Front Fork Oil Question

[MWB]

I know this doesnt go on this forum, however i find that most the fathers live on this site and have more tech knowledge. so here goes

hopefully not making anyone too angry

my sons TTR125, i have purchased the BBR Front springs and have just a question on Oil and levels here.

on the TTR site it recommends switching from the 5wt to the 15, however i remember someone telling me the heavier was not necessarily better for front forks, anyway just wondering what some thoughts are on this subject also

i dont have a manual but someone told me the oil level should be about 5 inches from the top , but is that with new springs in or out when this is measured??

Thanks for the help

[DaveJ]

There's nothing necessarily wrong with thicker oil, you just have to understand the effects and know that it does not stay thick forever.

Thicker oils are achieved by adding polymers, and when they shear, the oil becomes thin. Usually, the more you spend the better the oil and the longer it will last. Many applications, such as cruiser-street, don't have to worry much about this since the abuse is minimal.

However, a cheap high viscosity oil in a sever motocross application can be beat to death in about 10 to 15 minutes.

Often top tuners will talk about water like viscosities being best, and this is why. Consistency is key in suspension development so ultimately it's best to add or remove damping via internal modifications. Not always an option of course due to cost and such, so this is why their remains a market for such a product.

Also consider that both rebound and compression damping is effected (shifted) by a change in oil. So you may gain on one end, but introduce problems elsewhere.

DaveJ

[grayracer513]

Thicker oils are achieved by adding polymers, and when they shear, the oil becomes thin.

Thicker single grade suspension oils (there really isn't much advantage to a multigrade oil for this purpose) are made by using heavier base oils to start with. Virtually no one makes a single grade oil by adding anything to a lighter oil. It's expensive, and creates an inferior product.

[dpetteys]

15wt oil works very well in TTR 125 fork when you are using the higher rate BBR fork springs.

The forks are very simple damper rod forks, virtually the only way to tune damping is by increasing oil weight. Essentially the forks force oil through a small hole to create damping, by running thicker oil more damping force is created. Don't worry about the technical mumbo jumbo of oil weights, these are very simple forks, keep the tuning simple too.

As far as the oil height goes I believe I set mine at 100mm (4"). This is measured by bottoming the forks, with the springs out, and measuring the distance from the top of the upper tube to the oil. Just be sure that you have cycled the forks enough to get all of the air out off them before you take any measurements.

In case you don't know, by raising the oil height you will make the forks feel a little bit more progressive towards the bottom of the stroke. There are a few other minor benefits as well. I can get into more details later if you like.

DaveJ recommends using a good oil, which I certainly agree with. My personal favorites are silkolene and rock oil, but any decent oil will suffice. Again these forks are really simple, and regardless of what oil you use they will be an enomous improvement over stock.

[ample]

I'm planning to do the same thing with my wife's bike (BBR +30% springs and heavier fork oil). I plan to try and match the increase in spring rate with the increase in oil viscosity, so I'm going to try 10 wt first, and bump up to 15 wt if it's too bouncy.

My bad. The owners manunal shows 10wt. as stock fork oil viscosity, so I'll be running 15wt, and upping it to 20wt if its too bouncy.

[dpetteys]

Not to discourage you from trying 10wt. I certainly don't know what exactly you are trying to tune for, and it is not going to hurt anything or be that time consuming or expensive.

However, I think you will have a better chance of being on target if you start with 15wt. In my experience, I think you will find 10 wt to be too light. Everyone I know who has run the BBR springs has ended up with 15 or 20wt oil.

[DaveJ]

Thicker single grade suspension oils (there really isn't much advantage to a multigrade oil for this purpose) are made by using heavier base oils to start with. Virtually no one makes a single grade oil by adding anything to a lighter oil. It's expensive, and creates an inferior product.

Somewhere in my brain I recall a very detailed and professional publication that spoke of how suspension fluids and other oils are cooked up. Where that is now I'm not sure.

However, it was made clear that polymers were used to control viscosity and that they do shear which of course changes the "thickness" of the oil.

I would also assume that a lower cost process would be to have a single base stock in which one could formulate from to meet various markets.

And I have certainly gone through enough testing with various fluids to at least know that what went in at the start of the moto is not what came back out. It was my live for a few years so if the change was not related to polymers, than perhaps we should work out what it was.

In either case, I don't mind being corrected as long as it's substantiated.

Fire away.

DaveJ