Category: Marcos Modifications

I gained some motivation today and decided to brave the cold of the garage. It was a very productive day as I managed to convert 2 of my rear radius rods to be adjustable. I’ve already fitted them back onto the car and at some point in the future I will attempt to adjust the rear axle position to hopefully stop the car pulling to one side. However I don’t plan to do this till I’ve fitted some adjustable front wishbones.
The other item I was working on was making a bump steer gauge. If I get a chance I’ll try to remember to take a photo of it but it was pretty simple to make out of a few pieces of wood and some spare 16mm square tube. The most expensive part of it would have been the dial gauge but as I already had one on these it didn’t cost me anything.
From taking measurements of the actual bump steer I discovered that it wasn’t quite as bad as my original front suspension model, but it’s still not very good. Once I had the real measurements I made a slight adjustment to the model to get it to match the real data (my measurement of the steering rack height was not correct, only 0.6cm out). The graph below shows the original and updated model along with the real data and hopefully the ideal once the modifications have been completed. From the chart you can see that there is a slight curve to the data and this is caused by the rack being to wide. I plan to make the rack 5cm narrower, which is the most that can be realistically achieved. As luck would have it this pretty much removes the curve from the line. To make the line horizontal e.g. no bump steer I need to either raise the point the ball joint mounts onto the steering arm by 1.4cm or lower the steering rack by 1.4cm. Due to the design of the front chassis the easiest option will be to alter how the steering rack attaches to the steering arm.

Not much has really happened on the car front as it’s hard to get motivated when it’s so cold out in the garage. Currently I’m waiting for a new gasket set so I can fix the oil leak on the gearbox. One job I have made a start on is converting the rear radius rods to be adjustable. So far I’ve removed them from the car and I’m currently waiting for a machine shop to make a couple of threaded inserts that can be welded into the tube. With a bit of luck I should be able to get these next week. The other area I’ve been looking at is front suspension geometry. I have already decided that I plan to make the top wishbones on my car adjustable so I can set the camber and caster of the front suspension more accurately.
While doing some research on the internet I found this suspension analysis software. You can download the software and try the full version free of charge for 10 day. As it was free and I had nothing better to do with my time I made of few rough measurements of the front suspension and entered it into the software. The one issue/inaccuracy I’ve found with the software so far is that it expects ball joints to be used on the front upright. As the Marcos uses trunnions it’s not really possible to model the suspension 100% accurately. However with adjusting the pivot points I found that it makes very little difference to the results adjusting the bottom pivot point.
The one area I was really interested in calculating was the bump steer as it has been reported that this is a problem on Marcos cars. So far the only adjustments I’ve tried altering in the software is the rack width (distance between the ball joints on the rack) and the height that the track rod end attaches to the steering arm. There is a copy of an article from a MOC magazine in the download section of my site that discusses how to calculate the ideal steering rack width. It would be possible to make lots of other adjustments but most of these would be very difficult or impossible to actually implement due to the current design of the front of the car.
The photo below shows a chart comparing the bump steer with different modifications made. The 2 changes I made to the model were to shorten the rack width by 50mm. This value was chosen as it is the same amount as the width of the spacers that Marcos Car’s fitted to the steering rack to stop the wheels rubbing on the body. Although the calculations show that the rack should be made even shorter I think this would actually be impossible without having to adjust how the steering rack mounts to the car. The other modification I simulated was adjusting the height the track rod end attaches to the steering arm. In the simulation I raised the attachment point by 2.4cm to get the improvements I wanted. In practice this could be implemented very easily with a modification shown in the second photo below.
Now all this theory is great but the only real way of knowing if the simulation is accurate or not is to go and measure the bump steer on the car (looks like I’ll have to make myself a bump steer gauge now). If the real measurements are similar to the simulation I will at least know what to modify to improve the situation.

The brake upgrade is complete and there is a definite improvement. However I’m still not happy with the handling of the car. I think there are a number of issues that are causing this but the main problem with the Marcos is that if you adjust the suspension it affects the front wheel geometry. Since the front suspension wishbones are not adjustable you have no way of correcting the geometry when you adjust the springs/ride height. On a Triumph car the suspension mounts bolt on so it’s possible to place shims behind the mounts to correct the steering geometry, but when they made the Marcos they decided to weld them in place. I have basically come to the conclusion that I need to have adjustable top wishbones made. This means I can set the springs up correctly to get the correct ride height (maybe even have the corner weight set up). Once this is done I can then adjust the front suspension to get the correct camber and caster. I also think I need more adjustment on the rear axle as only one of my radius rods is adjustable so I will probably modify the rest so they can all be adjusted. Hopefully I can get these mods done over the next few weeks so I can then spend some time getting the suspension set up correctly.
The rebuilt gear box and overdrive also worked but it seems to have a bad oil leak between the overdrive and gearbox. When I was rebuilding it I noticed the aluminum overdrive to gearbox adapter was slightly warped but I was hopping using lots of gasket sealer would work. Turns out it didn’t so I will need to remove the gearbox again to replace the gasket and possibly have the adapter skimmed at the machine shop. Looks like nothing is ever simple on a Marcos!

So close but so far. I almost managed to complete the front brake upgrade but was unable to connect the brake pipes. I should have realized that the threads in the calipers wouldn’t be imperial so I now need to buy a couple of metric banjo bolts to complete the upgrade.

Not much to report really. All I did today was give a few items a coat of paint so if I get a chance tomorrow I can finally assemble the brakes. If this goes well I might even be able to take the car for a test drive. If this happens it will also give me a chance to test the rebuilt gearbox and overdrive as well as the new brakes.