I am installing a horn on my bobber my question is do I need to wire in a relay or can I just wire it into the ign then to a push button thanks

 

Use a relay.

From Popular Mechanics: The horn relay steps up the low current from the horn button to the higher current needed by the horns.

And from: http://www.rattlebars.com/mtz/hornrelay.html#basic

RELAY BASICS - 101
Some may not know what a relay is or how it works. Maybe a little parable. There once was a little man named Jon whose job it was to lift a weight of 50 lbs. He could do this all day long with no trouble. But, one day the boss came up and said that Jon now had to lift 200 lbs. But Jon knew he could not lift 200 lbs without eventually breaking his poor little back. So, he got his friend, Hercules to lift the 200 lbs each time that Jon tapped him on the shoulder. So Jon was able to lift the extra weight through Hercules' strength and Jon did not have to lift any weight at all.
Let's say you want to add some extra horns to your bike. Your horn switch (Jon) was designed little but it only had to supply current (50 lbs) to your dinky little stock horn and can do that easily. However, your new horns are bigger and require more current (200 lbs). If you simply hook up the new horns to your existing wiring and switch, then your switch will burn out rather quickly. How to get around this? Install a relay (Herculese). A relay is a mechanical or solid state SWITCH which is triggered (tapped on the shoulder) by current supplied to its trigger terminal (86). When current hits that terminal it closes the switch inside the relay, be it mechanical (by energizing a coil magnet) or solid state (by tripping a transistor). The other terminals of the relay then feed current from the INPUT terminal (30/51) through the now closed contacts to the OUTPUT terminal (87). Your old horn switch does not feed current to your new horns, it only triggers the switch inside the relay. The switch inside the relay feeds current to your horns from a completely different source (ie battery).
To wire up your new horns so that your little switch can work them, wire them as illustrated. Your old horn wire that went "hot" when you pushed the horn button is hooked to the TRIGGER terminal (86). When you hit the horn button, the button only needs to supply a small amount of current to trigger your relay. Hook up heavier wire thru a fuse directly from the battery to the INPUT (IN) terminal (30/51) and then hook your new horns "hot" to the OUTPUT (OUT) terminal (87). Terminal 85 is common ground (you can use your old horn ground). Now, instead of asking your weak and EXPENSIVE horn switch to work the increased load, your strong, CHEAP and easily replaceable relay does all the heavy lifting. (<A href="http://www.kpsec.freeuk.com/components/relay.htm" targer="_blank">Click here for more info) Relays will also fail after a time. Some are even rated by the number of times they will close a circuit before burning out (common relays are usually rated to 100,000 operations). The advantage is that a relay is normally cheaper and easier to replace than a switch. Using a relay also allows the use of smaller guage wire to the switch as well.

 

I don't think you do.
Look at a wiring diagram.
Starting at the ignition switch terminal marked 'I' (for ignition).
This terminal is hot when you switch on the key.
Following the wire from the terminal we go down to the 'accessory circuit breaker'. This is a circuit breaker, not a relay.
It works similar to a relay, but is basically an automatic switch which will reset itself after a problem.

Going through the circuit breaker. we come out on the other side with the 'hot' wire and follow it up to a common connection point.
Several wires meet together at this point.
Leaving that connection we go to the horn button, with the still 'hot' wire.

Pressing the horn button the 12V goes through the button and we proceed to the horn itself.
The horn output side is to ground, so when this 'hot' wire we are following reaches this point, the horn is going to start making some noise.
That is how the factory did it anyway.
pg

 

If you run multiple horns or change the stock horn to a high performance horn use a relay. Single stock horn you don't need it. I was trying to use the stock horn and a trumpet horn on my 68 and could only get one of them to work. Used a relay and now it sounds like a cruise ship horn. I wired the relay direct from the battery through a heavy duty fuse holder with the heavy wire. Works great.

 

I need to use a relay for these compressor air horns. Wired up as described by FilthyLucre. That is the compressor on the left side of the pic. As alrady pointed out, relay not needed for OEM horn.

 

This is just a horn from jp cycles nothing to brag about i am going to look at the electric for that year Thanks i just wanted to cheat if the current is small enough for the switch I don't know what the switchs are rated for