Fork oil

aitormartin

aitormartin

expert rider
Joined
Oct 16, 2020
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Spain
This tuesday I will take my bike to the shop to be serviced. It´s getting close to 7500km, it's more than 2 years old and it hasn't been used in almost a year... So I think I will change all the fluids.

I've had the chance to ride it for a week now and I feel like the front suspension compreses too much under braking. A friend of mine who has more experience with bikes than me told me the exact same thing. Do you think I should replace the oil in the forks with a thicker one?
 
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Here is my understanding:
It's going to dive under braking, I'd be more concerned with how the suspension works normally.
Brake dive, like heading into the corner is a long slow thing, compared to riding over a bump. As such, brake dive overwhelms any damping, and is really determined by spring rate, preload and a little by gas pressure. If your sag is too much, maybe find a way to add some preload, or a new spring, whichever is appropriate,
I wouldn't expect a thicker fluid to effect the total brake dive.

Damping might effect how quickly it dives and maybe overshoot... but not how far the total dive is after a millisecond of braking.

If there isn't an o-ring on your fork tube, put a zip tie around the tube, push it up and check out how far it moves during diferent situations. You want to use most of the travel over big bumps, but you don't want to bottom out unless it is the harshest imaginable. Maybe try braking hard on a smooth road, and then on a road with some ripples and see if you can see the difference in where the zip tie ends up. And is there still some room for the suspension to move when braking over the bumps? But test this very carefully!
 
Vintage Veloce said:
Here is my understanding:
It's going to dive under braking, I'd be more concerned with how the suspension works normally.
Brake dive, like heading into the corner is a long slow thing, compared to riding over a bump. As such, brake dive overwhelms any damping, and is really determined by spring rate, preload and a little by gas pressure. If your sag is too much, maybe find a way to add some preload, or a new spring, whichever is appropriate,
I wouldn't expect a thicker fluid to effect the total brake dive.

Damping might effect how quickly it dives and maybe overshoot... but not how far the total dive is after a millisecond of braking.

If there isn't an o-ring on your fork tube, put a zip tie around the tube, push it up and check out how far it moves during diferent situations. You want to use most of the travel over big bumps, but you don't want to bottom out unless it is the harshest imaginable. Maybe try braking hard on a smooth road, and then on a road with some ripples and see if you can see the difference in where the zip tie ends up. And is there still some room for the suspension to move when braking over the bumps? But test this very carefully!
Click to expand...
I totally agree with you on the replacing the oil vs replacing the springs. But I can't afford a new set of springs right now. Since I'm probably going to be changing it anyway, I thought thicker oil could make a difference.

Also, I thought thicker oil would change the way the dive feels rather than how much the suspension compresses.

I'm coming from cars, so I'm not too familiar with how replacing damper oil affects the feel.

Vintage Veloce said:
PS: I highly recommend this book:

Race Tech's Motorcycle Suspension Bible (Motorbooks Workshop): Thede, Paul, Parks, Lee: 9780760331408: Amazon.com: Books

Race Tech's Motorcycle Suspension Bible (Motorbooks Workshop) [Thede, Paul, Parks, Lee] on Amazon.com. *FREE* shipping on qualifying offers. Race Tech's Motorcycle Suspension Bible (Motorbooks Workshop)
www.amazon.com
Click to expand...
And thanks for the recomendation! I'm quite familiar with Vehicle Dynamics/Suspension on cars (Not an expert by any means). Would you recomend it anyways?
 
The book is a bit more dirt oriented, but still has good help for setup. I essentially paraphrased the troubleshooting brake dive section for you. I can't tell you if it is worth your 30 bucks... that depends on how much you like reference books and how valuable 30 bucks is to you. ;-)

Personally, I'd worry more about how the bike handles and performs over the roads and terrain you ride on. As long as it doesn't bottom the forks when braking and doesn't pogo or do something weird, I wouldn't worry about brake drive.
On the other hand, brake fluid is cheap... you could always just try it, and revert back if you don't like it. But it might cost a bunch if you are paying someone to do it for you.

Of course, you could just brake less... 👀
 
Alright! I think I'll order it, you're a good salesman god dammit haha.

And yes, 30 bucks is substantial for a student like me 🤣
 
Fork Oil is cheap to experiment with. 10wt or 15wt motor oil is $3 USD a quart, red ATF (auto transmission fluid) is 7.5wt. Thicker will slow the circuit under compression (braking). If you can’t budget for new springs, adding a simple preload spacer (scrap pipe of similar inner diameter - PVC or metal works) will stiffen up the front end across the range of travel.

If you have the fork apart to drain fluid, use the opportunity to add preload spacers. Easy and cheap way to tame the bottoming out until you can fit fresh higher quality springs.
 
Claytonroy said:
Fork Oil is cheap to experiment with. 10wt or 15wt motor oil is $3 USD a quart, red ATF (auto transmission fluid) is 7.5wt. Thicker will slow the circuit under compression (braking). If you can’t budget for new springs, adding a simple preload spacer (scrap pipe of similar inner diameter - PVC or metal works) will stiffen up the front end across the range of travel.

If you have the fork apart to drain fluid, use the opportunity to add preload spacers. Easy and cheap way to tame the bottoming out until you can fit fresh higher quality springs.
Click to expand...
For what it's worth: my understanding is that adding preload will reduce the sag, and thus there may be more distance left before the fork will bottom. But it doesn't actually make the forks any "stiffer", the spring rate for a given load remains the same. That said, I'd still definitely add preload if you don't have as much rider sag as you want.

Is it hard to change the fluid in these forks?

I have also heard that you can increase the stiffness a bit by adding more fluid (increasing the oil level) with some forks because it reduces the gas volume. That might be more effective at "stiffening" as opposed to "increasing the damping".
This might be the easiest experiment...
motocrossactionmag.com

HOW TO FIX YOUR FORKS WITH FORK OIL HEIGHT TRICKS - Motocross Action Magazine

Save hard-earned cash — Before sending your fork out to have them revalved, use this quick, easy and simple tuning trick
motocrossactionmag.com motocrossactionmag.com
 
Vintage Veloce said:
For what it's worth: my understanding is that adding preload will reduce the sag, and thus there may be more distance left before the fork will bottom. But it doesn't actually make the forks any "stiffer", the spring rate for a given load remains the same. That said, I'd still definitely add preload if you don't have as much rider sag as you want.

Is it hard to change the fluid in these forks?

I have also heard that you can increase the stiffness a bit by adding more fluid (increasing the oil level) with some forks because it reduces the gas volume. That might be more effective at "stiffening" as opposed to "increasing the damping".
This might be the easiest experiment...
motocrossactionmag.com

HOW TO FIX YOUR FORKS WITH FORK OIL HEIGHT TRICKS - Motocross Action Magazine

Save hard-earned cash — Before sending your fork out to have them revalved, use this quick, easy and simple tuning trick
motocrossactionmag.com motocrossactionmag.com
Click to expand...

With all respect to the discussion and “I have also heard” you are mistaken. As you said, adding preload to the spring will reduce rider sag, but you are removing total travel from the system.

Ideal spring rate for your riding style = no preload / max travel. But we don’t live in a perfect world, with consistent pavement, speeds, and conditions. Suspension is always a compromise. Pro tuning = keeping the rider in the controlled, middle of the stroke, with room for potholes and top out on wheelies 🏍
 
If you have a spring with 300mm of travel, and subtract 25mm of preload..there is not more distance before the bottom point.

Adding preload to a spring is effectively decreasing total travel.
 
Claytonroy said:
If you have a spring with 300mm of travel, and subtract 25mm of preload..there is not more distance before the bottom point.

Adding preload to a spring is effectively decreasing total travel.
Click to expand...
I'm definitely not an expert on modern forks. But does your statement assume that with the added preload all the coils bind and that stops the travel earlier? Is that typical?
If the whole spring doesn't bind, I would assume adding preload results in decreased sag and would result in more travel being available. What am I missing?
1606630138866.jpg
 
Finally I decided to keep the stock oil for now. I will keep riding like this until I decide what to do. This is also because I don't have the tools/space to do this myself right now.

But thanks for the feedback guys!
 
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