Colin Mill

The speedo sender is a 2 wire Hall effect sensor. I think the problem is that the way 2 wire probes work is to modulate their own power line and to allow them to do this they are fed via a resistor. Its not a very robust way of doing things - if they get partially shunted by water the thing is going to stop working. On mine the wires come out of the potting inside a sleeve but the other end of the sleeve is not sealed so I guess water can enter the unit down this sleeve. I'm going to seal up the end of the sleeve with silicone rubber and self amalgamating tape in a hope of keeping it dry.

Why do you think you want the 2.75:1 first rather than the 2.66:1 first? Caterham's 6 speeder has a 2.69:1 first. Assuming they got their sums right the 2.66:1 is closer to the 6 speeders first than 2.75:1 is. Also you would expect that the optimum first gear for a 5 speeder would be longer than the optimum first for a 6 speeder. All of which points to the 2.66:1 option. Edited by - Colin Mill on 7 Aug 2004 14:47:54

Seems to me you need rather a lot of input data before you can really optimise the ratios. I think a torque curve would be important as well as an idea what the traction limited acceleration is going to be on the rubber and surface of your choice. There are a few other things like how hard a time you are prepared to give the clutch in the initial launch. A tall first gear is good so long as you are prepared to hold the revs in the launch up at a level that allows you to be traction limited. The taller the first gear the higher the revs will need to be to have enough torque to (just) unstick the rear tyres and the longer you will have to slip it before you can let the clutch all the way in. With your engine you have a power loading of about 210Watts/kg. How much of that you can expect at the rear wheels I don't know. However allowing 5% for transmission losses and if you reckon on getting 0.8g acceleration (7.85m/s) then you can be traction limited up to about 25m/s (56mph). If the traction limited acceleration is 0.95g then the traction limit is 21.5m/s (48 mph). So the more traction you have the lower first gear you can make use of. I would reckon that a first gear that takes you to only 34 mph at max power will allow you to light the rears up with only 60% to 70% of max torque. With a torque curve you could see the minimum revs you would need to hold during the launch. I think the reason why you have bigger gaps in the lower ratios is down to the length of time one spends accelerating through the gear. In second you can afford to be a bit lower down the power curve just after the change than you can in third because you will spend less time in the less favourable part of the engine's power curve in the early part of second than you will in the early part of third if you had the same rpm drop in both gears. OK I think I have wittered on enough! Colin

Since I have not seen the hydraulic clutch set up could I just ask does it have a spring for the preload at the bell-housing end? Thanks Colin

So the unsprung weights of the two cars are indeed almost the same! Looks as if Disks + calipers are a lot hevier than drums + wheel cylinders.

Noger It would be interesting if you could weigh that axle - the info could help sort the confusion Colin

Thanks for the info - sad to say I already drilled for the repeaters but some day I'll get rid of them (gatepost job I think) 😬

Let me expand on my confusion. A DD tube set-up allows you to have the diff and half the drive shafts as sprung rather than unsprung mass. If the extra 25kg of the DD car is entirely in the back end then with a diff weighing 20kg and driveshafts weighing 14kg we have 27kg of sprung mass in the DD back end. If the LA is actually 25kg lighter then the net change in unsprung mass is 2kg. On this basis we have the following options:- Accept that the unsprung masses of the two cars are substantially the same. Accept that the extra weight in the DD car is not actually in the rear axle. Accept that the data is just wrong somewhere.

So the DD gear is as heavy without its diff as the full LA kit *confused*

Hi Chris Sure, but there is not much weight in the DD tube - its a fairly light fabrication and the ears are alloy (maybe 1kg each max). Against that the LA has cast half shaft casings. There can't be much difference there. Half-shafts and wheel bearings will be much the same in both cases. That brings you to 4 CVJs in the DD setup that don't exist in the LA - I just don't see how that adds up to 20kg difference unless the LA diff is a hell of a lot lighter than the Sierra diff. If the Sierra diff weighed 40kg I could never have held it in place for 4 or 5 minutes while my friends got the top bolt in place. The extra weight has to be somewhere else in the car. Colin

Rob - Thanks for that. I'm glad I can bin the bolt-on scheme. Grubbster - Many thanks for the kind offer. Unfortunately I'm near Manchester which would make it a good excuse for an extended blat were the car ready Is the Silkaflex similar to RTV i.e. has quite a bit of give in it even when cured? Thanks again Colin PS the great thing about being near Manchester is being near the "Cat & Fiddle" road 😬

Adam I'm kicking myself for not taking a much closer look at it before I dropped the engine in a couple of weeks back. I just thought at the time that it looked suitable for a 250cc bike engine not a car. Do you know what size engine the thing is designed to crank? Colin

I am just about to fit the cycle wings and hate the standard bolt-on method so I want to use either a glue on method (with RTV silicone or similar) or the BigHead method with zipstraps. Has anyone had any problems with the SVA test from using either of these methods? Thanks Colin

Adam Is it possible that the switched feed to the solenoid (brown/Red wire) is a switched earth - even this would be a bit strange as you would expect the solenoid to run between an unswitched positive and switched negative. With the thing wired up and working as yours is I would expect the solenoid to energise ok because its resistance is a lot less than the starter. This could be good as the solenoid current also going through the starter helps spin the starter to find a mesh with the ring gear. However, once it pulls in, its own contacts effectively bypass the coil. It would immediately drop out unless the friction etc on the starter holds it in. This would mean that letting go of the key would not stop the starter unless the engine fires and offloads it!? It would crank a dead engine till the battery died while you sat and listened to it with your arms folded. All very oddball - no wonder there are problems. Colin

Hi Adam I think the negative end of the solenoid could be grounded internally to the case. Colin

I'm curious about the DD being hevier than the LA. I know the Sierra diff is a bit of a lump but I bench-pressed it into place quite easily so it can't be that heavy 😬

Don't forget that the torque reaction of the propshaft is balanced by a load difference on the rear tyres on a LA. With a 150HP motor with say 190Nm torque and a 3.36:1 first gear this will give you about 50Kg (105lb) load difference between the rear tyres so you will loose traction on the right rear wheel long before you would in a DD car.

Tony It could be so. I'm not convinced about the argument against insulation - if you have to crank the thing for so long that the solenoid cooks itself you have other problems! An Ali shield with a free flow of air behind it would, I think, be better anyway. I thought the problem affected the magnetti starters as well as the magneton ones. Am I right? Colin

I'm none too keen on the second feed idea either as its quite a lot of trouble to go to. Lets face it, there are millions of cars out there with starters from the same sources and they, for the most part, work fine. It has to be something specific to the way its installed in the seven. The proximity of the exhaust apart the only thing left is the mounting - if I get the trouble I'll butcher the mounting holes with a round file and take it from there.

Since my everyday motor has synchro on reverse I'm going to have to keep reminding myself about this one - no trying to whip it into reverse while doing 5mph forward then! Given how simple the car is electrically I'm amazed how it can pack in more quirks than a fully loaded Jag on an off day. Current problem is trivial but a pain - the small diameter bit on the fixing point bushes on the 4-point harness are all too short so the belt ends are not swiveling once the bolts are torqued up - a simple job to fix with a lathe but doesn't anyone at the suppiers or CC check this sort of thing ? Colin

I wonder if the meshing of the ring gear and the starter pinion gets closer when the engine is hot. (differential expansion??) If so that would explain why the starter fails to go into mesh as easily and why shifting the ring gear around by pushing in gear sometimes works. If the mesh gets so close the thing can't go into mesh then pushing won't help. Has anyone tried opening out the starter mounting holes so that the mesh can be moved out a little? I wonder if it would be worth adding a second feed to the starter. Using a second relay and a resistor a reduced feed could be sent to the starter as soon as the key is turned (i.e. before the main contacts make) This would tend to turn the starter so that if the pinion was 'head butting' into the ring gear out of mesh it would turn until it did mesh. Trouble is you would need a fairly chunky resistor as I doubt the starter tries to turn with much less than 20 amps through it. Of course this would only work if the mesh didn't close up totally. Just a thought Colin

Have just done the first roll out and start-up. Nice! I avoided trying a 7 before buying so that this moment would be really interesting and I'm not disappointed. I thought they were supposed to be noisy but this is just perfect. With the aid of a mown field I've clocked up 5 miles (that's 3 real ones and 2 virtual ones!) Seems like mine has all the little bits of character BlatChat has lead me to expect - the singing stepper motor, the self advancing odometer, the run on with the fan going, the brakes that feel like an early servoless VW polo. The type 9 box is nice except that getting reverse is a bit tricky. I seem to have to lean on the stick and very gently let the clutch up till it snicks in. Is this normal and will it ease off with use? Colin

Hi Oliver Interesting. The method could be done with mechanical actuation of course. Colin

Hi Dan I think I have worked out what the synchro replacement bits are. I think they are direction switchable one way clutches (sprag clutches). In a normal sprag clutch you have a series of rollers sitting on ramps in the outer race while the inner race is plain. If you rotate the inner in the direction that drives the rollers up the ramps the thing jams up. Rotate it the other way and its free. Now if you had the rollers in between two ramps in opposite directions then the clutch could jam in both directions. However by caging the rollers you could stop the clutch jaming in a given direction by biasing the cage one way (away from the appropriate ramps). Bias the cage in the opposite direction and you reverse the direction of lock. You could set the direction of operation with a tiny biasing force so the things could be electromagnetically actuated. The biasing force would (I think) be too small to unlock the clutch until it was offloaded. Colin

Hi Oliver Any info on how the 'wizard bits' are actuated (electromagnetic, hydraulic or what??) Colin