Looking for some advice on tuning front forks

[navid]

I finally blew my left fork seals after 3 months of wheelie practice. I knew this was always a risk since I still land it pretty hard. I wanna take this opportunity to stiffen the front suspension from the stock pogo sticks.

- I wanna increase to 15W fork oil

- I'll keep stock springs (I'm 130lbs, so I don't think I need stiffer springs)

 

I wanted to know the advantages/disadvantages of cutting the spacers. I know some people follow Dave Moss religiously, but I don't want to blindly cut my spacers before figuring if this is even something I need. May you give some insight on this? Thanks.

 

[M. Hausknecht]

Good for you not wanting to blindly follow Dave Moss's recommendations. The spacers in the forks create a preload on the springs. Until that level of preload is exceeded, the spring doesn't begin to collapse. Example: imagine a 100lb/inch spring, so 100 pounds collapses the spring one inch. Put one inch of preload on the spring, and the result is that the first 100 pounds of load on the spring doesn't collapse it at all, rather than collapsing one inch. Put two inches of preload on the same spring, and the result is that the first 200 pounds of load on the spring doesn't collapse it at all, rather than collapsing two inches with no preload. Once the level of preload is exceeded, our spring will then collapse one inch per one hundred pounds. Preload doesn't change the spring's rate, it just changes the height of the spring when loaded.

Heavier fork oil adds compression and rebound damping which means the forks will collapse and extend more slowly but it doesn't change the rate of the spring. But slowing the fork collapse will result in less collapse if the force is limited in time, as it typically is when we ride. This is why on a bumpy road or going over pavement seams, heavier fork oil makes the suspension less compliant with the road, while also reducing the amount the bike moves round on the springs as you ride. The stock forks don't offer a lot of damping by design, so for riders who ride aggressively, heavier fork oil is a good tradeoff for worse grip on bumps. 

At 130lbs, the stock springs are a little stiff for you but only moderately so. If you're not bottoming the forks when you drop from a wheelie, if you cut the spacers, you'll use up more fork travel. Whether that is a good thing or not depends on the circumstances. On a race track or when riding fast on a curvy road (the context for Moss's recommendation to cut the fork spacers), fork collapse when on the brakes alters the chassis geometry in a way that helps the bike to turn better (steeper head angle and less trail). This is the primary idea behind trail-braking into a corner. along with the increased load and resultant traction it creates for the front tire.  

 

 

[navid]

1 hour ago, M. Hausknecht said:

Good for you not wanting to blindly follow Dave Moss's recommendations. The spacers in the forks create a preload on the springs. Until that level of preload is exceeded, the spring doesn't begin to collapse. Example: imagine a 100lb/inch spring, so 100 pounds collapses the spring one inch. Put one inch of preload on the spring, and the result is that the first 100 pounds of load on the spring doesn't collapse it at all, rather than collapsing one inch. Put two inches of preload on the same spring, and the result is that the first 200 pounds of load on the spring doesn't collapse it at all, rather than collapsing two inches with no preload. Once the level of preload is exceeded, our spring will then collapse one inch per one hundred pounds. Preload doesn't change the spring's rate, it just changes the height of the spring when loaded.

Heavier fork oil adds compression and rebound damping which means the forks will collapse and extend more slowly but it doesn't change the rate of the spring. But slowing the fork collapse will result in less collapse if the force is limited in time, as it typically is when we ride. This is why on a bumpy road or going over pavement seams, heavier fork oil makes the suspension less compliant with the road, while also reducing the amount the bike moves round on the springs as you ride. The stock forks don't offer a lot of damping by design, so for riders who ride aggressively, heavier fork oil is a good tradeoff for worse grip on bumps. 

At 130lbs, the stock springs are a little stiff for you but only moderately so. If you're not bottoming the forks when you drop from a wheelie, if you cut the spacers, you'll use up more fork travel. Whether that is a good thing or not depends on the circumstances. On a race track or when riding fast on a curvy road (the context for Moss's recommendation to cut the fork spacers), fork collapse when on the brakes alters the chassis geometry in a way that helps the bike to turn better (steeper head angle and less trail). This is the primary idea behind trail-braking into a corner. along with the increased load and resultant traction it creates for the front tire.  

 

 

I cannot thank you enough, I really appreciate your advice.

So that's why Dave asked that rider to cut the spacers. She's very light and the bike's spacers are made for a heavier average rider. That's also why my bike doesn't sink when I sit on it.

I'm pretty sure I bottomed my forks at least once when landing (my sister was recording). It's poor landing and I'll have to train it out. If I give it gas on landings, I can easily avoid it.

If I'm bottoming my forks already, it's probably very risky to cut spacers right? Id guess that'll eat up my fork's travel with a smaller impact.

[M. Hausknecht]

25 minutes ago, navid said:

I cannot thank you enough, I really appreciate your advice.

So that's why Dave asked that rider to cut the spacers. She's very light and the bike's spacers are made for a heavier average rider. That's also why my bike doesn't sink when I sit on it.

I'm pretty sure I bottomed my forks at least once when landing (my sister was recording). It's poor landing and I'll have to train it out. If I give it gas on landings, I can easily avoid it.

If I'm bottoming my forks already, it's probably very risky to cut spacers right? Id guess that'll eat up my fork's travel with a smaller impact.

Yes, if you're bottoming the forks now, cutting the fork spacers will make the bottoming worse. A way to reduce bottoming without changing springs or preload, add fork oil fluid to both forks. It will make the air gap in the forks smaller and the air will act like a stiff spring but only right at the end of travel. I'd try 20 cc more initially, then ride the bike and see what happens. If you're still bottoming pretty hard, add more fluid.

[navid]

23 minutes ago, M. Hausknecht said:

Yes, if you're bottoming the forks now, cutting the fork spacers will make the bottoming worse. A way to reduce bottoming without changing springs or preload, add fork oil fluid to both forks. It will make the air gap in the forks smaller and the air will act like a stiff spring but only right at the end of travel. I'd try 20 cc more initially, then ride the bike and see what happens. If you're still bottoming pretty hard, add more fluid.

Thanks, I'll try that.

 

Are you risking damaging the head stock by making your suspension stiffer? Would more of the shock of the wheelie landing gets transferred to that joint?

[M. Hausknecht]

Whatever load is on the front tire goes through the head stock. There isn't anywhere else for it to go. That is why if you wheelie a lot you should check your head stock bearings frequently. The frame on these bikes is wimpy; it flexes when loaded heavily.