Big Brother Posted July 7, 2004 Share Posted July 7, 2004 There were quite a few discussions a while ago after a string of CRB failures on K-series engined Sevens as to the lack of bearing preload on the standard clutch set up. (not sure the engine type makes that much difference to be honest) I have just converted from hydraulic to cable clutch and am about to fit a return spring. As this is a custom conversion/modification, I am free to use any type of spring. Can anyone enlighten me as to the prefered value of preload required. Also can you recommend a suitable (easily available) spring? Steve Se7en-Up! Now with added green! Link to comment Share on other sites More sharing options...
Jason Plato Posted July 8, 2004 Share Posted July 8, 2004 Wheres Nick Green 🤔 Nick is the CRB man Link to comment Share on other sites More sharing options...
Tony C Posted July 8, 2004 Share Posted July 8, 2004 The preload at the CRB should be between 80 and 100 Newtons (Roughly 10kg) On my 2003 K-Series I installed an additional pedal spring attached closer to the cable attachment and the spring tension required is 4kg to give 10kg at the CRB. BRG Brooklands SV 😬 It seems that perfection is achieved not when there is nothing more to add, but when there is nothing more to take away. (Antoine de Saint-Exupery) Link to comment Share on other sites More sharing options...
Mark Durrant Posted July 8, 2004 Share Posted July 8, 2004 Do a search and look for a posting by John Vine which will give you all the information you require. Mark D Su77on Se7ens Link to comment Share on other sites More sharing options...
Big Brother Posted July 8, 2004 Author Share Posted July 8, 2004 Cheers Mark, Copied from the CRB thread.... "The basic idea is to increase the CRB pre-load substantially by replacing the existing single clutch-pedal return spring with two of the same size, but pulling directly on the clevis pin and along the line of the clutch cable. (I'm assuming that your car has the same set-up as mine and the standard clutch actuating arm.) So, proceed as follows: 1. Obtain two clutch-pedal return springs from Caterham. Note that the spring often supplied (p/n 1454217) is too small; you need to ask for springs that are 100mm long overall, with a coiled section 30mm x 15mm (listed as "brake pedal springs", apparently -- Darren will know what to send). 2. Drill two 2mm holes in the top rear flange of the pedal box on the inboard side, just in front of the screw hole, so that they are in line with the clevis pin ends. These holes will be about 18mm apart and the inboard one will be about 3mm from the corner of the box. 3. Trim one end of each spring to about 30mm (that is, keep the straight bit), and make a sharp hook-shaped bend (25 deg) about 10mm from the end. 4. Insert these hooks into the 2mm holes from the underneath. 5. Make a full circular loop in the other end of each spring (so as to fit snugly over the clevis pin), ensuring that each spring (this is the tricky bit) is extended by 23-25mm when attached to the pin, and then trim off the excess spring wire. 6. Reassemble the pin and cable-eye such that one spring loop is under the pin shoulder and the other under the washer and R-clip. 7. Remove the original spring completely. 8. Replace the pedal box cover. This should hold the springs in place and prevent them dropping out in use. My reasoning for all this is as follows: At the clutch end, the mechanical advantage of the clutch cable over the CRB is about 2.5 (distance from CRB to fulcrum 110mm, and from cable to fulcrum 280mm). To apply a pre-load at the CRB of 100N (mid-point of INA's recommended 80-120N range), the cable needs to pull at 40N. So, assuming a frictionless cable arrangement, the combined pedal springs also need to pull at 40N along the line of the cable. 40N in imperial is 9lb (1N = 0.2248lb). Although Caterham quote the rate of the pedal spring as 7lb/in, I measured mine at 4.75lb/in, with each additional pound extending the spring by 1/4in. So, a 9lb total pull (4.5lb/spring) requires each spring to be extended by about 15/16in (that is, about 24mm). Apologies for the mixed units, but at least I'm following Caterham's own long-standing tradition! I hope all this makes sense. I think my reasoning is OK, but if anyone knows better, please put me straight! However, I make no claims whatever that this "solution" actually works in practice (although I'm about to find out, and will report back). It may also contradict Caterham's own recommendations and practice, so you proceed at your own risk. However, judging by BlatChat posts, premature CRB failure does seem to be a 7 trait, and the main reason (to my mind, at least) is that the standard set-up doesn't apply sufficient CRB pre-load. JV" Steve Se7en-Up! Now with added green! Link to comment Share on other sites More sharing options...
Nick Green Posted July 8, 2004 Share Posted July 8, 2004 As ever Dave, you are way ahead of me BTW, bit nervous of this "CRB man" label; just trying to make the CRB work properly, like everyone else. John Vine and I discussed ways of setting up these clutch springs. John went a more "scientific" way, while I went a more "empirical" way. My method is repeated below. I would say that the subtleties of when to take the reading off the spring balance, are possibly debatable with my method, so use at your own risk. I wanted to try and have some feeling of what force was actually being applied to the CRB. So there you go. BTW, if anyone sees anything wrong with the maths/theory, please do let me know! I'm always happy to be (politely!) corrected "80-120N (INA-recommended preloard for the CRB) translates (via division by 9.81 and multiplication by 2.2, I've been advised) to 18-27 lbs of pressure needing to be exerted by the CRB onto the clutch pressure plate. Allowing for the 2.25 lever ratio [of the clutch release arm] previously suggested (I cannot measure this on my car as it is still installed, but judging from photographs I have, it looks about right), that gives the force to be generated at the top of the release arm as 8-12 lbs... I put the clutch pedal spring back on to my car, fixed a cable tie around the release arm to allow more convenient hooking-on of the spring balance, and measured ~5lbs off the balance scale. I was having to pull the balance up at an angle of around 45 degrees to clear the bulkhead, so 5cos45 = ~3.5 lbs. Not enough, given our newly gained technical knowledge. On my clutch pedal, the hole in which the spring fixes, is halfway between the pivot bolt of the pedal and the point where the cable attaches. To cut the ramble down, I shortened the spring slightly and hooked it into the hole normally occupied by the R clip holding the clutch cable location pin in place on the pedal, to increase the spring's mechanical effectiveness. The force then measured was about 13 lbs, which tweaked as before, gives just over 9 lbs. The release lever was definitely putting up more resistance when moved directly by hand too." Edited by - Nick Green on 8 Jul 2004 11:16:43 Edited by - Nick Green on 8 Jul 2004 11:18:36 Edited by - Nick Green on 8 Jul 2004 11:19:33 Link to comment Share on other sites More sharing options...
John Vine Posted July 8, 2004 Share Posted July 8, 2004 Well, I was about to reply with chapter and verse, only to see that you guys had beaten me to it! Nick's approach was different to mine, but his solution works out just the same. The key point is to generate a pre-load in the 80-120N range. All I can add is that I've now done 6500 miles since my mod, and everything seems just fine (oh no, if that's not tempting providence...). For info, a little bird tells me that a CRB article from yours truly will be appearing in LF shortly (containing large chunks of my earlier posts). JV Link to comment Share on other sites More sharing options...
Big Brother Posted July 8, 2004 Author Share Posted July 8, 2004 John and Nick *thumbup* Many thanks for sharing the info. Steve Se7en-Up! Now with added green! Link to comment Share on other sites More sharing options...
Colin Mill Posted July 9, 2004 Share Posted July 9, 2004 Can I just check - is it being suggested that actually pushing the CRB into engagement with the clutch full time will cure the CRB failures? Link to comment Share on other sites More sharing options...
Nick Green Posted July 9, 2004 Share Posted July 9, 2004 Colin, yes that's exactly it. If you have the spring in place in the pedal box which looks as if it is there to keep the clutch pedal down, then preload is already being applied to the CRB. One or two of us have been in touch with INA about this and it turns out a certain amount of preload is indeed required. The bearing is designed to be constantly in contact with the clutch fingers, with a certain amount of force required to avoid any slip between the bearing face and the clutch fingers. Ford and others use self adjustment mechanisms and what have you, to put this preload on. Link to comment Share on other sites More sharing options...
Nick Green Posted July 9, 2004 Share Posted July 9, 2004 Having said that, not everyone suffers the problem, and I know one person who has tried my method and still had a CRB fail within a couple of thousand miles... However, INA *are* adamant that the bearing is designed to run with this preload, so who am I to argue ? Link to comment Share on other sites More sharing options...
Colin Mill Posted July 9, 2004 Share Posted July 9, 2004 Thanks Nick It all sounds really odd to me. Having had my ear clipped as a learner for resting my foot on the clutch and always adjusting cable clutches for free travel at the top I would really rather have the bearing come completely clear of the fingers in the released state. As I'm just putting my car together I have the option of playing about with this. I've just fitted a light compression spring over the bell housing end of the cable to act as a return spring with (hopefully!) just enough tension to to pull the cable back and push the bearing clear. At the top end I'm going to either fit an adjustable pedal stop or clamp a stop to the cable. I'm also going to pinch the idea of putting the top end cable nuts either side of the tube in the pedal-box and remove the spring existing spring from the pedal. This has got to be the only car I have seen without a stop to set the top of the pedal travel! Hopefully I can get enough use out of the car to find out if the method works or not. Best regards Colin Link to comment Share on other sites More sharing options...
Nick Green Posted July 9, 2004 Share Posted July 9, 2004 Colin, I asked the guy at INA about riding the clutch pedal and what was the difference, and he said it was still not a good idea. Causes a different type of wear. The preload is enough to let the bearing grip the clutch fingers and spin properly, without over-doing it. The bearing is designed to take the constant revolutions on the specified preload, apparently. FWIW, after my car knackered the first CRB, I put it all back together and just removed the spring from the clutch pedal. With one or two other tweaks to stop the clutch cable sheath becoming dislodged, the release arm would happily fly clear of the clutch, and I could even stick my foot under the pedal and lift it, to make sure the CRB was clear. Then I learnt of the preload thing. At the time, others on this forum said CRBs running preload was normal and that the spinning up from zero revs of the CRB on each gear change was bad for it. Perhaps different CRBs are designed for different forces or something. My unpreloaded CRB has lasted longer than others running the standard Caterham set up. (But then other people have done my sort of total mileage and more, with no problems at all. This is a very inconsistent problem...) My car's first CRB lasted 9k or so miles. On the second/current CRB, I have done 5k with an unsprung/zero preload clutch pedal, and have done the same again, more or less, with the increased preload modification... am heading into unknown territory now... I haven't had the gearbox and engine apart in that time to check the grip of the bearing on the plastic mounting plate. It is stiff when new. The bearing is designed to centralise once, when the engine first starts after installation, and should then never need to move again. If you take the CRB out after n thousand miles, and the bearing moves easily on the plastic mounting, then you've had overheating/slippage. It is one of the symptoms. N. PS. Just re-read one or two bits of the thread. Colin, I can't claim that this pre-load is a cure to the CRB failure problems that *everyone* has been experiencing. However, it does look like an area where the way Caterham utilise a particular component, is not quite in line with what the manufacturers of that part recommend, and there is evidence to show the lack of sufficient preload, *could* be the cause for a number of the failures... More a case of trying to eliminate potential causes... PPS. I'll add a too, before anyone thinks I mean anything else. Edited by - Nick Green on 9 Jul 2004 14:07:56 Edited by - Nick Green on 9 Jul 2004 14:12:50 Link to comment Share on other sites More sharing options...
Tony C Posted July 9, 2004 Share Posted July 9, 2004 Well, as the saying goes, "You can lead a horse to water ........" The information from the manufacturer of the CRB itself is that a preload of between 80 and 120 Newtons is required. I, like you, were brought-up on the "Do not rest your foot on the clutch pedal" advice. That stems from the days when CRB's were made of carbon and the carbon would wear at an increased rate if there was a preload. Our CRB's have a roller bearing. INA, the manufacturer, insist that if the CRB is not preloaded the shock load when you put your foot on the clutch with a stationary CRB will decrease its service life due to the sudden acceleration of the bearing and the shock alignment. It's your CRB and of course it's up to you ..... BRG Brooklands SV 😬 It seems that perfection is achieved not when there is nothing more to add, but when there is nothing more to take away. (Antoine de Saint-Exupery) Link to comment Share on other sites More sharing options...
Colin Mill Posted July 9, 2004 Share Posted July 9, 2004 Hi Tony Even I'm not old enough to remember carbon CRBs first hand! 8-) I can see that they have several competing problems to deal with. With a low pre-load the release fingers will just skitter over the face of the bearing and the face will get badly worn. Also the forces will be to low to overcome the friction of the self-aligning slide. Increase the preload and at least its the bearing is doing the turning and it should self align. But can a sealed grease-packed race do engine rpm for 100k miles? It would really help to know what the failed bearings actually look like. Is it the race itself, the front face, or the plastic carrier part that has suffered? Best regards Colin Link to comment Share on other sites More sharing options...
Tony C Posted July 9, 2004 Share Posted July 9, 2004 'Fraid you'll have to get that info from Nick, John or Rob. I never saw mine, just heard the awful noise it made BRG Brooklands SV 😬 It seems that perfection is achieved not when there is nothing more to add, but when there is nothing more to take away. (Antoine de Saint-Exupery) Link to comment Share on other sites More sharing options...
Nick Green Posted July 9, 2004 Share Posted July 9, 2004 Hi Colin. I think the short answer to your question and list of possible symptoms is: leave it long enough and it is "all of the above". I have done a little delve through my email archive and come up with the following comments/observations from INA on one of John Vine's CRBs. The other was too knackered to be able to diagnose. John, I think you have posted this on BC before. If not, sorry! 😬 Observations: "Bearing face is polished and shows discolouration" "Signs of grease egress from bearing, at the interface between the bearing and the plastic housing" "Bearing can be moved in a radial direction around the plastic housing with relative ease" "Bearing still operates in a smooth, noise-free condition" Conclusions: "Failure can be attributed to insufficient bearing pre-load resulting in skidding between bearing contact face and the clutch cover. This would lead to heat generation (discoloured contact face) and subsequent grease loss due to the elevated operating temperatures" "We do not have any information on this application to verify the installation parameters for our bearing" "The bearing will operate more effectively when a pre-load of 80-120N is applied" "The ease with which the bearing can be moved in a radial action around the plastic housing can be attributed to the low pre-load. This feature is designed to locate the centre of the clutch and remain in this location for the remaining life of the clutch. It is not designed for repeated adjustment" The INA guy told John he felt the squealing when the pedal was pressed was most likely due to the metal housing rotating against the plastic carrier. He also added that the typical clutch operating force was 800-1600N, so a pre-load of 80-120N was insignificant in operating terms. And of course, once you start losing grease, it is only a matter of time before it all falls to bits. Hope this all helps! Edited by - Nick Green on 9 Jul 2004 15:59:58 Link to comment Share on other sites More sharing options...
noddy Posted July 9, 2004 Share Posted July 9, 2004 What about the posibility of the CRB failing because the release arm is not pushing the bearing square onto clutch cover 🤔 could this be one of the reasons for the frequent failures 🤔 R400 .......... I love it Link to comment Share on other sites More sharing options...
John Vine Posted July 9, 2004 Share Posted July 9, 2004 Many thanks to Nick and others for saving me the trouble of searching! As I hinted in an earlier post, this topic is to feature in a forthcoming issue of LF, although a lot of the content has now been posted here! So, be ready for a feeling of deja vu when that issue of LF hits your doormat! BTW, the CRB in question here is INA P/N F-202994, Caterham P/N AB57, and is also the one used in the Ford Sierra and elsewhere. Re resting the left foot on the clutch pedal: Yes, this will increase the CRB pre-load, but I imagine it's probably bad practice because it's likely to generate too much end-load on the crank thrust washers, something that they weren't designed to handle. JV Link to comment Share on other sites More sharing options...
John Vine Posted July 9, 2004 Share Posted July 9, 2004 Noddy, INA commented to me that it was sometimes possible to feel this through the clutch pedal when the foot is applied lightly, but they didn't suggest that it was a significant possible cause of failure. However, you may well have something there. JV Link to comment Share on other sites More sharing options...
Colin Mill Posted July 9, 2004 Share Posted July 9, 2004 Many thanks to you all for the info I'm coming to the conclusion I'm going to have to go with the preload. I did a few measurements on my car and came up with the dimensions mentioned earlier. The spring gives a cable tension of 1.1kgf (10.8N) so I reckon that gives a preload of only 27.5N at the bearing. Since the self centering slide takes (by my crude measurement with a spring balance) 5kgf (49N) of side load to move it I can see why a preload of only 27N is going to be ineffective in keeping the bearing centred - the friction with the fingers would not be enough to create a big enough side force. However I don't see why it should need constant centering . It should go right initially and stay right. Perhaps the preload is needed to create additional friction between the bearing and its carrier to prevent it rotating on the carrier. (Since the bearing is grease filled the torque transmitted by the bearing at 6k must be quite big just from pushing the grease about that fast.) It seems that we are paying a high price for having +-1mm of self alignment in the bearing. It sounds as if the thing would last for ever if the thing could be accurately lined up with the clutch without the nasty plastic bit! Colin Link to comment Share on other sites More sharing options...
John Vine Posted July 10, 2004 Share Posted July 10, 2004 Colin, I'm coming to the conclusion I'm going to have to go with the preload.I think that's the correct conclusion, and accords with INA's advice. What I'm not sure of right now is whether the chaps at the factory have come to the same conclusion. The spring gives a cable tension of 1.1kgf (10.8N) so I reckon that gives a preload of only 27.5N at the bearing.Ah! There you have it! My contention is that the standard spring assembly is incapable of generating the necessary pre-load. On my car, I calculated that it would produce about 15N maximum. However I don't see why it should need constant centering . It should go right initially and stay right.You're right. The idea is that the CRB centres itself once only, and then remains in that position. But it will do this only if it runs in constant contact with the diaphragm fingers, otherwise it will attempt to re-centre itself on every clutch application. I believe it's the initial "off-centre" rotation that encourages this centring action. Perhaps the preload is needed to create additional friction between the bearing and its carrier to prevent it rotating on the carrier.The purpose of the pre-load is to keep the CRB running at crank speed at all times, thus preventing its having to speed up to crank speed every time the clutch is pressed. Without this pre-load, the CRB pressure face skims over the diaphragm fingers, generating a lot of heat. This, in turn, leads to loss of grease from the bearing, and eventually to total failure. The bearing should not rotate on its carrier. If it does, that's a symptom of overheating. According to INA, it's this movement that causes the all-too-familiar squealing noise. It seems that we are paying a high price for having +-1mm of self alignment in the bearing. That would be true if the bearing had to re-centre itself on every clutch application. But, as explained above, it's designed to centre itself once only, and then stay centred thereafter. the nasty plastic bit!I don't understand this comment, though. Do you mean the bearing carrier, perhaps? JV Link to comment Share on other sites More sharing options...
Colin Mill Posted July 10, 2004 Share Posted July 10, 2004 Hi John Yes, its the carrier I don't like much. I used a thermostatic soldering iron on the plastic to try to get an idea of the melting point and it seems the plastic is softening at about 190C. The air temperature in the bell housing could easily be up around 100C so it would not take a lot of dissipation in the bearing for it to reach the melting point of the plastic. The bearing weighs 167g so the thermal capacity must be no more than 70J/C. I did a quick sum on the heat that could be generated in fault conditions and found that a 10% slip on the front face at a preload of 30N and 6000rpm could give 43watts of heating. 5 minutes of this would warm the bearing up by about 180C and the carrier would be history. From what you were saying about the post-mortem on one of the bearings the carrier is the bit that has really had it because they reckoned the bearing itself was still turning smoothly. Once the lateral friction between the bearing and the carrier has reduced I would expect the bearing can whirl in the carrier causing more heating. Best regards Colin Link to comment Share on other sites More sharing options...
John Vine Posted July 11, 2004 Share Posted July 11, 2004 Colin, I think your calculations bear out exactly what INA told me about the danger of friction-induced heat. I suffered two CRB failures. The first was so bad that INA couldn't say what had caused the failure. The second showed signs of severe overheating, but was still operating. I'm not qualified to comment on the durability temperature-wise of the plastic carrier, except to say that INA themselves feel it is well up to the job, provided there is sufficient pre-load to prevent the depredations of friction-induced heat. JV Link to comment Share on other sites More sharing options...
Colin Mill Posted July 11, 2004 Share Posted July 11, 2004 John I'm just glad I bought the engine hoist rather than just hireing it! 8-)= Colin Link to comment Share on other sites More sharing options...
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