out.
Road racing rate for 200lbs is more like 1.0kg
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Sometimes what you think is happeneing ain't what's really happening. The ziptie doesn't lie though.

That's my free advice, likely not worth the cost.
True to an extent. It can give misinformation though. Best way (in my limited world) that I found how to get the most out of it is to push it up and do 1 lap. Take it relatively easy except in one corner. The one where you notice the problem. Come in and check it then. The tie will only tell you the lowest point ever. However, overall, you are correct sir.
FWIW, I am running a .95 and I am 180 w/out gear. Different bike, but still.


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August 20, 2012, 05:10 PM

Get the correct spring and then have your fork oil changed and set to the right height. Then set your sag and start trying to dial in your rebound and compression. If you set it up wrong it will not handle good at all. Might even get worse.


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August 21, 2012, 09:38 AM

Thanks for the advice guys. The bottoming is zip tie confirmed and the sag was set by Kenny @ MRP. Plan of action:

Step 1 Get less fat
Step 2 Get stiffer springs
Step 3 Start all over using the steps windblown laid out


-Tommy
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