egret Posted March 26 Share Posted March 26 I'm thinking out loud here, so please correct me if I'm wrong (and don't be upset if I'm attempting to teach you to suck eggs)! I belive the goal is to have the rocker arm movement be symmetrical around horizontal*. I.e. fully open and fully closed valve are equal angles away from horizontal. This way you keep the angles as small as possible and minimise any unwanted horizontal forces. I think the order would be: Measure pushrod (cam lobe) travel, and calculate/Measure valve stem movement based on rocker geometry. Calculate 1/2 way point in valve movement cycle. Shim the pedistals to make the rockers sit horizontal when the valve is at this half way point. Set pushrod lengths based on this rocker arm geometry. Check for binding Check some more for binding! *I'm not certain this is correct, it might be that with the valve closed the forces required to move the valve are lower than the forces to move it when fully open, so you might want to limit horizontal forces at fully open valve geometry. I assume the roller rockers will help on the valve stem end, but the pushrod geometry can be tricky too. I assume pushrods at too high an angle is a contributing factor to your issue. Link to comment Share on other sites More sharing options...
Escadrille Ecosse Posted March 26 Share Posted March 26 1 hour ago, egret said: I belive the goal is to have the rocker arm movement be symmetrical around horizontal*. I.e. fully open and fully closed valve are equal angles away from horizontal. This way you keep the angles as small as possible and minimise any unwanted horizontal forces. Essentially this is what we're trying to achieve. Geometrically as the rocker moves about the axis of the shaft the effective horizontal length (between the shaft and the point where the rocker sits on the valve head) changes. From a minimum at the top and bottom of the stroke to a maximum at some point between. Ideally at half valve lift. This obviously causes the tip of the rocker/roller to move side to side across the top of the valve stem. To minimise side loads on the valve stem and guides the aim is to minimise the side to side travel of the rocker tip and to keep it as nearly centred on the valve as possible within the restrictions of space available in the engine. Installing a higher lift cam and/or increased ratio rockers increases the range of motion of the rocker tip and valve so increasing the side loads on the valve and potentially leading to the rocker tip 'falling off' one side or other of the valve stem - generally the side nearest the rocker shaft. Larger lift cams also often mean there is insufficient room left in the ball adjusters to cope. The simplest/cheapest thing to do here is to keep the existing pushrods and shim the rocker shaft up to accommodate the larger cam lobes. Unfortunately this actually makes the geometry of the rocker to the valve worse and moves the centre of the side to side movement over the valve nearer to the rocker shaft. Inreases the side loading on the valve stem, increasing wear and valve rock. This is exacerbated by the use of high ratio rockers. Somewhat counterintuitively the actual solution is to use shorter pushrods and even SHORTEN the rocker pedestals for extreme cam/rocker combinations. Ideally you would also move the rocker shaft further away from the valves but this is not really practical. The other potential issue with shimming the rocker shaft to cope with high lift cams is that it causes the downtroke of the rocker at full lift to become extreme and combined with having the ball adjusters near the limit of adjustment it is possible for the cup on the top of the pushrod to come in contact with the underside of the rocker and lift off the ball. Best case there is a bit of 'twang' in the pushrod and it just increases wear on the cam and valvetrain. Worst case things get bent, pop out, break. I found with the Spitfire that with the 40-80 0.305" lift cam from Tony that the 1.6 rockers were unneccesary and that I needed shortened tubular pushrods to keep the geometry sensible without pedestal shims and give sufficient space around the ball adjuster. @JohnD suggest you check the above with yours. Don't use pedestal shims. Do use solid spacers on the rocker shaft if not doing so already. The geometry check across the valve stem is as you say with engineer's blue. If you keep your 1.6 rockers than you will almost certainly want to shorten the pedestals. Tubular pushrods are pretty simple. They come standard length and you simply remove the cup end, file/turn down the tube as required and refit the cup. They are just a press fit. Very reliable too. Just make sure NOTHING binds over the full valve cycle as unlike the standard pushrods they won't act as sacrifical members for the rest of the valvetrain. 1 2 Link to comment Share on other sites More sharing options...
egret Posted March 26 Share Posted March 26 (edited) @Escadrille Ecosse of course! The fixed point is the top of the movement, when the valve is closed, so you want to shorten the pedistal to drop the pivot point. More lift = more open, as you can't get more closed than closed! Edit to show a quick sketch I put together to helped me understand a little better. It shows the push rod on left, valve on right. Blue lines being a notional standard geometry, and green being a higher lift geometry. I have somewhat picked numbers out of thin air to allow my brain to see it, but I show 1.5: 1 rocker arms and probably a rather extreme example of about 30% lift increase for the green- the blue is 15° max rocker rotation away from the horizontal with the green being 20°. It shows how rocker pedista, and pushrods need to become shorter to accommodate higher lift and how all the geometry moves away from optimal as the angles increase. Trying to deliver this higher lift without moving the rocker pivot point would be even worse (second image). Edited March 26 by egret 2 1 Link to comment Share on other sites More sharing options...
JohnD Posted March 26 Author Share Posted March 26 Egret, Colin, Thank you! I've actually printed your responses out, as I'm old fashioned about learning from a screen! I'll give it a try! John 1 Link to comment Share on other sites More sharing options...
PeteStupps Posted March 26 Share Posted March 26 What an illuminating discussion, thanks Colin for your explanation and Egret for the illustration. Every day's a school day on Sideways 1 Link to comment Share on other sites More sharing options...
egret Posted March 26 Share Posted March 26 Just realised shimming is a compromised solution for when you skim the head and effectively increase the pushrod length to the point you run out of adjustment in the rockers. Link to comment Share on other sites More sharing options...
JohnD Posted March 26 Author Share Posted March 26 (edited) Absolutely, Pete! Like in NHS "Multidisciplinary Team Meetings" bring all the specialists concerned with a patient together, or the Common Room of a University or School all the subjects, there are people here who know things that you don't and which you need! It's possible that more things have been learnt over a coffee or a glass than ever in the lab, library or lecture theatre! As to progress, this afternoon I made an adjustable length pushrod, as suggested by RR, mainly because I could without much new learning. Back to the books and paper tonight! Thanks to all who have educated me! John Edited March 26 by JohnD Link to comment Share on other sites More sharing options...
Hamish Posted March 26 Share Posted March 26 John I have no expertise to impart but I do rate tr enterprises and they are expanding their parts shop they have these on eBay and the description suggests good for non injection skimmed heads https://www.ebay.co.uk/itm/185331814482?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=BO0FK8KtTe2&sssrc=4429486&ssuid=33-Tg7STSaq&var=&widget_ver=artemis&media=COPY Link to comment Share on other sites More sharing options...
RedRooster Posted March 26 Share Posted March 26 1 hour ago, JohnD said: s to progress, this afternoon I made an adjustable length pushrod, as suggested by RR, mainly because I could without much new learning. Back to the books and paper tonight! John Good that's how John Sleath showed me how to do it. https://www.john-sleath.com/ RR Link to comment Share on other sites More sharing options...
JohnD Posted March 26 Author Share Posted March 26 What a pity, RR, my firewall lights up with a security warning "Privacy error" when I follow your link. Hamish, good idea, but I'll wait and see what the length calculations lead to. JOhn Link to comment Share on other sites More sharing options...
Escadrille Ecosse Posted March 26 Share Posted March 26 2 hours ago, egret said: Just realised shimming is a compromised solution for when you skim the head and effectively increase the pushrod length to the point you run out of adjustment in the rockers. Although we're talking about a pretty massive skim. And once again this adversely affects the valve/rocker geometry. Link to comment Share on other sites More sharing options...
Nick Jones Posted March 26 Share Posted March 26 All I can say is that it’s perhaps surprising that mine works ok….. 1 Link to comment Share on other sites More sharing options...
Escadrille Ecosse Posted March 26 Share Posted March 26 2 minutes ago, Nick Jones said: All I can say is that it’s perhaps surprising that mine works ok….. Nah. Unless you're trying to run a head skimmed to 11/1 CR with flat top pistons, 0.5" valve lift, 300+ duration and sustained 7000+ rpm it all works quite adequately well. It's fine... Link to comment Share on other sites More sharing options...
egret Posted March 26 Share Posted March 26 Perhaps this is an area where unless it's horribly wrong it's basically fine? Link to comment Share on other sites More sharing options...
Escadrille Ecosse Posted March 26 Share Posted March 26 Just now, egret said: Perhaps this is an area where unless it's horribly wrong it's basically fine? This ^^^^^^ Link to comment Share on other sites More sharing options...
michaeljf Posted March 26 Share Posted March 26 Hello, When we did mine there was a lot of testing the grey matter to get the calculations right! regards Michael. (Roller,both ends rockers,mod. head, decked block, chrome moly push rods, different valves/valve springs etc., Link to comment Share on other sites More sharing options...
Escadrille Ecosse Posted March 26 Share Posted March 26 And just be very wary when using roller rockers of the increased risk of the top of the pushrod hitting the much thicker area round the ball adjuster Link to comment Share on other sites More sharing options...
Nick Jones Posted March 26 Share Posted March 26 I’m using the standard rockers and pushrods. Quite a bit of valve lift. Not sure how thin the head is compared to the original but as it started off as a 2500S one and is now on a 2L at 10.5:1, so it’s had an epic skim. It’s done almost 30k in this form and has seen the rev limiter a few times so I guess it’s close enough. 1 Link to comment Share on other sites More sharing options...
JohnD Posted March 27 Author Share Posted March 27 OK, progress. To supplement the above good advice, I find that the Revington TR website includes a way to calculate if the rocker shaft is the correct height above the valves for good geometry, as explained by m'learned friends above. See: https://www.revingtontr.com/information-sheets/is0066-rocker-geometry I've sent most of today trying to follow it. I think that it aims to find that the central axis of the rocker shaft is halfway between the closed and open heights of the valves - would m'learned friends agree? - and every iteration of my measurements shows that it is in fact above that level. By how much? I've done the measurements half a dozen times, and never got the same result twice, but about 2.5mm. Certainly, a spirit level on a rocker shows that at rest, closed valve, the rocker top is level (I know that's not level with the pivot points at either end, but anyway) so that the arc of its travel is all below the horizontal, when (I think, M'learneds?) it should be horizontal halfway through that arc. Lower pedestals would go towards correcting that, so first task would be to have 2.5mm removed from their bases. This then brings us to the length of the push rods, but I can't see how I can check that with my adjustable rod (below) until the shaft is that much lower. M'learneds? Please? JOhn Link to comment Share on other sites More sharing options...
Escadrille Ecosse Posted March 27 Share Posted March 27 (edited) 3 hours ago, JohnD said: This then brings us to the length of the push rods, but I can't see how I can check that with my adjustable rod (below) until the shaft is that much lower. M'learneds? Please? Essentially correct there John. The rocker shaft needs to be lowered as you have found. Regards pushrods you need to account for the increased depth of the tail of the roller rockers, the 'cup' in the underside that surrounds the adjuster ball and clearance for the head of the pushrod so it doesn't strike the rocker. As the space around the head of the pushrod is not easily measured it's best to do this once the rocker heights are set. You can play around a bit with ball adjuster heights. Also with the tube nut type pushrods you need to check clearances on them too. There isn't actually that much room for the envelope the pushrods occupy when in motion. Edited March 27 by Escadrille Ecosse Link to comment Share on other sites More sharing options...
JohnD Posted March 28 Author Share Posted March 28 Thanks, Colin! When I've had the pedestals skimmed, and I fit that adjustable rod, I'll need to set that so that the rocker has the valve end up at rest and equally down when the valve is open. Thinking aloud, I can adjust it for length in position, until with the rocker adjuster screw in mid range, the gap between rocker and valve stem is correct. I can then adjust rod length and adjuster screw until the rod is an available length, or else I find that it has to be bespoke! Is that the way to go? Or is there a formal, 'classroom' method as detailed by Revington? John Link to comment Share on other sites More sharing options...
JohnD Posted April 15 Author Share Posted April 15 The CSCChas just published a race report on Donington, in which there is no mention- none deserved! - of SofS. BUT, they have also posted a short video on YouTube, AND SofS IS IN THE INTRO!!!!! Momentarily, but TWICE! WHOOOOOHOO! See: 2 Link to comment Share on other sites More sharing options...
Steve 13-60 Posted Tuesday at 05:37 PM Share Posted Tuesday at 05:37 PM Any news on the fix yet? And also the cause of the problem? Link to comment Share on other sites More sharing options...
JohnD Posted Tuesday at 06:42 PM Author Share Posted Tuesday at 06:42 PM I explained it above, Steve! After (long after) a significant skim, and fitting a set of roller rockers without thinking about the height of the pedastles, my push rods were too long. Correcting that, with careful measurement to derive the correct height and length, and I hope, all is now well. Certainly, in quick test up and down my road, it's pulling like a train. We'll find out this weekend. I was slow to enter because of the above, so I have a reserve place at Oulton Park this Saturday, But I will be able to practice, and if there is a little more than normal attrition, race! Qualy at 1005, race at 1430! All Sidewayzers welcome in the Paddock! John Link to comment Share on other sites More sharing options...
egret Posted Tuesday at 07:07 PM Share Posted Tuesday at 07:07 PM (edited) Sadly a little far from this part of the country for me to drop by (evening cub camp commitments with my eldest). Good luck, and fingers crossed it's running better than ever! Edited Tuesday at 07:08 PM by egret Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now