Heavy Clutch
 

Oh the ecstasy..............and then the agony.
My new years resolution is to make some progress this year on my Cabrio.
I have completed the modifications to my pedal box and battery tray, and have re-installed them both.
Tonight came the exciting bit of connecting the accelerator pedal up - really pleased : nice smooth action, nice weight to pedal, travel slightly long, but will help to make it smooth in traffic.

Then the anticipation of the clutch. It took a while to bleed through, with my son pressing the pedal while I used the slave cylinder bleed and evetually we sorted the fluid supply. But when we tested the pedal ..............what a disappointment. The weight is too heavy - after a few trips around town I will have a left leg weight lifters would be proud of!!

Essentially I have maintained the Marlin geometry, and used the standard BMW master cylinder - as per Rob Martin's Sportster. Rob, is your clutch heavy?
Mine reminds me of the 70's Escort Mexico clutch - it was real he-man effort to use.
So, what to do now?
Patrick - I know you changed your master cylinder to a Willen(?)- what brought you to that decision?
Jason - how does your clutch feel with the Willen (?) clutch?

I'm really p'd off now, and I was so excited......................
............and I haven't tried the brakes yet, but I guess I will not know how hard they are until I put it on the road. Robin, you used the Metro servo - is it strong enough to give good brakes, or do you need a strong right foot to bring it to a quick stop?

Oh well, tomorrow is an other day..............

Mike

Mike - It feels heavy with the wilwood master. I was wondering if the geometery was wrong, and was going to tinker with it later on. I know I can change the length of the rod, but I don't know if that will make any difference

http://www.greatoldone.co.uk/Sportst...lutchcylon.JPG

Moving the push rod down to the lower hole won't work either as there would be less mechanical advantage and the clutch would feel even heavier!

Didn't someone fit a 'helper' spring to their clutch?

It's heavy - but not that bad that I feel I need to do something about it.

I'm not impressed with the brakes. I am not sure if I have a "weak" servo or if thay are all like that. I have a 2nd hand servo at the moment but I bought a new (old stock) servo from Ebay which I intend to fit shortly. I hope that will improve matters.

Robin

Aha - I knew I'd seen it somewhere - Simon's Cabrio:

http://farm1.static.flickr.com/121/3...80ed9c9e14.jpg

He used a helper spring from his donor car.

Jason
I don't think altering the length of the rod will make much, if any difference. However, I do think moving the push rod in to a lower hole will improve the feel significantly. It will increase the mechanical advantage to the pedal which will make it feel lighter, but the travel of the pedal will increase to compensate. If my school trigonometry, and physics is right, I believe lowering the hole by 30% of the distance between pedal pivot and rod pivot should reduce effort by 30%. However, I also think it will increase pedal travel by 30% too?
I have seen Simon's helper spring, but he has a modified Sierra pedal box which seems to position the pedals closer to the driver, allowing room for the helper spring between the bulkhead and the pedal. I can not see how we can fit this in to the limited space in the Marlin pedal box?
Do you know what bore size your Wilwood master cylinder is?

Anyones' thoughts will be appreciated , as always!
Many thanks for your ideas
Mike

Mike - I guess Simon used part 21 on this diagram:

http://www.realoem.com/bmw/showparts...30&hg=35&fg=05

I'm sure (if it works - Simon will be able to tell us) that you'd be able to get one from the breakers yard, or a new one from the BMW parts desk.

Mind you, the £ equivilant of $44 for a spring is a bit steep.

I believe the wilwood is .75 inches... I seem to remeber that being stamped on the casting.

Exactly the opposite.

Jason
I have one of these springs/connectors from a breaker taken months ago, (when I saw Simon fit his), just in case - I was hoping not to have to fit it! I could leave it, but I know it will bug me every time I drive the car, so I will change it now while it is easier to do. Trouble is, the geometry has to be just right, and I do not have a donor car to take the dimensions and angles from.

Oh great joy............
Mike

Ah. So I have it in the wrong hole.... :madgrin:

I wonder it there's any way of moving the rod to the lower hole without changing the position of the cylinder... I have quite a bit of bracing arounf there now, and it'd be a pain to move it all (probably mean taking the pedal box out again. Not something I really want to do this close to IVA)

Let me see what I can do with mine - if I can get mine to be less heavy, a wilwwod cylinder mounted in a similar fashion may be an easier fix.

The Wilwood will only change the weight if the bore is smaller than the BMW, but I am sure mine is not bigger than 0.75 inches
Mike

http://farm1.static.flickr.com/121/3...80ed9c9e14.jpg


Is the spring not working to return the pedal in this picture, or is the travel such that the spring initially works against the downward stroke, but once past the point of maximum compression, starts to extend and assist?

Yes that's right, the spring initially works against the downward stroke, but once past the point of maximum compression, it starts to extend and assist.

I copied the geometry for the helper spring arrangement from the BMW pedal box and it works a treat (i.e. nice and light when the pedal is depressed over half way).

The BMW helper spring assembly also has a threaded central rod to allow the spring seat position to be adjusted and hence further change the pedal load.

My car has the Sierra pedal box and it's a tight squeeze to fit but its definitely worth the effort.

Simon

I binned mine because the standard pedal box is rubbish. I've separated out the elements and re-enforced the box. The throttle & brake have much more metal to keep them solid. The clutch has it's own ali cage that is sits in. The scuttle is also re-enforced.

All this has let to a much more precise clutch - I changed the leverage of the pedal making it much easier to use.

I've spent the last couple of hours tinkering with the clutch cylinder.

After having a heavy bout of tourettes whilst having my head buried in the foot well, I've lengthened the stroke of the piston by winding out the saddle that acts on the pedal. I tried it on the lower hole, and it doesn't like it at all. The angles are all wrong.

However, the lengthened rod seems to have some effect, as it's not as heavy as it was.

I have another idea, but i'm not sure how effective it will be

Hi Mike,
I used the BMW master cylinder and the Marlin supplied pedal box with no mods or helper spring.
The clutch is firm but not heavy and i'm not a big bloke. I haven't felt the need to change anything even after my long trips.

Are you using a standard clutch mechanism? Could your slave cylinder be seized? Maybe the release bearing is sticking?
Have you checked that the clutch is actually disengaging?

I think one of the 5exi builders recently had a brand new, but faulty, clutch cover assembly that wouldn't dis-engaged!

Might be worth checking out 'down below' before modding the pedal assembly?

...peter

It'll put loads into the rod it isn't designed to take - not a good plan. The offset load put into the shaft at the pedal end will try and bend the rod, assuming the point at which the rod contacts the master cylinder's piston can react against it. So either the rod will dislocate from the master cylinder's piston, or the rod will be forced to bend.

Jason

I have had alook at my geometry and believe moving the rod to the lower hole will make it at least 30% easier. If your master cylinder can cope with the change of angle, I would love to know how much better it feels.
I have checked the throw of my pedal on my current arrangement and it is only 70-75mm. I checked my Audi, and it is over 170mm. No wonder I think the clutch is heavy! I plan to re-organise my geometry (ie re-fit the master cylinder in a new position) and connect the plunger rod in the lower hole, and see what differenc it makes. At least this is a fairly simple change to make.

Mike

Mike - I tried it in the lower hole, and the angle the rod makes is too great.

After alackofspeed's advice, I'll ditch my first plan, and switch to plan B

I'm going to try and drill some other mounting holes in situ to swing the cylinder to a flatter angle, so that the rod isn't at such an acute angle when it travels.

I found it was quite easy to unhook the cylinder in it's box section frame and let it dangle whilst I messed with the rod earlier, so I'll get it out the way and drill other holes with the 90deg attachment I have for my drill. Shouldn't take too long, but I'm betting my tourettes will flare up again. :D

Jason

With your method of mounting the master cylinder it will be easy to check the effects of lowering the rod on the pedal. If you use a spacer in between the lower cylinder mount and your bracket - such as a nut, or several washers - it should alter the angle enough for you to test the effect on the pedal weight when the rod is placed in the lower pedal hole. Only if you are then happy, do you need to carry out the long term fix to the cylinder mount.
Have you measured the throw of your pedal with the current layout?
My pedal only depresses 70-75mm. I checked my Auidi and it depresses around 170mm - no wonder the Cabrio feels so hard! I think if we go to the lower hole the travel will still only be around 100mm, which should be quite acceptable.
Let me know how you get on.

I have removed my steering column, and the pedal box, so I'm currently on Pedal Box version 4 - and I thought I had been clever designing out the problems Patrick had with the flimsy design!! Oh well we can all keep learning!!
Mike