Undersized drive pulley issues

[benebob]

We put an undersized drive pully on the race car this past week and have found that the steering has now gone to boarderline acceptable for track conditions. VERY TIGHT! I'm assuming that if we put a smaller pulley on the PS pump our issue will be solved but wanna see if anybody has done this and how small did ya go to return it to a decently assisted steering box. Also, where'd you snake the pully from?

[dcarrb]

If you've downsized the pulley on one end, seems to me you'd have to upsize the other one to restore the proper drive ratio.

dcb

[Phast SVX]

Originally Posted by dcarrb

If you've downsized the pulley on one end, seems to me you'd have to upsize the other one to restore the proper drive ratio.

dcb

the ratio will not change, but i have a ahard time believing that the pulley has created little flow. remember, the pump dosnt create pressure, jsut flow. All the pressure is created in the rack itself with the seals etc.
phil

[benebob]

Rack is as leak free as a 92 SVX original rack can get. No fluid has been added since we got it and there are no visable leaks. Remember it is spinning about a 1/4th less so there is a 1/4 less flow so pressure should be down from that. It worked just fine the day before only difference was the pulley.

The pulley on the PS would have to go smaller (against your logic but at least your thinking). Imagine it like this. Smaller crank pulley means it is spinning the belt less per revolution of the engine. Going bigger on a sub pulley like the ps pump will cause the ps pump to spin even slower as the drive belt is moving slower and now the bigger pulley has a larger circumference but if this belt is smaller it will in turn spin faster with less rev. from the engine. Here endith your physics lesson for the day. Just think, if all us car nuts had physics questions like this we'd all be rocket scientists like Bill.

[McTaff]

Hey champ, that's a good summary, but I'll break it up a little to explain a little more. This might help others to decide what they want.

Drive pulley on a belt-pulley system:
We will call the source pulley of the power the "Drive" pulley. This is the pulley that will supply the force to turn the belt.
We will call the receiving pulley of the power the "Slave" pulley. This pulley is turned by the belt and thus powers secondary devices.

Each slave pulley in the system can be treated seperately as a lever and fulcrum system. Force applied is at the Drive end, and work done is at the Slave end.

====
On a per revolution basis, increasing the size of a drive pulley will increase the number of times the belt turns for each revolution of the drive pulley.

This decreases mechanical advantage (like moving your fulcrum closer to the applied force), but increases the distance travelled (the other end of your lever moves further). This means it will react less favourably to high load situations, as the transmitted force has less mechanical advantage, like trying to use a shorter lever. (Not really a problem with drive belts in cars, though)

Decreasing the size of the drive pulley will decrease the number of times the belt travels.

This increases mechanical advantage, but decreases the distance travelled.

Now remember that doing this to the Drive pulley changes the "fulcrum" for ALL YOUR SLAVE PULLEYS.

To re-adjust each slave pulley INDIVIDUALLY, the opposite is now in effect, because you are changing the other side of the lever. (By making them smaller, it is like a "Claytons making the drive pulley bigger".) This means you need to reverse the formula.... i.e:

Making the Slave smaller has the same effect as making the Drive bigger, and vice-versa.
====

You need to work out the ratio of Drive change and apply it to the Slave and bring it back towards 1:1 maybe half way. If you went from circumference X to circumference Y on the Drive, work out the ratio X:Y and reverse-apply that to the Slave pulley.

That will give you the size of the Slave pulley you would need to return it to the original specs.

All you need to do then is to pick a size maybe halfway between the stock one you have, and the size you'd need. (Unfortunately this is a black art, and it's rather like inventing a recipe... you have a fair idea what the ingredients do, but no idea what the end result would be like.) This would be a rule of thumb. But I'd say halfway between them would balance it out - if it's close but not quite perfect you'd already have a fair idea because you've already crunched the numbers.

Hope that helps the people who like to fiddle.

[Earthworm]

I just think of a bicycle chain

[Beav]

Originally Posted by dcarrb

If you've downsized the pulley on one end, seems to me you'd have to upsize the other one to restore the proper drive ratio.

dcb

Uh...no. Keeping the same ratio is done by reducing the size of both pulleys. Increasing the diameter of the p/s pulley would only provide a further underdriven ratio.

[Beav]

Originally Posted by Phast SVX

the ratio will not change, but i have a ahard time believing that the pulley has created little flow. remember, the pump dosnt create pressure, jsut flow. All the pressure is created in the rack itself with the seals etc.
phil

Without adequate flow, e.g. a slow turning pump, pressure can be hard to come by. You have to maintain a certain amount of flow before pressure can be built and maintained.

R&P p/s works by having pressure on both sides of the rack piston and depleting the pressure on one side when the steering wheel is turned. This allows for quicker response than applying pressure to one side when assist is called for. No pressure = no assist. Low flow doesn't allow for very quick pressure recovery. Rule of thumb:since he did x, y has become an issue. More than likely x was the cause of y.

[oab_au]

I'd just put the bigger crank pulley back on.

Harvey.

[Earthworm]

Now did you replace OEM or OEM-size lightweight?

[benebob]

Originally Posted by Earthworm

Now did you replace OEM or OEM-size lightweight?

OEM size to a cheap aftermarket one (the kind that Hocrest say's he's seen pictures of melted). It came with LeakingOil's SVX and she really doesn't have a need for it or desire for it so we thought we'd give it a wirl to see if it does anything or hurts (which so far I think the later is the answer but I'm not giving up on it just yet).

[McTaff]

Originally Posted by Beav

Uh...no. Keeping the same ratio is done by reducing the size of both pulleys. Increasing the diameter of the p/s pulley would only provide a further underdriven ratio.

Which is what I said, right?

I don't even remember what I wrote. I think I need a kebab, and to stop working at work (and concentrate on forumtime)

[svxhunter]

We run an underdrive pulley on the SVreX. Never noticed any difference in the steering.

[dcarrb]

As I've already demonstrated that I know nothing about this particular topic, I figure it can't hurt to ask: How exactly is an underdrive pulley supposed to enhance the performance of the engine?

dcb

[b3lha]

Originally Posted by dcarrb

As I've already demonstrated that I know nothing about this particular topic, I figure it can't hurt to ask: How exactly is an underdrive pulley supposed to enhance the performance of the engine?

dcb

The theory goes that the engine produces a fixed amount of power. Some goes to drive things like the alternator and steering. The rest goes to the wheels. If you change the pulley to divert less of the power to the alternator and steering, then more will go to the wheels.

But then you may have problems with heavier steering and poor charging. You could possibly fix those problems, but in doing so you might just be reclaiming the power that you diverted to the wheels.