College Project

[ParaHandy]

Re: Sprung v unsprung weight

You *must* have suspension. Frighten yourself by towing your trailer empty at reasonable speed on typical UK "A" road and watch how much it moves. Don't forget that you may have to support the mast on the trailer.

Torsion bar suspension - v. simple with few parts reqd? Was used on Morris Minors c1960.

 

[tcm]

Re: fight breaks out in staffroom

Bearings are not destroyed by a bit of salty water. Further, these bearings wil be operating way below spec as they have to fit a relatively large dia axle, at tiny loads, and 2-3 revs a second motorway speeds won't warm them much if at all.

I-section can have at least as good a modulus as square tube. Angle not nec so good. But both can be "welded all round" for better joint, and more importantly can be rinsed so no seawater anywhere. Angle a good cheap option for an assembly not eally needs huge torsional rigidity. Weight advantage of composite somewhat irrelevant if chucking a boat on and dragging arond the roads.

Copying the competition re box section not relevant, IMHO, unless trying to make a mousetrap nearly as good as that other one over there. Most motorbikes had separate gearboxes, most car chassis were separate, until someone tried something else. For a college project (and in real life) try something new if there are good reasons. Shouldn't it be a BIT innovative? ;-)

 

[vyv_cox]

Bearing corrosion

Bearings are destroyed by miniscule amounts of condensed fresh water, never mind salt. Once the bearing is static, corrosion of the raceways occurs and fatigue pits nucleate at the corrosion pits. The vast majority of bearings fail by abuse, either corrosion or particulates. This is a most fortunate fact, as it has kept me in employment for the past 25 years.

 

[tcm]

Re: Bearing corrosion

I bow to your expertise, not doubting that damage is done to bearing. I just felt that it wd be an acceptable level of damage, given fairly (very) low load conditions in other respects. Agreed, new bearings wd be needed every n seasons, increasing requirement for new bearings due to pitting of raceways, and hence continued prosperity of all in that industry;-)

 

[Guest]

Re: College Project (Eureka)

-Thanks - Good idea. I'm trying to think of a way that suspension could somehow be incorporated with the wheels and the bearings.

LW

 

[brian_neale]

Re: College Project (Eureka)

I built a trailer for a Mirror some years back. Simple T-shape with a single welded joint, box section around 40mm, couple of steel strips to triangulate the joint. Suspension was a pair of units from Indespension, but the problem was actually getting units that worked with so little weight, so even with the boat on it tended to bounce a lot. The launching trailer was a piece of 4x2 timber with wheels on the end, and a U-shaped handle made from polythene pipe and plumbing fittings that was pinned into the cross bar, so easily taken apart for transport (it all went inside the boat, strapped down to stop it moving).

Don't forget that you also need mudguards to make it legal, and that complicates the construction! Of course, it could all be one-piece moulded carbon fibre, a la Pete Goss and his catamaran. Ceramic wheel bearings to avoid immersion problems? Wave-piercing trailer, anyone?

 

[Avocet]

Re: Sprung v unsprung weight

You will still need suspension to protect the boat from shock damage but I guess you could have a flexible part to the trailer and a stiff part. Why not have a transverse leaf spring in a composite material with a wheel on each end?

(Actually, this won't help the low ground clearance. Maybe that's a daft idea!)

Try to do something like that though. The wheels could go on two flexible trailing links instead.

 

[Guest]

Re: Sprung v unsprung weight

But what if the trailer was made from composite rod in such a way that the whole trailer flexed, thereby acting as it's own suspension.

LW

 

[tcm]

Re: Sprung v unsprung weight

This compsite stuff v expensive, LW. Also, suspension must be a finite part of the vehicle, towed or not. You can't have the whole things as "suspension" otherwise the sprung weight is only the boat, and the trailer will twang about. Much better to have the axles connected to the frame by some leaf springs, rubber mounts, to minimise unsprung weight, and then a rigid frame.

 

[vyv_cox]

Re: Sprung v unsprung weight

Unfortunately that's how they all are. I thought the idea of the project was to come up with a novel idea based upon the properties of materials not previously available. Like a pair of wheels on a single carbon fibre rod each, clamped to the side of the boat. Or something.

 

[ParaHandy]

Re: Sprung v unsprung weight

Oooh! Be careful. As a very young short person, I decided to build a go-kart. Father worked for John Brown shipyard who had copious quantities if 1.5” mild steel box section so designed & built the chassis, attached Villiers 200cc 2 stroke and proudly towed it to scrutineer who lived close by. He laughed. Said that if ever we hit a fellow competitor, machine would likely destroy it (actually engine couldn’t blow skin off rice pudding and that was before). Much chastened, then built v. light alloy tubular frame and fitted Zundapp 200cc short stroke. Due to miscalculation of compression ratio and subsequent welding of combustion chamber, this required towing at 30mph before it started but it did go. Like sh*t off a shovel before piston got eaten. It was tested on an old aerodrome shared with herd of cows who’d crapped everywhere and she was so low that it scooped the crud up and splattered it all over you. Anyway, the point is that this chassis flexed sufficiently to almost keep the 4 wheels on the road but as soon as you entered a corner, centrifugal force pushed the weight over onto the outward side, lifting the inboard wheels and, with slicks, the machine's centrifugal force slipped it sideways. You don't have that advantage. You want to get the CofG as low as possible and as close to the axle height as possible so that the wheel tends not to toe in. A bit like gybing a laser!

Your outboard wheel, without sufficient movement, will likely break traction, skip, grip quite violently and then flip it over. I can't see you being able to so arrange the chassis that allows the wheels sufficient independent movement whilst rigidly holding the boat in position. You can complicate the design (so that old farts like me can criticise - we're trying to help) but our lifelong experience has been to keep it simple.