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Author Topic:   Trailer Spring Rate

fittongr posted 08-31-2014 10:56 PM ET (US)
Should the maximum spring capacity be the same as the total sprung weight of the load? Example: trailer capacity 1,700-lbs = sprung weight 1,700-lbs
Jefecinco posted 09-01-2014 10:38 AM ET (US)
I don't know. I've always replaced with a "like" item. If I was going to fabricate a new trailer I'd seek advice from a local fabricator. I guess a spring should be selected on the basis of load rather than trailer capacity. If the spring does not flex when it should it is too strong. If it flexes excessively under load it is too weak.
If the springs on my trailer need replacement I will probably replace the axle with a torsion axle due to doing 99% of my boating in salt water. An option for small boat trailers is a single leaf spring.
I'm currently shopping for a new trailer to replace the one that was manufactured in the 1980s. It needs new springs and tires, has had numerous welds to repair rust and does not fit the boat as well as it should. Rather than put a lot of money and effort into a trailer that is bound to require a lot of attention in the future I'll spend about $1800 for a new custom aluminum I beam trailer equipped with aluminum wheels, all stainless steel hardware, a spare mounted, LED lights, welded aluminum bunk and keel roller brackets, and torsion axle with a wide track to lower the frame and boat between the aluminum (not plastic) fenders.
That is a lot of money to sink into a 33 year old Sport 13 but it is the last boat I'll own and if I become unable to use it I'll just take my time restoring it and tinkering with it. Who knows, maybe some day it'll be a great garage find for someone.
Butch
elvis posted 09-01-2014 02:10 PM ET (US)
I just finished rebuilding my aluminum magic tilt trailer for my 87 Guardian 18, and I used similar springs as the trailer came with...1750 per side with new 3500 lb axle. I spent about $500 on the rebuild, shopped hard, and chose sturdy bilt trailers.
http://www.sturdybuiltonline.com . I had questions too, asked them..guy was extremely helpful, shipped me new axle, u bolts, springs, etc in like 3 days. Good stuff. You may want to call them and do some research...lots of other trailer places out there too, I'm sure plenty of advice!
fittongr posted 09-01-2014 06:33 PM ET (US)
thanks for your input. my boat is a 1969 13' std. with a 25 hp yamaha 2 cycle with tiller steering.
boat 325 lbs. (with mahogany - actual weight)
motor 104 lbs.
fuel 50 lbs. (single 6 gal. tank)
equip. 100 lbs. (est.)
trailer 100 lbs. (est.)(sprung - not axle & wheels)
total 679 lbs. (est.)
my trailer is not original to the boat. it's a 1999 magic trail. i cut 18" off the tonge to make it fit my whaler. i'm sure it was made for a boat and motor much heavier than mine. i feel that my lightweight boat doesn't even depress the springs. it bounces. i'm thinking of replacing the rusty springs with 2 - 500 lbs. leaf springs (total 1000 lbs.)
original question: if the load is 679 lbs. what would be the optimal spring rate for safety & control.
note: i bought 2 new 6 ply 4.80 x 12 tires. they require 90 lbs. of air. should i run less air?
thanks again for you advice.
AZdave posted 09-03-2014 01:45 AM ET (US)
Your method is logically correct, as you are including the parts of the trailer above the springs. I suspect that the trailer frame, bunks and fittings might be more than 100 lb. but you still have a safety margin with 1000 lb. of spring capacity. I try to keep trailer tires near their rated maximum inflation. I think the tires do not flex as much and run cooler (a major consideration in AZ).
Jerry Townsend posted 09-03-2014 12:16 PM ET (US)
fittongr - The springs should be rated above the sprung weight - and at least 50% more (I apologize as I am away from home and from my books and don't have that information firmly in my mind). This is because the your boat et al. is a dynamic load and the bouncing as it is towed induces the increased loading.
You want some spring action to soften the impacts to your boat. Using multi-leaf springs, lubricate the surfaces of the spring leaves.
Regarding your question on the tire pressure - if they REQUIRE 90 psi - that is what you should use. But, the less pressure, the softer the suspension system is. ---- Jerry/Idaho
fittongr posted 09-05-2014 08:42 PM ET (US)
jerry/idaho - thanks for your response. i agree that the 6 ply 90 lb. tires should be run at full inflation. i don't know what the rating on my current springs are, but they have 3 leafs and are way to stiff for my boat. the resource information you have is what i'm looking for. "the optimal ratio of spring rate to sprung weight". if you get a chance, please let me know what your resource material says on the matter or direct me to where i can read it for myself. and thanks again for your interest in my problem. ....glen

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Author	Topic: Trailer Spring Rate
fittongr	posted 08-31-2014 10:56 PM ET (US) Should the maximum spring capacity be the same as the total sprung weight of the load? Example: trailer capacity 1,700-lbs = sprung weight 1,700-lbs
Jefecinco	posted 09-01-2014 10:38 AM ET (US) I don't know. I've always replaced with a "like" item. If I was going to fabricate a new trailer I'd seek advice from a local fabricator. I guess a spring should be selected on the basis of load rather than trailer capacity. If the spring does not flex when it should it is too strong. If it flexes excessively under load it is too weak. If the springs on my trailer need replacement I will probably replace the axle with a torsion axle due to doing 99% of my boating in salt water. An option for small boat trailers is a single leaf spring. I'm currently shopping for a new trailer to replace the one that was manufactured in the 1980s. It needs new springs and tires, has had numerous welds to repair rust and does not fit the boat as well as it should. Rather than put a lot of money and effort into a trailer that is bound to require a lot of attention in the future I'll spend about $1800 for a new custom aluminum I beam trailer equipped with aluminum wheels, all stainless steel hardware, a spare mounted, LED lights, welded aluminum bunk and keel roller brackets, and torsion axle with a wide track to lower the frame and boat between the aluminum (not plastic) fenders. That is a lot of money to sink into a 33 year old Sport 13 but it is the last boat I'll own and if I become unable to use it I'll just take my time restoring it and tinkering with it. Who knows, maybe some day it'll be a great garage find for someone. Butch
elvis	posted 09-01-2014 02:10 PM ET (US) I just finished rebuilding my aluminum magic tilt trailer for my 87 Guardian 18, and I used similar springs as the trailer came with...1750 per side with new 3500 lb axle. I spent about $500 on the rebuild, shopped hard, and chose sturdy bilt trailers. http://www.sturdybuiltonline.com . I had questions too, asked them..guy was extremely helpful, shipped me new axle, u bolts, springs, etc in like 3 days. Good stuff. You may want to call them and do some research...lots of other trailer places out there too, I'm sure plenty of advice!
fittongr	posted 09-01-2014 06:33 PM ET (US) thanks for your input. my boat is a 1969 13' std. with a 25 hp yamaha 2 cycle with tiller steering. boat 325 lbs. (with mahogany - actual weight) motor 104 lbs. fuel 50 lbs. (single 6 gal. tank) equip. 100 lbs. (est.) trailer 100 lbs. (est.)(sprung - not axle & wheels) total 679 lbs. (est.) my trailer is not original to the boat. it's a 1999 magic trail. i cut 18" off the tonge to make it fit my whaler. i'm sure it was made for a boat and motor much heavier than mine. i feel that my lightweight boat doesn't even depress the springs. it bounces. i'm thinking of replacing the rusty springs with 2 - 500 lbs. leaf springs (total 1000 lbs.) original question: if the load is 679 lbs. what would be the optimal spring rate for safety & control. note: i bought 2 new 6 ply 4.80 x 12 tires. they require 90 lbs. of air. should i run less air? thanks again for you advice.
AZdave	posted 09-03-2014 01:45 AM ET (US) Your method is logically correct, as you are including the parts of the trailer above the springs. I suspect that the trailer frame, bunks and fittings might be more than 100 lb. but you still have a safety margin with 1000 lb. of spring capacity. I try to keep trailer tires near their rated maximum inflation. I think the tires do not flex as much and run cooler (a major consideration in AZ).
Jerry Townsend	posted 09-03-2014 12:16 PM ET (US) fittongr - The springs should be rated above the sprung weight - and at least 50% more (I apologize as I am away from home and from my books and don't have that information firmly in my mind). This is because the your boat et al. is a dynamic load and the bouncing as it is towed induces the increased loading. You want some spring action to soften the impacts to your boat. Using multi-leaf springs, lubricate the surfaces of the spring leaves. Regarding your question on the tire pressure - if they REQUIRE 90 psi - that is what you should use. But, the less pressure, the softer the suspension system is. ---- Jerry/Idaho
fittongr	posted 09-05-2014 08:42 PM ET (US) jerry/idaho - thanks for your response. i agree that the 6 ply 90 lb. tires should be run at full inflation. i don't know what the rating on my current springs are, but they have 3 leafs and are way to stiff for my boat. the resource information you have is what i'm looking for. "the optimal ratio of spring rate to sprung weight". if you get a chance, please let me know what your resource material says on the matter or direct me to where i can read it for myself. and thanks again for your interest in my problem. ....glen